Attributes in Clang

Introduction

This page lists the attributes currently supported by Clang.

AMD GPU Attributes

amdgpu_flat_work_group_size

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
amdgpu_flat_work_group_size	clang::amdgpu_flat_work_group_size					Yes

The flat work-group size is the number of work-items in the work-group size specified when the kernel is dispatched. It is the product of the sizes of the x, y, and z dimension of the work-group.

Clang supports the __attribute__((amdgpu_flat_work_group_size(<min>, <max>))) attribute for the AMDGPU target. This attribute may be attached to a kernel function definition and is an optimization hint.

<min> parameter specifies the minimum flat work-group size, and <max> parameter specifies the maximum flat work-group size (must be greater than <min>) to which all dispatches of the kernel will conform. Passing 0, 0 as <min>, <max> implies the default behavior (128, 256).

If specified, the AMDGPU target backend might be able to produce better machine code for barriers and perform scratch promotion by estimating available group segment size.

An error will be given if:

• Specified values violate subtarget specifications;

• Specified values are not compatible with values provided through other attributes.

amdgpu_num_sgpr

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
amdgpu_num_sgpr	clang::amdgpu_num_sgpr					Yes

Clang supports the __attribute__((amdgpu_num_sgpr(<num_sgpr>))) and __attribute__((amdgpu_num_vgpr(<num_vgpr>))) attributes for the AMDGPU target. These attributes may be attached to a kernel function definition and are an optimization hint.

If these attributes are specified, then the AMDGPU target backend will attempt to limit the number of SGPRs and/or VGPRs used to the specified value(s). The number of used SGPRs and/or VGPRs may further be rounded up to satisfy the allocation requirements or constraints of the subtarget. Passing 0 as num_sgpr and/or num_vgpr implies the default behavior (no limits).

These attributes can be used to test the AMDGPU target backend. It is recommended that the amdgpu_waves_per_eu attribute be used to control resources such as SGPRs and VGPRs since it is aware of the limits for different subtargets.

An error will be given if:

• Specified values violate subtarget specifications;

• Specified values are not compatible with values provided through other attributes;

• The AMDGPU target backend is unable to create machine code that can meet the request.

amdgpu_num_vgpr

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
amdgpu_num_vgpr	clang::amdgpu_num_vgpr					Yes

Clang supports the __attribute__((amdgpu_num_sgpr(<num_sgpr>))) and __attribute__((amdgpu_num_vgpr(<num_vgpr>))) attributes for the AMDGPU target. These attributes may be attached to a kernel function definition and are an optimization hint.

If these attributes are specified, then the AMDGPU target backend will attempt to limit the number of SGPRs and/or VGPRs used to the specified value(s). The number of used SGPRs and/or VGPRs may further be rounded up to satisfy the allocation requirements or constraints of the subtarget. Passing 0 as num_sgpr and/or num_vgpr implies the default behavior (no limits).

These attributes can be used to test the AMDGPU target backend. It is recommended that the amdgpu_waves_per_eu attribute be used to control resources such as SGPRs and VGPRs since it is aware of the limits for different subtargets.

An error will be given if:

• Specified values violate subtarget specifications;

• Specified values are not compatible with values provided through other attributes;

• The AMDGPU target backend is unable to create machine code that can meet the request.

amdgpu_waves_per_eu

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
amdgpu_waves_per_eu	clang::amdgpu_waves_per_eu					Yes

A compute unit (CU) is responsible for executing the wavefronts of a work-group. It is composed of one or more execution units (EU), which are responsible for executing the wavefronts. An EU can have enough resources to maintain the state of more than one executing wavefront. This allows an EU to hide latency by switching between wavefronts in a similar way to symmetric multithreading on a CPU. In order to allow the state for multiple wavefronts to fit on an EU, the resources used by a single wavefront have to be limited. For example, the number of SGPRs and VGPRs. Limiting such resources can allow greater latency hiding, but can result in having to spill some register state to memory.

Clang supports the __attribute__((amdgpu_waves_per_eu(<min>[, <max>]))) attribute for the AMDGPU target. This attribute may be attached to a kernel function definition and is an optimization hint.

<min> parameter specifies the requested minimum number of waves per EU, and optional <max> parameter specifies the requested maximum number of waves per EU (must be greater than <min> if specified). If <max> is omitted, then there is no restriction on the maximum number of waves per EU other than the one dictated by the hardware for which the kernel is compiled. Passing 0, 0 as <min>, <max> implies the default behavior (no limits).

If specified, this attribute allows an advanced developer to tune the number of wavefronts that are capable of fitting within the resources of an EU. The AMDGPU target backend can use this information to limit resources, such as number of SGPRs, number of VGPRs, size of available group and private memory segments, in such a way that guarantees that at least <min> wavefronts and at most <max> wavefronts are able to fit within the resources of an EU. Requesting more wavefronts can hide memory latency but limits available registers which can result in spilling. Requesting fewer wavefronts can help reduce cache thrashing, but can reduce memory latency hiding.

This attribute controls the machine code generated by the AMDGPU target backend to ensure it is capable of meeting the requested values. However, when the kernel is executed, there may be other reasons that prevent meeting the request, for example, there may be wavefronts from other kernels executing on the EU.

An error will be given if:

• Specified values violate subtarget specifications;

• Specified values are not compatible with values provided through other attributes;

• The AMDGPU target backend is unable to create machine code that can meet the request.

Calling Conventions

Clang supports several different calling conventions, depending on the target platform and architecture. The calling convention used for a function determines how parameters are passed, how results are returned to the caller, and other low-level details of calling a function.

aarch64_vector_pcs

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
aarch64_vector_pcs	clang::aarch64_vector_pcs	clang::aarch64_vector_pcs

On AArch64 targets, this attribute changes the calling convention of a function to preserve additional floating-point and Advanced SIMD registers relative to the default calling convention used for AArch64.

This means it is more efficient to call such functions from code that performs extensive floating-point and vector calculations, because fewer live SIMD and FP registers need to be saved. This property makes it well-suited for e.g. floating-point or vector math library functions, which are typically leaf functions that require a small number of registers.

However, using this attribute also means that it is more expensive to call a function that adheres to the default calling convention from within such a function. Therefore, it is recommended that this attribute is only used for leaf functions.

For more information, see the documentation for aarch64_vector_pcs on the Arm Developer website.

fastcall

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
fastcall	gnu::fastcall	gnu::fastcall		__fastcall _fastcall

On 32-bit x86 targets, this attribute changes the calling convention of a function to use ECX and EDX as register parameters and clear parameters off of the stack on return. This convention does not support variadic calls or unprototyped functions in C, and has no effect on x86_64 targets. This calling convention is supported primarily for compatibility with existing code. Users seeking register parameters should use the regparm attribute, which does not require callee-cleanup. See the documentation for __fastcall on MSDN.

ms_abi

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
ms_abi	gnu::ms_abi	gnu::ms_abi

On non-Windows x86_64 targets, this attribute changes the calling convention of a function to match the default convention used on Windows x86_64. This attribute has no effect on Windows targets or non-x86_64 targets.

pcs

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
pcs	gnu::pcs	gnu::pcs

On ARM targets, this attribute can be used to select calling conventions similar to stdcall on x86. Valid parameter values are “aapcs” and “aapcs-vfp”.

preserve_all

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
preserve_all	clang::preserve_all	clang::preserve_all

On X86-64 and AArch64 targets, this attribute changes the calling convention of a function. The preserve_all calling convention attempts to make the code in the caller even less intrusive than the preserve_most calling convention. This calling convention also behaves identical to the C calling convention on how arguments and return values are passed, but it uses a different set of caller/callee-saved registers. This removes the burden of saving and recovering a large register set before and after the call in the caller. If the arguments are passed in callee-saved registers, then they will be preserved by the callee across the call. This doesn’t apply for values returned in callee-saved registers.

• On X86-64 the callee preserves all general purpose registers, except for R11. R11 can be used as a scratch register. Furthermore it also preserves all floating-point registers (XMMs/YMMs).

The idea behind this convention is to support calls to runtime functions that don’t need to call out to any other functions.

This calling convention, like the preserve_most calling convention, will be used by a future version of the Objective-C runtime and should be considered experimental at this time.

preserve_most

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
preserve_most	clang::preserve_most	clang::preserve_most

On X86-64 and AArch64 targets, this attribute changes the calling convention of a function. The preserve_most calling convention attempts to make the code in the caller as unintrusive as possible. This convention behaves identically to the C calling convention on how arguments and return values are passed, but it uses a different set of caller/callee-saved registers. This alleviates the burden of saving and recovering a large register set before and after the call in the caller. If the arguments are passed in callee-saved registers, then they will be preserved by the callee across the call. This doesn’t apply for values returned in callee-saved registers.

• On X86-64 the callee preserves all general purpose registers, except for R11. R11 can be used as a scratch register. Floating-point registers (XMMs/YMMs) are not preserved and need to be saved by the caller.

The idea behind this convention is to support calls to runtime functions that have a hot path and a cold path. The hot path is usually a small piece of code that doesn’t use many registers. The cold path might need to call out to another function and therefore only needs to preserve the caller-saved registers, which haven’t already been saved by the caller. The preserve_most calling convention is very similar to the cold calling convention in terms of caller/callee-saved registers, but they are used for different types of function calls. coldcc is for function calls that are rarely executed, whereas preserve_most function calls are intended to be on the hot path and definitely executed a lot. Furthermore preserve_most doesn’t prevent the inliner from inlining the function call.

This calling convention will be used by a future version of the Objective-C runtime and should therefore still be considered experimental at this time. Although this convention was created to optimize certain runtime calls to the Objective-C runtime, it is not limited to this runtime and might be used by other runtimes in the future too. The current implementation only supports X86-64 and AArch64, but the intention is to support more architectures in the future.

regcall

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
regcall	gnu::regcall	gnu::regcall		__regcall

On x86 targets, this attribute changes the calling convention to __regcall convention. This convention aims to pass as many arguments as possible in registers. It also tries to utilize registers for the return value whenever it is possible.

regparm

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
regparm	gnu::regparm	gnu::regparm

On 32-bit x86 targets, the regparm attribute causes the compiler to pass the first three integer parameters in EAX, EDX, and ECX instead of on the stack. This attribute has no effect on variadic functions, and all parameters are passed via the stack as normal.

stdcall

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
stdcall	gnu::stdcall	gnu::stdcall		__stdcall _stdcall

On 32-bit x86 targets, this attribute changes the calling convention of a function to clear parameters off of the stack on return. This convention does not support variadic calls or unprototyped functions in C, and has no effect on x86_64 targets. This calling convention is used widely by the Windows API and COM applications. See the documentation for __stdcall on MSDN.

thiscall

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
thiscall	gnu::thiscall	gnu::thiscall		__thiscall _thiscall

On 32-bit x86 targets, this attribute changes the calling convention of a function to use ECX for the first parameter (typically the implicit this parameter of C++ methods) and clear parameters off of the stack on return. This convention does not support variadic calls or unprototyped functions in C, and has no effect on x86_64 targets. See the documentation for __thiscall on MSDN.

vectorcall

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
vectorcall	clang::vectorcall	clang::vectorcall		__vectorcall _vectorcall

On 32-bit x86 and x86_64 targets, this attribute changes the calling convention of a function to pass vector parameters in SSE registers.

On 32-bit x86 targets, this calling convention is similar to __fastcall. The first two integer parameters are passed in ECX and EDX. Subsequent integer parameters are passed in memory, and callee clears the stack. On x86_64 targets, the callee does not clear the stack, and integer parameters are passed in RCX, RDX, R8, and R9 as is done for the default Windows x64 calling convention.

On both 32-bit x86 and x86_64 targets, vector and floating point arguments are passed in XMM0-XMM5. Homogeneous vector aggregates of up to four elements are passed in sequential SSE registers if enough are available. If AVX is enabled, 256 bit vectors are passed in YMM0-YMM5. Any vector or aggregate type that cannot be passed in registers for any reason is passed by reference, which allows the caller to align the parameter memory.

See the documentation for __vectorcall on MSDN for more details.

Consumed Annotation Checking

Clang supports additional attributes for checking basic resource management properties, specifically for unique objects that have a single owning reference. The following attributes are currently supported, although the implementation for these annotations is currently in development and are subject to change.

callable_when

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
callable_when	clang::callable_when					Yes

Use __attribute__((callable_when(...))) to indicate what states a method may be called in. Valid states are unconsumed, consumed, or unknown. Each argument to this attribute must be a quoted string. E.g.:

__attribute__((callable_when("unconsumed", "unknown")))

consumable

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
consumable	clang::consumable					Yes

Each class that uses any of the typestate annotations must first be marked using the consumable attribute. Failure to do so will result in a warning.

This attribute accepts a single parameter that must be one of the following: unknown, consumed, or unconsumed.

param_typestate

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
param_typestate	clang::param_typestate					Yes

This attribute specifies expectations about function parameters. Calls to an function with annotated parameters will issue a warning if the corresponding argument isn’t in the expected state. The attribute is also used to set the initial state of the parameter when analyzing the function’s body.

return_typestate

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
return_typestate	clang::return_typestate					Yes

The return_typestate attribute can be applied to functions or parameters. When applied to a function the attribute specifies the state of the returned value. The function’s body is checked to ensure that it always returns a value in the specified state. On the caller side, values returned by the annotated function are initialized to the given state.

When applied to a function parameter it modifies the state of an argument after a call to the function returns. The function’s body is checked to ensure that the parameter is in the expected state before returning.

set_typestate

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
set_typestate	clang::set_typestate					Yes

Annotate methods that transition an object into a new state with __attribute__((set_typestate(new_state))). The new state must be unconsumed, consumed, or unknown.

test_typestate

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
test_typestate	clang::test_typestate					Yes

Use __attribute__((test_typestate(tested_state))) to indicate that a method returns true if the object is in the specified state..

Customizing Swift Import

Clang supports additional attributes for customizing how APIs are imported into Swift.

swift_async

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_async	clang::swift_async	clang::swift_async				Yes

The swift_async attribute specifies if and how a particular function or Objective-C method is imported into a swift async method. For instance:

@interface MyClass : NSObject
-(void)notActuallyAsync:(int)p1 withCompletionHandler:(void (^)())handler
    __attribute__((swift_async(none)));

-(void)actuallyAsync:(int)p1 callThisAsync:(void (^)())fun
    __attribute__((swift_async(swift_private, 1)));
@end

Here, notActuallyAsync:withCompletionHandler would have been imported as async (because it’s last parameter’s selector piece is withCompletionHandler) if not for the swift_async(none) attribute. Conversely, actuallyAsync:callThisAsync wouldn’t have been imported as async if not for the swift_async attribute because it doesn’t match the naming convention.

When using swift_async to enable importing, the first argument to the attribute is either swift_private or not_swift_private to indicate whether the function/method is private to the current framework, and the second argument is the index of the completion handler parameter.

swift_async_error

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_async_error	clang::swift_async_error	clang::swift_async_error				Yes

The swift_async_error attribute specifies how an error state will be represented in a swift async method. It’s a bit analogous to the swift_error attribute for the generated async method. The swift_async_error attribute can indicate a variety of different ways of representing an error.

• __attribute__((swift_async_error(zero_argument, N))), specifies that the async method is considered to have failed if the Nth argument to the completion handler is zero.

• __attribute__((swift_async_error(nonzero_argument, N))), specifies that the async method is considered to have failed if the Nth argument to the completion handler is non-zero.

• __attribute__((swift_async_error(nonnull_error))), specifies that the async method is considered to have failed if the NSError * argument to the completion handler is non-null.

• __attribute__((swift_async_error(none))), specifies that the async method cannot fail.

For instance:

@interface MyClass : NSObject
-(void)asyncMethod:(void (^)(char, int, float))handler
    __attribute__((swift_async(swift_private, 1)))
    __attribute__((swift_async_error(zero_argument, 2)));
@end

Here, the swift_async attribute specifies that handler is the completion handler for this method, and the swift_async_error attribute specifies that the int parameter is the one that represents the error.

swift_async_name

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_async_name						Yes

The swift_async_name attribute provides the name of the async overload for the given declaration in Swift. If this attribute is absent, the name is transformed according to the algorithm built into the Swift compiler.

The argument is a string literal that contains the Swift name of the function or method. The name may be a compound Swift name. The function or method with such an attribute must have more than zero parameters, as its last parameter is assumed to be a callback that’s eliminated in the Swift async name.

@interface URL
+ (void) loadContentsFrom:(URL *)url callback:(void (^)(NSData *))data __attribute__((__swift_async_name__("URL.loadContentsFrom(_:)")))
@end

swift_attr

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_attr						Yes

The swift_attr provides a Swift-specific annotation for the declaration to which the attribute appertains to. It can be used on any declaration in Clang. This kind of annotation is ignored by Clang as it doesn’t have any semantic meaning in languages supported by Clang. The Swift compiler can interpret these annotations according to its own rules when importing C or Objective-C declarations.

swift_bridge

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_bridge

The swift_bridge attribute indicates that the declaration to which the attribute appertains is bridged to the named Swift type.

__attribute__((__objc_root__))
@interface Base
- (instancetype)init;
@end

__attribute__((__swift_bridge__("BridgedI")))
@interface I : Base
@end

In this example, the Objective-C interface I will be made available to Swift with the name BridgedI. It would be possible for the compiler to refer to I still in order to bridge the type back to Objective-C.

swift_bridged

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_bridged_typedef						Yes

The swift_bridged_typedef attribute indicates that when the typedef to which the attribute appertains is imported into Swift, it should refer to the bridged Swift type (e.g. Swift’s String) rather than the Objective-C type as written (e.g. NSString).

@interface NSString;
typedef NSString *AliasedString __attribute__((__swift_bridged_typedef__));

extern void acceptsAliasedString(AliasedString _Nonnull parameter);

In this case, the function acceptsAliasedString will be imported into Swift as a function which accepts a String type parameter.

swift_error

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_error						Yes

The swift_error attribute controls whether a particular function (or Objective-C method) is imported into Swift as a throwing function, and if so, which dynamic convention it uses.

All of these conventions except none require the function to have an error parameter. Currently, the error parameter is always the last parameter of type NSError** or CFErrorRef*. Swift will remove the error parameter from the imported API. When calling the API, Swift will always pass a valid address initialized to a null pointer.

• swift_error(none) means that the function should not be imported as throwing. The error parameter and result type will be imported normally.

• swift_error(null_result) means that calls to the function should be considered to have thrown if they return a null value. The return type must be a pointer type, and it will be imported into Swift with a non-optional type. This is the default error convention for Objective-C methods that return pointers.

• swift_error(zero_result) means that calls to the function should be considered to have thrown if they return a zero result. The return type must be an integral type. If the return type would have been imported as Bool, it is instead imported as Void. This is the default error convention for Objective-C methods that return a type that would be imported as Bool.

• swift_error(nonzero_result) means that calls to the function should be considered to have thrown if they return a non-zero result. The return type must be an integral type. If the return type would have been imported as Bool, it is instead imported as Void.

• swift_error(nonnull_error) means that calls to the function should be considered to have thrown if they leave a non-null error in the error parameter. The return type is left unmodified.

swift_name

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_name

The swift_name attribute provides the name of the declaration in Swift. If this attribute is absent, the name is transformed according to the algorithm built into the Swift compiler.

The argument is a string literal that contains the Swift name of the function, variable, or type. When renaming a function, the name may be a compound Swift name. For a type, enum constant, property, or variable declaration, the name must be a simple or qualified identifier.

@interface URL
- (void) initWithString:(NSString *)s __attribute__((__swift_name__("URL.init(_:)")))
@end

void __attribute__((__swift_name__("squareRoot()"))) sqrt(double v) {
}

swift_newtype

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_newtype swift_wrapper						Yes

The swift_newtype attribute indicates that the typedef to which the attribute appertains is imported as a new Swift type of the typedef’s name. Previously, the attribute was spelt swift_wrapper. While the behaviour of the attribute is identical with either spelling, swift_wrapper is deprecated, only exists for compatibility purposes, and should not be used in new code.

• swift_newtype(struct) means that a Swift struct will be created for this typedef.

• swift_newtype(enum) means that a Swift enum will be created for this typedef.

  // Import UIFontTextStyle as an enum type, with enumerated values being
  // constants.
  typedef NSString * UIFontTextStyle __attribute__((__swift_newtype__(enum)));
  
  // Import UIFontDescriptorFeatureKey as a structure type, with enumerated
  // values being members of the type structure.
  typedef NSString * UIFontDescriptorFeatureKey __attribute__((__swift_newtype__(struct)));
  

swift_objc_members

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_objc_members						Yes

This attribute indicates that Swift subclasses and members of Swift extensions of this class will be implicitly marked with the @objcMembers Swift attribute, exposing them back to Objective-C.

swift_private

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_private

Declarations marked with the swift_private attribute are hidden from the framework client but are still made available for use within the framework or Swift SDK overlay.

The purpose of this attribute is to permit a more idomatic implementation of declarations in Swift while hiding the non-idiomatic one.

Declaration Attributes

Owner

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
	gsl::Owner					Yes

Note

This attribute is experimental and its effect on analysis is subject to change in a future version of clang.

The attribute [[gsl::Owner(T)]] applies to structs and classes that own an object of type T:

class [[gsl::Owner(int)]] IntOwner {
private:
  int value;
public:
  int *getInt() { return &value; }
};

The argument T is optional and is ignored. This attribute may be used by analysis tools and has no effect on code generation. A void argument means that the class can own any type.

See Pointer for an example.

Pointer

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
	gsl::Pointer					Yes

Note

This attribute is experimental and its effect on analysis is subject to change in a future version of clang.

The attribute [[gsl::Pointer(T)]] applies to structs and classes that behave like pointers to an object of type T:

class [[gsl::Pointer(int)]] IntPointer {
private:
  int *valuePointer;
public:
  int *getInt() { return &valuePointer; }
};

The argument T is optional and is ignored. This attribute may be used by analysis tools and has no effect on code generation. A void argument means that the pointer can point to any type.

Example: When constructing an instance of a class annotated like this (a Pointer) from an instance of a class annotated with [[gsl::Owner]] (an Owner), then the analysis will consider the Pointer to point inside the Owner. When the Owner’s lifetime ends, it will consider the Pointer to be dangling.

int f() {
  IntPointer P;
  if (true) {
    IntOwner O(7);
    P = IntPointer(O); // P "points into" O
  } // P is dangling
  return P.get(); // error: Using a dangling Pointer.
}

__single_inhertiance, __multiple_inheritance, __virtual_inheritance

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
				__single_inheritance __multiple_inheritance __virtual_inheritance __unspecified_inheritance

This collection of keywords is enabled under -fms-extensions and controls the pointer-to-member representation used on *-*-win32 targets.

The *-*-win32 targets utilize a pointer-to-member representation which varies in size and alignment depending on the definition of the underlying class.

However, this is problematic when a forward declaration is only available and no definition has been made yet. In such cases, Clang is forced to utilize the most general representation that is available to it.

These keywords make it possible to use a pointer-to-member representation other than the most general one regardless of whether or not the definition will ever be present in the current translation unit.

This family of keywords belong between the class-key and class-name:

struct __single_inheritance S;
int S::*i;
struct S {};

This keyword can be applied to class templates but only has an effect when used on full specializations:

template <typename T, typename U> struct __single_inheritance A; // warning: inheritance model ignored on primary template
template <typename T> struct __multiple_inheritance A<T, T>; // warning: inheritance model ignored on partial specialization
template <> struct __single_inheritance A<int, float>;

Note that choosing an inheritance model less general than strictly necessary is an error:

struct __multiple_inheritance S; // error: inheritance model does not match definition
int S::*i;
struct S {};

asm

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
				asm __asm__

This attribute can be used on a function or variable to specify its symbol name.

On some targets, all C symbols are prefixed by default with a single character, typically _. This was done historically to distinguish them from symbols used by other languages. (This prefix is also added to the standard Itanium C++ ABI prefix on “mangled” symbol names, so that e.g. on such targets the true symbol name for a C++ variable declared as int cppvar; would be __Z6cppvar; note the two underscores.) This prefix is not added to the symbol names specified by the asm attribute; programmers wishing to match a C symbol name must compensate for this.

For example, consider the following C code:

int var1 asm("altvar") = 1;  // "altvar" in symbol table.
int var2 = 1; // "_var2" in symbol table.

void func1(void) asm("altfunc");
void func1(void) {} // "altfunc" in symbol table.
void func2(void) {} // "_func2" in symbol table.

Clang’s implementation of this attribute is compatible with GCC’s, documented here.

While it is possible to use this attribute to name a special symbol used internally by the compiler, such as an LLVM intrinsic, this is neither recommended nor supported and may cause the compiler to crash or miscompile. Users who wish to gain access to intrinsic behavior are strongly encouraged to request new builtin functions.

deprecated

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
deprecated	gnu::deprecated deprecated	gnu::deprecated deprecated	deprecated

The deprecated attribute can be applied to a function, a variable, or a type. This is useful when identifying functions, variables, or types that are expected to be removed in a future version of a program.

Consider the function declaration for a hypothetical function f:

void f(void) __attribute__((deprecated("message", "replacement")));

When spelled as __attribute__((deprecated)), the deprecated attribute can have two optional string arguments. The first one is the message to display when emitting the warning; the second one enables the compiler to provide a Fix-It to replace the deprecated name with a new name. Otherwise, when spelled as [[gnu::deprecated]] or [[deprecated]], the attribute can have one optional string argument which is the message to display when emitting the warning.

empty_bases

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
			empty_bases

The empty_bases attribute permits the compiler to utilize the empty-base-optimization more frequently. This attribute only applies to struct, class, and union types. It is only supported when using the Microsoft C++ ABI.

enum_extensibility

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
enum_extensibility	clang::enum_extensibility	clang::enum_extensibility				Yes

Attribute enum_extensibility is used to distinguish between enum definitions that are extensible and those that are not. The attribute can take either closed or open as an argument. closed indicates a variable of the enum type takes a value that corresponds to one of the enumerators listed in the enum definition or, when the enum is annotated with flag_enum, a value that can be constructed using values corresponding to the enumerators. open indicates a variable of the enum type can take any values allowed by the standard and instructs clang to be more lenient when issuing warnings.

enum __attribute__((enum_extensibility(closed))) ClosedEnum {
  A0, A1
};

enum __attribute__((enum_extensibility(open))) OpenEnum {
  B0, B1
};

enum __attribute__((enum_extensibility(closed),flag_enum)) ClosedFlagEnum {
  C0 = 1 << 0, C1 = 1 << 1
};

enum __attribute__((enum_extensibility(open),flag_enum)) OpenFlagEnum {
  D0 = 1 << 0, D1 = 1 << 1
};

void foo1() {
  enum ClosedEnum ce;
  enum OpenEnum oe;
  enum ClosedFlagEnum cfe;
  enum OpenFlagEnum ofe;

  ce = A1;           // no warnings
  ce = 100;          // warning issued
  oe = B1;           // no warnings
  oe = 100;          // no warnings
  cfe = C0 | C1;     // no warnings
  cfe = C0 | C1 | 4; // warning issued
  ofe = D0 | D1;     // no warnings
  ofe = D0 | D1 | 4; // no warnings
}

external_source_symbol

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
external_source_symbol	clang::external_source_symbol	clang::external_source_symbol				Yes

The external_source_symbol attribute specifies that a declaration originates from an external source and describes the nature of that source.

The fact that Clang is capable of recognizing declarations that were defined externally can be used to provide better tooling support for mixed-language projects or projects that rely on auto-generated code. For instance, an IDE that uses Clang and that supports mixed-language projects can use this attribute to provide a correct ‘jump-to-definition’ feature. For a concrete example, consider a protocol that’s defined in a Swift file:

@objc public protocol SwiftProtocol {
  func method()
}

This protocol can be used from Objective-C code by including a header file that was generated by the Swift compiler. The declarations in that header can use the external_source_symbol attribute to make Clang aware of the fact that SwiftProtocol actually originates from a Swift module:

__attribute__((external_source_symbol(language="Swift",defined_in="module")))
@protocol SwiftProtocol
@required
- (void) method;
@end

Consequently, when ‘jump-to-definition’ is performed at a location that references SwiftProtocol, the IDE can jump to the original definition in the Swift source file rather than jumping to the Objective-C declaration in the auto-generated header file.

The external_source_symbol attribute is a comma-separated list that includes clauses that describe the origin and the nature of the particular declaration. Those clauses can be:

language=string-literal

The name of the source language in which this declaration was defined.

defined_in=string-literal

The name of the source container in which the declaration was defined. The exact definition of source container is language-specific, e.g. Swift’s source containers are modules, so defined_in should specify the Swift module name.

generated_declaration

This declaration was automatically generated by some tool.

The clauses can be specified in any order. The clauses that are listed above are all optional, but the attribute has to have at least one clause.

flag_enum

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
flag_enum	clang::flag_enum	clang::flag_enum				Yes

This attribute can be added to an enumerator to signal to the compiler that it is intended to be used as a flag type. This will cause the compiler to assume that the range of the type includes all of the values that you can get by manipulating bits of the enumerator when issuing warnings.

layout_version

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
			layout_version

The layout_version attribute requests that the compiler utilize the class layout rules of a particular compiler version. This attribute only applies to struct, class, and union types. It is only supported when using the Microsoft C++ ABI.

lto_visibility_public

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
lto_visibility_public	clang::lto_visibility_public	clang::lto_visibility_public				Yes

See LTO Visibility.

managed

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
managed			__managed__			Yes

The __managed__ attribute can be applied to a global variable declaration in HIP. A managed variable is emitted as an undefined global symbol in the device binary and is registered by __hipRegisterManagedVariable in init functions. The HIP runtime allocates managed memory and uses it to define the symbol when loading the device binary. A managed variable can be accessed in both device and host code.

novtable

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
			novtable

This attribute can be added to a class declaration or definition to signal to the compiler that constructors and destructors will not reference the virtual function table. It is only supported when using the Microsoft C++ ABI.

ns_error_domain

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
ns_error_domain						Yes

In Cocoa frameworks in Objective-C, one can group related error codes in enums and categorize these enums with error domains.

The ns_error_domain attribute indicates a global NSString or CFString constant representing the error domain that an error code belongs to. For pointer uniqueness and code size this is a constant symbol, not a literal.

The domain and error code need to be used together. The ns_error_domain attribute links error codes to their domain at the source level.

This metadata is useful for documentation purposes, for static analysis, and for improving interoperability between Objective-C and Swift. It is not used for code generation in Objective-C.

For example:

#define NS_ERROR_ENUM(_type, _name, _domain)  \
  enum _name : _type _name; enum __attribute__((ns_error_domain(_domain))) _name : _type

extern NSString *const MyErrorDomain;
typedef NS_ERROR_ENUM(unsigned char, MyErrorEnum, MyErrorDomain) {
  MyErrFirst,
  MyErrSecond,
};

objc_boxable

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
objc_boxable	clang::objc_boxable	clang::objc_boxable				Yes

Structs and unions marked with the objc_boxable attribute can be used with the Objective-C boxed expression syntax, @(...).

Usage: __attribute__((objc_boxable)). This attribute can only be placed on a declaration of a trivially-copyable struct or union:

struct __attribute__((objc_boxable)) some_struct {
  int i;
};
union __attribute__((objc_boxable)) some_union {
  int i;
  float f;
};
typedef struct __attribute__((objc_boxable)) _some_struct some_struct;

// ...

some_struct ss;
NSValue *boxed = @(ss);

objc_direct

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
objc_direct	clang::objc_direct	clang::objc_direct				Yes

The objc_direct attribute can be used to mark an Objective-C method as being direct. A direct method is treated statically like an ordinary method, but dynamically it behaves more like a C function. This lowers some of the costs associated with the method but also sacrifices some of the ordinary capabilities of Objective-C methods.

A message send of a direct method calls the implementation directly, as if it were a C function, rather than using ordinary Objective-C method dispatch. This is substantially faster and potentially allows the implementation to be inlined, but it also means the method cannot be overridden in subclasses or replaced dynamically, as ordinary Objective-C methods can.

Furthermore, a direct method is not listed in the class’s method lists. This substantially reduces the code-size overhead of the method but also means it cannot be called dynamically using ordinary Objective-C method dispatch at all; in particular, this means that it cannot override a superclass method or satisfy a protocol requirement.

Because a direct method cannot be overridden, it is an error to perform a super message send of one.

Although a message send of a direct method causes the method to be called directly as if it were a C function, it still obeys Objective-C semantics in other ways:

• If the receiver is nil, the message send does nothing and returns the zero value for the return type.

• A message send of a direct class method will cause the class to be initialized, including calling the +initialize method if present.

• The implicit _cmd parameter containing the method’s selector is still defined. In order to minimize code-size costs, the implementation will not emit a reference to the selector if the parameter is unused within the method.

Symbols for direct method implementations are implicitly given hidden visibility, meaning that they can only be called within the same linkage unit.

It is an error to do any of the following:

• declare a direct method in a protocol,

• declare an override of a direct method with a method in a subclass,

• declare an override of a non-direct method with a direct method in a subclass,

• declare a method with different directness in different class interfaces, or

• implement a non-direct method (as declared in any class interface) with a direct method.

If any of these rules would be violated if every method defined in an @implementation within a single linkage unit were declared in an appropriate class interface, the program is ill-formed with no diagnostic required. If a violation of this rule is not diagnosed, behavior remains well-defined; this paragraph is simply reserving the right to diagnose such conflicts in the future, not to treat them as undefined behavior.

Additionally, Clang will warn about any @selector expression that names a selector that is only known to be used for direct methods.

For the purpose of these rules, a “class interface” includes a class’s primary @interface block, its class extensions, its categories, its declared protocols, and all the class interfaces of its superclasses.

An Objective-C property can be declared with the direct property attribute. If a direct property declaration causes an implicit declaration of a getter or setter method (that is, if the given method is not explicitly declared elsewhere), the method is declared to be direct.

Some programmers may wish to make many methods direct at once. In order to simplify this, the objc_direct_members attribute is provided; see its documentation for more information.

objc_direct_members

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
objc_direct_members	clang::objc_direct_members	clang::objc_direct_members				Yes

The objc_direct_members attribute can be placed on an Objective-C @interface or @implementation to mark that methods declared therein should be considered direct by default. See the documentation for objc_direct for more information about direct methods.

When objc_direct_members is placed on an @interface block, every method in the block is considered to be declared as direct. This includes any implicit method declarations introduced by property declarations. If the method redeclares a non-direct method, the declaration is ill-formed, exactly as if the method was annotated with the objc_direct attribute.

When objc_direct_members is placed on an @implementation block, methods defined in the block are considered to be declared as direct unless they have been previously declared as non-direct in any interface of the class. This includes the implicit method definitions introduced by synthesized properties, including auto-synthesized properties.

objc_non_runtime_protocol

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
objc_non_runtime_protocol	clang::objc_non_runtime_protocol	clang::objc_non_runtime_protocol				Yes

The objc_non_runtime_protocol attribute can be used to mark that an Objective-C protocol is only used during static type-checking and doesn’t need to be represented dynamically. This avoids several small code-size and run-time overheads associated with handling the protocol’s metadata. A non-runtime protocol cannot be used as the operand of a @protocol expression, and dynamic attempts to find it with objc_getProtocol will fail.

If a non-runtime protocol inherits from any ordinary protocols, classes and derived protocols that declare conformance to the non-runtime protocol will dynamically list their conformance to those bare protocols.

objc_nonlazy_class

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
objc_nonlazy_class	clang::objc_nonlazy_class	clang::objc_nonlazy_class				Yes

This attribute can be added to an Objective-C @interface or @implementation declaration to add the class to the list of non-lazily initialized classes. A non-lazy class will be initialized eagerly when the Objective-C runtime is loaded. This is required for certain system classes which have instances allocated in non-standard ways, such as the classes for blocks and constant strings. Adding this attribute is essentially equivalent to providing a trivial +load method but avoids the (fairly small) load-time overheads associated with defining and calling such a method.

objc_runtime_name

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
objc_runtime_name	clang::objc_runtime_name	clang::objc_runtime_name				Yes

By default, the Objective-C interface or protocol identifier is used in the metadata name for that object. The objc_runtime_name attribute allows annotated interfaces or protocols to use the specified string argument in the object’s metadata name instead of the default name.

Usage: __attribute__((objc_runtime_name("MyLocalName"))). This attribute can only be placed before an @protocol or @interface declaration:

__attribute__((objc_runtime_name("MyLocalName")))
@interface Message
@end

objc_runtime_visible

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
objc_runtime_visible	clang::objc_runtime_visible	clang::objc_runtime_visible				Yes

This attribute specifies that the Objective-C class to which it applies is visible to the Objective-C runtime but not to the linker. Classes annotated with this attribute cannot be subclassed and cannot have categories defined for them.

objc_subclassing_restricted

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
objc_subclassing_restricted	clang::objc_subclassing_restricted	clang::objc_subclassing_restricted				Yes

This attribute can be added to an Objective-C @interface declaration to ensure that this class cannot be subclassed.

preferred_name

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
preferred_name	clang::preferred_name

The preferred_name attribute can be applied to a class template, and specifies a preferred way of naming a specialization of the template. The preferred name will be used whenever the corresponding template specialization would otherwise be printed in a diagnostic or similar context.

The preferred name must be a typedef or type alias declaration that refers to a specialization of the class template (not including any type qualifiers). In general this requires the template to be declared at least twice. For example:

template<typename T> struct basic_string;
using string = basic_string<char>;
using wstring = basic_string<wchar_t>;
template<typename T> struct [[clang::preferred_name(string),
                              clang::preferred_name(wstring)]] basic_string {
  // ...
};

selectany

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
selectany	gnu::selectany	gnu::selectany	selectany

This attribute appertains to a global symbol, causing it to have a weak definition ( linkonce ), allowing the linker to select any definition.

For more information see gcc documentation or msvc documentation.

transparent_union

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
transparent_union	gnu::transparent_union	gnu::transparent_union

This attribute can be applied to a union to change the behavior of calls to functions that have an argument with a transparent union type. The compiler behavior is changed in the following manner:

• A value whose type is any member of the transparent union can be passed as an argument without the need to cast that value.

• The argument is passed to the function using the calling convention of the first member of the transparent union. Consequently, all the members of the transparent union should have the same calling convention as its first member.

Transparent unions are not supported in C++.

trivial_abi

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
trivial_abi	clang::trivial_abi					Yes

The trivial_abi attribute can be applied to a C++ class, struct, or union. It instructs the compiler to pass and return the type using the C ABI for the underlying type when the type would otherwise be considered non-trivial for the purpose of calls. A class annotated with trivial_abi can have non-trivial destructors or copy/move constructors without automatically becoming non-trivial for the purposes of calls. For example:

// A is trivial for the purposes of calls because ``trivial_abi`` makes the
// user-provided special functions trivial.
struct __attribute__((trivial_abi)) A {
  ~A();
  A(const A &);
  A(A &&);
  int x;
};

// B's destructor and copy/move constructor are considered trivial for the
// purpose of calls because A is trivial.
struct B {
  A a;
};

If a type is trivial for the purposes of calls, has a non-trivial destructor, and is passed as an argument by value, the convention is that the callee will destroy the object before returning.

Attribute trivial_abi has no effect in the following cases:

• The class directly declares a virtual base or virtual methods.

• Copy constructors and move constructors of the class are all deleted.

• The class has a base class that is non-trivial for the purposes of calls.

• The class has a non-static data member whose type is non-trivial for the purposes of calls, which includes:

  • classes that are non-trivial for the purposes of calls

  • __weak-qualified types in Objective-C++

  • arrays of any of the above

using_if_exists

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
using_if_exists	clang::using_if_exists

The using_if_exists attribute applies to a using-declaration. It allows programmers to import a declaration that potentially does not exist, instead deferring any errors to the point of use. For instance:

namespace empty_namespace {};
__attribute__((using_if_exists))
using empty_namespace::does_not_exist; // no error!

does_not_exist x; // error: use of unresolved 'using_if_exists'

The C++ spelling of the attribte ([[clang::using_if_exists]]) is also supported as a clang extension, since ISO C++ doesn’t support attributes in this position. If the entity referred to by the using-declaration is found by name lookup, the attribute has no effect. This attribute is useful for libraries (primarily, libc++) that wish to redeclare a set of declarations in another namespace, when the availability of those declarations is difficult or impossible to detect at compile time with the preprocessor.

Field Attributes

no_unique_address

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
	no_unique_address

The no_unique_address attribute allows tail padding in a non-static data member to overlap other members of the enclosing class (and in the special case when the type is empty, permits it to fully overlap other members). The field is laid out as if a base class were encountered at the corresponding point within the class (except that it does not share a vptr with the enclosing object).

Example usage:

template<typename T, typename Alloc> struct my_vector {
  T *p;
  [[no_unique_address]] Alloc alloc;
  // ...
};
static_assert(sizeof(my_vector<int, std::allocator<int>>) == sizeof(int*));

[[no_unique_address]] is a standard C++20 attribute. Clang supports its use in C++11 onwards.

Function Attributes

#pragma omp declare simd

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
					omp declare simd

The declare simd construct can be applied to a function to enable the creation of one or more versions that can process multiple arguments using SIMD instructions from a single invocation in a SIMD loop. The declare simd directive is a declarative directive. There may be multiple declare simd directives for a function. The use of a declare simd construct on a function enables the creation of SIMD versions of the associated function that can be used to process multiple arguments from a single invocation from a SIMD loop concurrently. The syntax of the declare simd construct is as follows:

#pragma omp declare simd [clause[[,] clause] ...] new-line
[#pragma omp declare simd [clause[[,] clause] ...] new-line]
[...]
function definition or declaration

where clause is one of the following:

simdlen(length)
linear(argument-list[:constant-linear-step])
aligned(argument-list[:alignment])
uniform(argument-list)
inbranch
notinbranch

#pragma omp declare target

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
					omp declare target

The declare target directive specifies that variables and functions are mapped to a device for OpenMP offload mechanism.

The syntax of the declare target directive is as follows:

#pragma omp declare target new-line
declarations-definition-seq
#pragma omp end declare target new-line

or

#pragma omp declare target (extended-list) new-line

or

#pragma omp declare target clause[ [,] clause ... ] new-line

where clause is one of the following:

to(extended-list)
link(list)
device_type(host | nohost | any)

#pragma omp declare variant

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
					omp declare variant

The declare variant directive declares a specialized variant of a base function and specifies the context in which that specialized variant is used. The declare variant directive is a declarative directive. The syntax of the declare variant construct is as follows:

#pragma omp declare variant(variant-func-id) clause new-line
[#pragma omp declare variant(variant-func-id) clause new-line]
[...]
function definition or declaration

where clause is one of the following:

match(context-selector-specification)

and where variant-func-id is the name of a function variant that is either a base language identifier or, for C++, a template-id.

Clang provides the following context selector extensions, used via implementation={extension(EXTENSION)}:

match_all
match_any
match_none
disable_implicit_base
allow_templates

The match extensions change when the entire context selector is considered a match for an OpenMP context. The default is all, with none no trait in the selector is allowed to be in the OpenMP context, with any a single trait in both the selector and OpenMP context is sufficient. Only a single match extension trait is allowed per context selector. The disable extensions remove default effects of the begin declare variant applied to a definition. If disable_implicit_base is given, we will not introduce an implicit base function for a variant if no base function was found. The variant is still generated but will never be called, due to the absence of a base function and consequently calls to a base function. The allow extensions change when the begin declare variant effect is applied to a definition. If allow_templates is given, template function definitions are considered as specializations of existing or assumed template declarations with the same name. The template parameters for the base functions are used to instantiate the specialization.

_Noreturn

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
				_Noreturn

A function declared as _Noreturn shall not return to its caller. The compiler will generate a diagnostic for a function declared as _Noreturn that appears to be capable of returning to its caller. Despite being a type specifier, the _Noreturn attribute cannot be specified on a function pointer type.

abi_tag

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
abi_tag	gnu::abi_tag					Yes

The abi_tag attribute can be applied to a function, variable, class or inline namespace declaration to modify the mangled name of the entity. It gives the ability to distinguish between different versions of the same entity but with different ABI versions supported. For example, a newer version of a class could have a different set of data members and thus have a different size. Using the abi_tag attribute, it is possible to have different mangled names for a global variable of the class type. Therefore, the old code could keep using the old mangled name and the new code will use the new mangled name with tags.

acquire_capability, acquire_shared_capability

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
acquire_capability acquire_shared_capability exclusive_lock_function shared_lock_function	clang::acquire_capability clang::acquire_shared_capability

Marks a function as acquiring a capability.

alloc_align

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
alloc_align	gnu::alloc_align	gnu::alloc_align

Use __attribute__((alloc_align(<alignment>)) on a function declaration to specify that the return value of the function (which must be a pointer type) is at least as aligned as the value of the indicated parameter. The parameter is given by its index in the list of formal parameters; the first parameter has index 1 unless the function is a C++ non-static member function, in which case the first parameter has index 2 to account for the implicit this parameter.

// The returned pointer has the alignment specified by the first parameter.
void *a(size_t align) __attribute__((alloc_align(1)));

// The returned pointer has the alignment specified by the second parameter.
void *b(void *v, size_t align) __attribute__((alloc_align(2)));

// The returned pointer has the alignment specified by the second visible
// parameter, however it must be adjusted for the implicit 'this' parameter.
void *Foo::b(void *v, size_t align) __attribute__((alloc_align(3)));

Note that this attribute merely informs the compiler that a function always returns a sufficiently aligned pointer. It does not cause the compiler to emit code to enforce that alignment. The behavior is undefined if the returned pointer is not sufficiently aligned.

alloc_size

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
alloc_size	gnu::alloc_size	gnu::alloc_size

The alloc_size attribute can be placed on functions that return pointers in order to hint to the compiler how many bytes of memory will be available at the returned pointer. alloc_size takes one or two arguments.

• alloc_size(N) implies that argument number N equals the number of available bytes at the returned pointer.

• alloc_size(N, M) implies that the product of argument number N and argument number M equals the number of available bytes at the returned pointer.

Argument numbers are 1-based.

An example of how to use alloc_size

void *my_malloc(int a) __attribute__((alloc_size(1)));
void *my_calloc(int a, int b) __attribute__((alloc_size(1, 2)));

int main() {
  void *const p = my_malloc(100);
  assert(__builtin_object_size(p, 0) == 100);
  void *const a = my_calloc(20, 5);
  assert(__builtin_object_size(a, 0) == 100);
}

Note

This attribute works differently in clang than it does in GCC. Specifically, clang will only trace const pointers (as above); we give up on pointers that are not marked as const. In the vast majority of cases, this is unimportant, because LLVM has support for the alloc_size attribute. However, this may cause mildly unintuitive behavior when used with other attributes, such as enable_if.

allocator

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
			allocator

The __declspec(allocator) attribute is applied to functions that allocate memory, such as operator new in C++. When CodeView debug information is emitted (enabled by clang -gcodeview or clang-cl /Z7), Clang will attempt to record the code offset of heap allocation call sites in the debug info. It will also record the type being allocated using some local heuristics. The Visual Studio debugger uses this information to profile memory usage.

This attribute does not affect optimizations in any way, unlike GCC’s __attribute__((malloc)).

always_inline, __force_inline

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
always_inline	gnu::always_inline	gnu::always_inline		__forceinline		Yes

Inlining heuristics are disabled and inlining is always attempted regardless of optimization level.

Does not guarantee that inline substitution actually occurs.

See also the Microsoft Docs on Inline Functions, the GCC Common Function Attribute docs, and the GCC Inline docs.

artificial

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
artificial	gnu::artificial	gnu::artificial

The artificial attribute can be applied to an inline function. If such a function is inlined, the attribute indicates that debuggers should associate the resulting instructions with the call site, rather than with the corresponding line within the inlined callee.

assert_capability, assert_shared_capability

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
assert_capability assert_shared_capability	clang::assert_capability clang::assert_shared_capability

Marks a function that dynamically tests whether a capability is held, and halts the program if it is not held.

assume

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
assume	clang::assume	clang::assume				Yes

Clang supports the __attribute__((assume("assumption"))) attribute to provide additional information to the optimizer. The string-literal, here “assumption”, will be attached to the function declaration such that later analysis and optimization passes can assume the “assumption” to hold. This is similar to __builtin_assume but instead of an expression that can be assumed to be non-zero, the assumption is expressed as a string and it holds for the entire function.

A function can have multiple assume attributes and they propagate from prior declarations to later definitions. Multiple assumptions are aggregated into a single comma separated string. Thus, one can provide multiple assumptions via a comma separated string, i.a., __attribute__((assume("assumption1,assumption2"))).

While LLVM plugins might provide more assumption strings, the default LLVM optimization passes are aware of the following assumptions:

"omp_no_openmp"
"omp_no_openmp_routines"
"omp_no_parallelism"

The OpenMP standard defines the meaning of OpenMP assumptions (“omp_XYZ” is spelled “XYZ” in the OpenMP 5.1 Standard).

assume_aligned

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
assume_aligned	gnu::assume_aligned	gnu::assume_aligned				Yes

Use __attribute__((assume_aligned(<alignment>[,<offset>])) on a function declaration to specify that the return value of the function (which must be a pointer type) has the specified offset, in bytes, from an address with the specified alignment. The offset is taken to be zero if omitted.

// The returned pointer value has 32-byte alignment.
void *a() __attribute__((assume_aligned (32)));

// The returned pointer value is 4 bytes greater than an address having
// 32-byte alignment.
void *b() __attribute__((assume_aligned (32, 4)));

Note that this attribute provides information to the compiler regarding a condition that the code already ensures is true. It does not cause the compiler to enforce the provided alignment assumption.

availability

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
availability	clang::availability	clang::availability				Yes

The availability attribute can be placed on declarations to describe the lifecycle of that declaration relative to operating system versions. Consider the function declaration for a hypothetical function f:

void f(void) __attribute__((availability(macos,introduced=10.4,deprecated=10.6,obsoleted=10.7)));

The availability attribute states that f was introduced in macOS 10.4, deprecated in macOS 10.6, and obsoleted in macOS 10.7. This information is used by Clang to determine when it is safe to use f: for example, if Clang is instructed to compile code for macOS 10.5, a call to f() succeeds. If Clang is instructed to compile code for macOS 10.6, the call succeeds but Clang emits a warning specifying that the function is deprecated. Finally, if Clang is instructed to compile code for macOS 10.7, the call fails because f() is no longer available.

The availability attribute is a comma-separated list starting with the platform name and then including clauses specifying important milestones in the declaration’s lifetime (in any order) along with additional information. Those clauses can be:

introduced=version

The first version in which this declaration was introduced.

deprecated=version

The first version in which this declaration was deprecated, meaning that users should migrate away from this API.

obsoleted=version

The first version in which this declaration was obsoleted, meaning that it was removed completely and can no longer be used.

unavailable

This declaration is never available on this platform.

message=string-literal

Additional message text that Clang will provide when emitting a warning or error about use of a deprecated or obsoleted declaration. Useful to direct users to replacement APIs.

replacement=string-literal

Additional message text that Clang will use to provide Fix-It when emitting a warning about use of a deprecated declaration. The Fix-It will replace the deprecated declaration with the new declaration specified.

Multiple availability attributes can be placed on a declaration, which may correspond to different platforms. For most platforms, the availability attribute with the platform corresponding to the target platform will be used; any others will be ignored. However, the availability for watchOS and tvOS can be implicitly inferred from an iOS availability attribute. Any explicit availability attributes for those platforms are still preferred over the implicitly inferred availability attributes. If no availability attribute specifies availability for the current target platform, the availability attributes are ignored. Supported platforms are:

ios

Apple’s iOS operating system. The minimum deployment target is specified by the -mios-version-min=*version* or -miphoneos-version-min=*version* command-line arguments.

macos

Apple’s macOS operating system. The minimum deployment target is specified by the -mmacosx-version-min=*version* command-line argument. macosx is supported for backward-compatibility reasons, but it is deprecated.

tvos

Apple’s tvOS operating system. The minimum deployment target is specified by the -mtvos-version-min=*version* command-line argument.

watchos

Apple’s watchOS operating system. The minimum deployment target is specified by the -mwatchos-version-min=*version* command-line argument.

A declaration can typically be used even when deploying back to a platform version prior to when the declaration was introduced. When this happens, the declaration is weakly linked, as if the weak_import attribute were added to the declaration. A weakly-linked declaration may or may not be present a run-time, and a program can determine whether the declaration is present by checking whether the address of that declaration is non-NULL.

The flag strict disallows using API when deploying back to a platform version prior to when the declaration was introduced. An attempt to use such API before its introduction causes a hard error. Weakly-linking is almost always a better API choice, since it allows users to query availability at runtime.

If there are multiple declarations of the same entity, the availability attributes must either match on a per-platform basis or later declarations must not have availability attributes for that platform. For example:

void g(void) __attribute__((availability(macos,introduced=10.4)));
void g(void) __attribute__((availability(macos,introduced=10.4))); // okay, matches
void g(void) __attribute__((availability(ios,introduced=4.0))); // okay, adds a new platform
void g(void); // okay, inherits both macos and ios availability from above.
void g(void) __attribute__((availability(macos,introduced=10.5))); // error: mismatch

When one method overrides another, the overriding method can be more widely available than the overridden method, e.g.,:

@interface A
- (id)method __attribute__((availability(macos,introduced=10.4)));
- (id)method2 __attribute__((availability(macos,introduced=10.4)));
@end

@interface B : A
- (id)method __attribute__((availability(macos,introduced=10.3))); // okay: method moved into base class later
- (id)method __attribute__((availability(macos,introduced=10.5))); // error: this method was available via the base class in 10.4
@end

Starting with the macOS 10.12 SDK, the API_AVAILABLE macro from <os/availability.h> can simplify the spelling:

@interface A
- (id)method API_AVAILABLE(macos(10.11)));
- (id)otherMethod API_AVAILABLE(macos(10.11), ios(11.0));
@end

Availability attributes can also be applied using a #pragma clang attribute. Any explicit availability attribute whose platform corresponds to the target platform is applied to a declaration regardless of the availability attributes specified in the pragma. For example, in the code below, hasExplicitAvailabilityAttribute will use the macOS availability attribute that is specified with the declaration, whereas getsThePragmaAvailabilityAttribute will use the macOS availability attribute that is applied by the pragma.

#pragma clang attribute push (__attribute__((availability(macOS, introduced=10.12))), apply_to=function)
void getsThePragmaAvailabilityAttribute(void);
void hasExplicitAvailabilityAttribute(void) __attribute__((availability(macos,introduced=10.4)));
#pragma clang attribute pop

For platforms like watchOS and tvOS, whose availability attributes can be implicitly inferred from an iOS availability attribute, the logic is slightly more complex. The explicit and the pragma-applied availability attributes whose platform corresponds to the target platform are applied as described in the previous paragraph. However, the implicitly inferred attributes are applied to a declaration only when there is no explicit or pragma-applied availability attribute whose platform corresponds to the target platform. For example, the function below will receive the tvOS availability from the pragma rather than using the inferred iOS availability from the declaration:

#pragma clang attribute push (__attribute__((availability(tvOS, introduced=12.0))), apply_to=function)
void getsThePragmaTVOSAvailabilityAttribute(void) __attribute__((availability(iOS,introduced=11.0)));
#pragma clang attribute pop

The compiler is also able to apply implicitly inferred attributes from a pragma as well. For example, when targeting tvOS, the function below will receive a tvOS availability attribute that is implicitly inferred from the iOS availability attribute applied by the pragma:

#pragma clang attribute push (__attribute__((availability(iOS, introduced=12.0))), apply_to=function)
void infersTVOSAvailabilityFromPragma(void);
#pragma clang attribute pop

The implicit attributes that are inferred from explicitly specified attributes whose platform corresponds to the target platform are applied to the declaration even if there is an availability attribute that can be inferred from a pragma. For example, the function below will receive the tvOS, introduced=11.0 availability that is inferred from the attribute on the declaration rather than inferring availability from the pragma:

#pragma clang attribute push (__attribute__((availability(iOS, unavailable))), apply_to=function)
void infersTVOSAvailabilityFromAttributeNextToDeclaration(void)
  __attribute__((availability(iOS,introduced=11.0)));
#pragma clang attribute pop

Also see the documentation for @available

btf_decl_tag

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
btf_decl_tag	clang::btf_decl_tag	clang::btf_decl_tag				Yes

Clang supports the __attribute__((btf_decl_tag("ARGUMENT"))) attribute for all targets. This attribute may be attached to a struct/union, struct/union field, function, function parameter, variable or typedef declaration. If -g is specified, the ARGUMENT info will be preserved in IR and be emitted to dwarf. For BPF targets, the ARGUMENT info will be emitted to .BTF ELF section too.

callback

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
callback	clang::callback	clang::callback				Yes

The callback attribute specifies that the annotated function may invoke the specified callback zero or more times. The callback, as well as the passed arguments, are identified by their parameter name or position (starting with 1!) in the annotated function. The first position in the attribute identifies the callback callee, the following positions declare describe its arguments. The callback callee is required to be callable with the number, and order, of the specified arguments. The index 0, or the identifier this, is used to represent an implicit “this” pointer in class methods. If there is no implicit “this” pointer it shall not be referenced. The index ‘-1’, or the name “__”, represents an unknown callback callee argument. This can be a value which is not present in the declared parameter list, or one that is, but is potentially inspected, captured, or modified. Parameter names and indices can be mixed in the callback attribute.

The callback attribute, which is directly translated to callback metadata <http://llvm.org/docs/LangRef.html#callback-metadata>, make the connection between the call to the annotated function and the callback callee. This can enable interprocedural optimizations which were otherwise impossible. If a function parameter is mentioned in the callback attribute, through its position, it is undefined if that parameter is used for anything other than the actual callback. Inspected, captured, or modified parameters shall not be listed in the callback metadata.

Example encodings for the callback performed by pthread_create are shown below. The explicit attribute annotation indicates that the third parameter (start_routine) is called zero or more times by the pthread_create function, and that the fourth parameter (arg) is passed along. Note that the callback behavior of pthread_create is automatically recognized by Clang. In addition, the declarations of __kmpc_fork_teams and __kmpc_fork_call, generated for #pragma omp target teams and #pragma omp parallel, respectively, are also automatically recognized as broker functions. Further functions might be added in the future.

__attribute__((callback (start_routine, arg)))
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                   void *(*start_routine) (void *), void *arg);

__attribute__((callback (3, 4)))
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                   void *(*start_routine) (void *), void *arg);

carries_dependency

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
carries_dependency	carries_dependency					Yes

The carries_dependency attribute specifies dependency propagation into and out of functions.

When specified on a function or Objective-C method, the carries_dependency attribute means that the return value carries a dependency out of the function, so that the implementation need not constrain ordering upon return from that function. Implementations of the function and its caller may choose to preserve dependencies instead of emitting memory ordering instructions such as fences.

Note, this attribute does not change the meaning of the program, but may result in generation of more efficient code.

cf_consumed

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
cf_consumed	clang::cf_consumed	clang::cf_consumed				Yes

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

cf_returns_not_retained

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
cf_returns_not_retained	clang::cf_returns_not_retained	clang::cf_returns_not_retained

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

cf_returns_retained

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
cf_returns_retained	clang::cf_returns_retained	clang::cf_returns_retained

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

cfi_canonical_jump_table

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
cfi_canonical_jump_table	clang::cfi_canonical_jump_table	clang::cfi_canonical_jump_table				Yes

Use __attribute__((cfi_canonical_jump_table)) on a function declaration to make the function’s CFI jump table canonical. See the CFI documentation for more details.

clang::builtin_alias, clang_builtin_alias

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
clang_builtin_alias	clang::builtin_alias	clang::builtin_alias				Yes

This attribute is used in the implementation of the C intrinsics. It allows the C intrinsic functions to be declared using the names defined in target builtins, and still be recognized as clang builtins equivalent to the underlying name. For example, riscv_vector.h declares the function vadd with __attribute__((clang_builtin_alias(__builtin_rvv_vadd_vv_i8m1))). This ensures that both functions are recognized as that clang builtin, and in the latter case, the choice of which builtin to identify the function as can be deferred until after overload resolution.

This attribute can only be used to set up the aliases for certain ARM/RISC-V C intrinsic functions; it is intended for use only inside arm_*.h and riscv_*.h and is not a general mechanism for declaring arbitrary aliases for clang builtin functions.

clang_arm_builtin_alias

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
__clang_arm_builtin_alias	clang::__clang_arm_builtin_alias	clang::__clang_arm_builtin_alias				Yes

This attribute is used in the implementation of the ACLE intrinsics. It allows the intrinsic functions to be declared using the names defined in ACLE, and still be recognized as clang builtins equivalent to the underlying name. For example, arm_mve.h declares the function vaddq_u32 with __attribute__((__clang_arm_mve_alias(__builtin_arm_mve_vaddq_u32))), and similarly, one of the type-overloaded declarations of vaddq will have the same attribute. This ensures that both functions are recognized as that clang builtin, and in the latter case, the choice of which builtin to identify the function as can be deferred until after overload resolution.

This attribute can only be used to set up the aliases for certain Arm intrinsic functions; it is intended for use only inside arm_*.h and is not a general mechanism for declaring arbitrary aliases for clang builtin functions.

In order to avoid duplicating the attribute definitions for similar purpose for other architecture, there is a general form for the attribute clang_builtin_alias.

cmse_nonsecure_entry

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
cmse_nonsecure_entry						Yes

This attribute declares a function that can be called from non-secure state, or from secure state. Entering from and returning to non-secure state would switch to and from secure state, respectively, and prevent flow of information to non-secure state, except via return values. See ARMv8-M Security Extensions: Requirements on Development Tools - Engineering Specification Documentation for more information.

code_seg

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
			code_seg

The __declspec(code_seg) attribute enables the placement of code into separate named segments that can be paged or locked in memory individually. This attribute is used to control the placement of instantiated templates and compiler-generated code. See the documentation for __declspec(code_seg) on MSDN.

convergent

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
convergent	clang::convergent	clang::convergent				Yes

The convergent attribute can be placed on a function declaration. It is translated into the LLVM convergent attribute, which indicates that the call instructions of a function with this attribute cannot be made control-dependent on any additional values.

In languages designed for SPMD/SIMT programming model, e.g. OpenCL or CUDA, the call instructions of a function with this attribute must be executed by all work items or threads in a work group or sub group.

This attribute is different from noduplicate because it allows duplicating function calls if it can be proved that the duplicated function calls are not made control-dependent on any additional values, e.g., unrolling a loop executed by all work items.

Sample usage:

void convfunc(void) __attribute__((convergent));
// Setting it as a C++11 attribute is also valid in a C++ program.
// void convfunc(void) [[clang::convergent]];

cpu_dispatch

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
cpu_dispatch	clang::cpu_dispatch	clang::cpu_dispatch	cpu_dispatch			Yes

The cpu_specific and cpu_dispatch attributes are used to define and resolve multiversioned functions. This form of multiversioning provides a mechanism for declaring versions across translation units and manually specifying the resolved function list. A specified CPU defines a set of minimum features that are required for the function to be called. The result of this is that future processors execute the most restrictive version of the function the new processor can execute.

Function versions are defined with cpu_specific, which takes one or more CPU names as a parameter. For example:

// Declares and defines the ivybridge version of single_cpu.
__attribute__((cpu_specific(ivybridge)))
void single_cpu(void){}

// Declares and defines the atom version of single_cpu.
__attribute__((cpu_specific(atom)))
void single_cpu(void){}

// Declares and defines both the ivybridge and atom version of multi_cpu.
__attribute__((cpu_specific(ivybridge, atom)))
void multi_cpu(void){}

A dispatching (or resolving) function can be declared anywhere in a project’s source code with cpu_dispatch. This attribute takes one or more CPU names as a parameter (like cpu_specific). Functions marked with cpu_dispatch are not expected to be defined, only declared. If such a marked function has a definition, any side effects of the function are ignored; trivial function bodies are permissible for ICC compatibility.

// Creates a resolver for single_cpu above.
__attribute__((cpu_dispatch(ivybridge, atom)))
void single_cpu(void){}

// Creates a resolver for multi_cpu, but adds a 3rd version defined in another
// translation unit.
__attribute__((cpu_dispatch(ivybridge, atom, sandybridge)))
void multi_cpu(void){}

Note that it is possible to have a resolving function that dispatches based on more or fewer options than are present in the program. Specifying fewer will result in the omitted options not being considered during resolution. Specifying a version for resolution that isn’t defined in the program will result in a linking failure.

It is also possible to specify a CPU name of generic which will be resolved if the executing processor doesn’t satisfy the features required in the CPU name. The behavior of a program executing on a processor that doesn’t satisfy any option of a multiversioned function is undefined.

cpu_specific

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
cpu_specific	clang::cpu_specific	clang::cpu_specific	cpu_specific			Yes

The cpu_specific and cpu_dispatch attributes are used to define and resolve multiversioned functions. This form of multiversioning provides a mechanism for declaring versions across translation units and manually specifying the resolved function list. A specified CPU defines a set of minimum features that are required for the function to be called. The result of this is that future processors execute the most restrictive version of the function the new processor can execute.

Function versions are defined with cpu_specific, which takes one or more CPU names as a parameter. For example:

// Declares and defines the ivybridge version of single_cpu.
__attribute__((cpu_specific(ivybridge)))
void single_cpu(void){}

// Declares and defines the atom version of single_cpu.
__attribute__((cpu_specific(atom)))
void single_cpu(void){}

// Declares and defines both the ivybridge and atom version of multi_cpu.
__attribute__((cpu_specific(ivybridge, atom)))
void multi_cpu(void){}

A dispatching (or resolving) function can be declared anywhere in a project’s source code with cpu_dispatch. This attribute takes one or more CPU names as a parameter (like cpu_specific). Functions marked with cpu_dispatch are not expected to be defined, only declared. If such a marked function has a definition, any side effects of the function are ignored; trivial function bodies are permissible for ICC compatibility.

// Creates a resolver for single_cpu above.
__attribute__((cpu_dispatch(ivybridge, atom)))
void single_cpu(void){}

// Creates a resolver for multi_cpu, but adds a 3rd version defined in another
// translation unit.
__attribute__((cpu_dispatch(ivybridge, atom, sandybridge)))
void multi_cpu(void){}

Note that it is possible to have a resolving function that dispatches based on more or fewer options than are present in the program. Specifying fewer will result in the omitted options not being considered during resolution. Specifying a version for resolution that isn’t defined in the program will result in a linking failure.

It is also possible to specify a CPU name of generic which will be resolved if the executing processor doesn’t satisfy the features required in the CPU name. The behavior of a program executing on a processor that doesn’t satisfy any option of a multiversioned function is undefined.

diagnose_as_builtin

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
diagnose_as_builtin	clang::diagnose_as_builtin	clang::diagnose_as_builtin				Yes

The diagnose_as_builtin attribute indicates that Fortify diagnostics are to be applied to the declared function as if it were the function specified by the attribute. The builtin function whose diagnostics are to be mimicked should be given. In addition, the order in which arguments should be applied must also be given.

For example, the attribute can be used as follows.

__attribute__((diagnose_as_builtin(__builtin_memset, 3, 2, 1)))
void *mymemset(int n, int c, void *s) {
  // ...
}

This indicates that calls to mymemset should be diagnosed as if they were calls to __builtin_memset. The arguments 3, 2, 1 indicate by index the order in which arguments of mymemset should be applied to __builtin_memset. The third argument should be applied first, then the second, and then the first. Thus (when Fortify warnings are enabled) the call mymemset(n, c, s) will diagnose overflows as if it were the call __builtin_memset(s, c, n).

For variadic functions, the variadic arguments must come in the same order as they would to the builtin function, after all normal arguments. For instance, to diagnose a new function as if it were sscanf, we can use the attribute as follows.

__attribute__((diagnose_as_builtin(sscanf, 1, 2)))
int mysscanf(const char *str, const char *format, ...)  {
  // ...
}

Then the call mysscanf(“abc def”, “%4s %4s”, buf1, buf2) will be diagnosed as if it were the call sscanf(“abc def”, “%4s %4s”, buf1, buf2).

This attribute cannot be applied to non-static member functions.

diagnose_if

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
diagnose_if

The diagnose_if attribute can be placed on function declarations to emit warnings or errors at compile-time if calls to the attributed function meet certain user-defined criteria. For example:

int abs(int a)
  __attribute__((diagnose_if(a >= 0, "Redundant abs call", "warning")));
int must_abs(int a)
  __attribute__((diagnose_if(a >= 0, "Redundant abs call", "error")));

int val = abs(1); // warning: Redundant abs call
int val2 = must_abs(1); // error: Redundant abs call
int val3 = abs(val);
int val4 = must_abs(val); // Because run-time checks are not emitted for
                          // diagnose_if attributes, this executes without
                          // issue.

diagnose_if is closely related to enable_if, with a few key differences:

• Overload resolution is not aware of diagnose_if attributes: they’re considered only after we select the best candidate from a given candidate set.

• Function declarations that differ only in their diagnose_if attributes are considered to be redeclarations of the same function (not overloads).

• If the condition provided to diagnose_if cannot be evaluated, no diagnostic will be emitted.

Otherwise, diagnose_if is essentially the logical negation of enable_if.

As a result of bullet number two, diagnose_if attributes will stack on the same function. For example:

int foo() __attribute__((diagnose_if(1, "diag1", "warning")));
int foo() __attribute__((diagnose_if(1, "diag2", "warning")));

int bar = foo(); // warning: diag1
                 // warning: diag2
int (*fooptr)(void) = foo; // warning: diag1
                           // warning: diag2

constexpr int supportsAPILevel(int N) { return N < 5; }
int baz(int a)
  __attribute__((diagnose_if(!supportsAPILevel(10),
                             "Upgrade to API level 10 to use baz", "error")));
int baz(int a)
  __attribute__((diagnose_if(!a, "0 is not recommended.", "warning")));

int (*bazptr)(int) = baz; // error: Upgrade to API level 10 to use baz
int v = baz(0); // error: Upgrade to API level 10 to use baz

Query for this feature with __has_attribute(diagnose_if).

disable_sanitizer_instrumentation

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
disable_sanitizer_instrumentation	clang::disable_sanitizer_instrumentation	clang::disable_sanitizer_instrumentation				Yes

Use the disable_sanitizer_instrumentation attribute on a function, Objective-C method, or global variable, to specify that no sanitizer instrumentation should be applied.

This is not the same as __attribute__((no_sanitize(...))), which depending on the tool may still insert instrumentation to prevent false positive reports.

disable_tail_calls

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
disable_tail_calls	clang::disable_tail_calls	clang::disable_tail_calls				Yes

The disable_tail_calls attribute instructs the backend to not perform tail call optimization inside the marked function.

For example:

int callee(int);

int foo(int a) __attribute__((disable_tail_calls)) {
  return callee(a); // This call is not tail-call optimized.
}

Marking virtual functions as disable_tail_calls is legal.

int callee(int);

class Base {
public:
  [[clang::disable_tail_calls]] virtual int foo1() {
    return callee(); // This call is not tail-call optimized.
  }
};

class Derived1 : public Base {
public:
  int foo1() override {
    return callee(); // This call is tail-call optimized.
  }
};

enable_if

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
enable_if						Yes

Note

Some features of this attribute are experimental. The meaning of multiple enable_if attributes on a single declaration is subject to change in a future version of clang. Also, the ABI is not standardized and the name mangling may change in future versions. To avoid that, use asm labels.

The enable_if attribute can be placed on function declarations to control which overload is selected based on the values of the function’s arguments. When combined with the overloadable attribute, this feature is also available in C.

int isdigit(int c);
int isdigit(int c) __attribute__((enable_if(c <= -1 || c > 255, "chosen when 'c' is out of range"))) __attribute__((unavailable("'c' must have the value of an unsigned char or EOF")));

void foo(char c) {
  isdigit(c);
  isdigit(10);
  isdigit(-10);  // results in a compile-time error.
}

The enable_if attribute takes two arguments, the first is an expression written in terms of the function parameters, the second is a string explaining why this overload candidate could not be selected to be displayed in diagnostics. The expression is part of the function signature for the purposes of determining whether it is a redeclaration (following the rules used when determining whether a C++ template specialization is ODR-equivalent), but is not part of the type.

The enable_if expression is evaluated as if it were the body of a bool-returning constexpr function declared with the arguments of the function it is being applied to, then called with the parameters at the call site. If the result is false or could not be determined through constant expression evaluation, then this overload will not be chosen and the provided string may be used in a diagnostic if the compile fails as a result.

Because the enable_if expression is an unevaluated context, there are no global state changes, nor the ability to pass information from the enable_if expression to the function body. For example, suppose we want calls to strnlen(strbuf, maxlen) to resolve to strnlen_chk(strbuf, maxlen, size of strbuf) only if the size of strbuf can be determined:

__attribute__((always_inline))
static inline size_t strnlen(const char *s, size_t maxlen)
  __attribute__((overloadable))
  __attribute__((enable_if(__builtin_object_size(s, 0) != -1))),
                           "chosen when the buffer size is known but 'maxlen' is not")))
{
  return strnlen_chk(s, maxlen, __builtin_object_size(s, 0));
}

Multiple enable_if attributes may be applied to a single declaration. In this case, the enable_if expressions are evaluated from left to right in the following manner. First, the candidates whose enable_if expressions evaluate to false or cannot be evaluated are discarded. If the remaining candidates do not share ODR-equivalent enable_if expressions, the overload resolution is ambiguous. Otherwise, enable_if overload resolution continues with the next enable_if attribute on the candidates that have not been discarded and have remaining enable_if attributes. In this way, we pick the most specific overload out of a number of viable overloads using enable_if.

void f() __attribute__((enable_if(true, "")));  // #1
void f() __attribute__((enable_if(true, ""))) __attribute__((enable_if(true, "")));  // #2

void g(int i, int j) __attribute__((enable_if(i, "")));  // #1
void g(int i, int j) __attribute__((enable_if(j, ""))) __attribute__((enable_if(true)));  // #2

In this example, a call to f() is always resolved to #2, as the first enable_if expression is ODR-equivalent for both declarations, but #1 does not have another enable_if expression to continue evaluating, so the next round of evaluation has only a single candidate. In a call to g(1, 1), the call is ambiguous even though #2 has more enable_if attributes, because the first enable_if expressions are not ODR-equivalent.

Query for this feature with __has_attribute(enable_if).

Note that functions with one or more enable_if attributes may not have their address taken, unless all of the conditions specified by said enable_if are constants that evaluate to true. For example:

const int TrueConstant = 1;
const int FalseConstant = 0;
int f(int a) __attribute__((enable_if(a > 0, "")));
int g(int a) __attribute__((enable_if(a == 0 || a != 0, "")));
int h(int a) __attribute__((enable_if(1, "")));
int i(int a) __attribute__((enable_if(TrueConstant, "")));
int j(int a) __attribute__((enable_if(FalseConstant, "")));

void fn() {
  int (*ptr)(int);
  ptr = &f; // error: 'a > 0' is not always true
  ptr = &g; // error: 'a == 0 || a != 0' is not a truthy constant
  ptr = &h; // OK: 1 is a truthy constant
  ptr = &i; // OK: 'TrueConstant' is a truthy constant
  ptr = &j; // error: 'FalseConstant' is a constant, but not truthy
}

Because enable_if evaluation happens during overload resolution, enable_if may give unintuitive results when used with templates, depending on when overloads are resolved. In the example below, clang will emit a diagnostic about no viable overloads for foo in bar, but not in baz:

double foo(int i) __attribute__((enable_if(i > 0, "")));
void *foo(int i) __attribute__((enable_if(i <= 0, "")));
template <int I>
auto bar() { return foo(I); }

template <typename T>
auto baz() { return foo(T::number); }

struct WithNumber { constexpr static int number = 1; };
void callThem() {
  bar<sizeof(WithNumber)>();
  baz<WithNumber>();
}

This is because, in bar, foo is resolved prior to template instantiation, so the value for I isn’t known (thus, both enable_if conditions for foo fail). However, in baz, foo is resolved during template instantiation, so the value for T::number is known.

enforce_tcb

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
enforce_tcb	clang::enforce_tcb	clang::enforce_tcb				Yes

The enforce_tcb attribute can be placed on functions to enforce that a

trusted compute base (TCB) does not call out of the TCB. This generates a warning every time a function not marked with an enforce_tcb attribute is called from a function with the enforce_tcb attribute. A function may be a part of multiple TCBs. Invocations through function pointers are currently not checked. Builtins are considered to a part of every TCB.

• enforce_tcb(Name) indicates that this function is a part of the TCB named Name

enforce_tcb_leaf

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
enforce_tcb_leaf	clang::enforce_tcb_leaf	clang::enforce_tcb_leaf				Yes

The enforce_tcb_leaf attribute satisfies the requirement enforced by

enforce_tcb for the marked function to be in the named TCB but does not continue to check the functions called from within the leaf function.

• enforce_tcb_leaf(Name) indicates that this function is a part of the TCB named Name

error, warning

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
error warning	gnu::error gnu::warning	gnu::error gnu::warning				Yes

The error and warning function attributes can be used to specify a custom diagnostic to be emitted when a call to such a function is not eliminated via optimizations. This can be used to create compile time assertions that depend on optimizations, while providing diagnostics pointing to precise locations of the call site in the source.

__attribute__((warning("oh no"))) void dontcall();
void foo() {
  if (someCompileTimeAssertionThatsTrue)
    dontcall(); // Warning

  dontcall(); // Warning

  if (someCompileTimeAssertionThatsFalse)
    dontcall(); // No Warning
  sizeof(dontcall()); // No Warning
}

exclude_from_explicit_instantiation

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
exclude_from_explicit_instantiation	clang::exclude_from_explicit_instantiation	clang::exclude_from_explicit_instantiation				Yes

The exclude_from_explicit_instantiation attribute opts-out a member of a class template from being part of explicit template instantiations of that class template. This means that an explicit instantiation will not instantiate members of the class template marked with the attribute, but also that code where an extern template declaration of the enclosing class template is visible will not take for granted that an external instantiation of the class template would provide those members (which would otherwise be a link error, since the explicit instantiation won’t provide those members). For example, let’s say we don’t want the data() method to be part of libc++’s ABI. To make sure it is not exported from the dylib, we give it hidden visibility:

// in <string>
template <class CharT>
class basic_string {
public:
  __attribute__((__visibility__("hidden")))
  const value_type* data() const noexcept { ... }
};

template class basic_string<char>;

Since an explicit template instantiation declaration for basic_string<char> is provided, the compiler is free to assume that basic_string<char>::data() will be provided by another translation unit, and it is free to produce an external call to this function. However, since data() has hidden visibility and the explicit template instantiation is provided in a shared library (as opposed to simply another translation unit), basic_string<char>::data() won’t be found and a link error will ensue. This happens because the compiler assumes that basic_string<char>::data() is part of the explicit template instantiation declaration, when it really isn’t. To tell the compiler that data() is not part of the explicit template instantiation declaration, the exclude_from_explicit_instantiation attribute can be used:

// in <string>
template <class CharT>
class basic_string {
public:
  __attribute__((__visibility__("hidden")))
  __attribute__((exclude_from_explicit_instantiation))
  const value_type* data() const noexcept { ... }
};

template class basic_string<char>;

Now, the compiler won’t assume that basic_string<char>::data() is provided externally despite there being an explicit template instantiation declaration: the compiler will implicitly instantiate basic_string<char>::data() in the TUs where it is used.

This attribute can be used on static and non-static member functions of class templates, static data members of class templates and member classes of class templates.

export_name

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
export_name	clang::export_name	clang::export_name				Yes

Clang supports the __attribute__((export_name(<name>))) attribute for the WebAssembly target. This attribute may be attached to a function declaration, where it modifies how the symbol is to be exported from the linked WebAssembly.

WebAssembly functions are exported via string name. By default when a symbol is exported, the export name for C/C++ symbols are the same as their C/C++ symbol names. This attribute can be used to override the default behavior, and request a specific string name be used instead.

flatten

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
flatten	gnu::flatten	gnu::flatten				Yes

The flatten attribute causes calls within the attributed function to be inlined unless it is impossible to do so, for example if the body of the callee is unavailable or if the callee has the noinline attribute.

force_align_arg_pointer

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
force_align_arg_pointer	gnu::force_align_arg_pointer	gnu::force_align_arg_pointer

Use this attribute to force stack alignment.

Legacy x86 code uses 4-byte stack alignment. Newer aligned SSE instructions (like ‘movaps’) that work with the stack require operands to be 16-byte aligned. This attribute realigns the stack in the function prologue to make sure the stack can be used with SSE instructions.

Note that the x86_64 ABI forces 16-byte stack alignment at the call site. Because of this, ‘force_align_arg_pointer’ is not needed on x86_64, except in rare cases where the caller does not align the stack properly (e.g. flow jumps from i386 arch code).

__attribute__ ((force_align_arg_pointer))
void f () {
  ...
}

format

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
format	gnu::format	gnu::format

Clang supports the format attribute, which indicates that the function accepts a printf or scanf-like format string and corresponding arguments or a va_list that contains these arguments.

Please see GCC documentation about format attribute to find details about attribute syntax.

Clang implements two kinds of checks with this attribute.

1. Clang checks that the function with the format attribute is called with a format string that uses format specifiers that are allowed, and that arguments match the format string. This is the -Wformat warning, it is on by default.

2. Clang checks that the format string argument is a literal string. This is the -Wformat-nonliteral warning, it is off by default.

   Clang implements this mostly the same way as GCC, but there is a difference for functions that accept a va_list argument (for example, vprintf). GCC does not emit -Wformat-nonliteral warning for calls to such functions. Clang does not warn if the format string comes from a function parameter, where the function is annotated with a compatible attribute, otherwise it warns. For example:

   __attribute__((__format__ (__scanf__, 1, 3)))
   void foo(const char* s, char *buf, ...) {
     va_list ap;
     va_start(ap, buf);
   
     vprintf(s, ap); // warning: format string is not a string literal
   }
   

   In this case we warn because s contains a format string for a scanf-like function, but it is passed to a printf-like function.

   If the attribute is removed, clang still warns, because the format string is not a string literal.

   Another example:

   __attribute__((__format__ (__printf__, 1, 3)))
   void foo(const char* s, char *buf, ...) {
     va_list ap;
     va_start(ap, buf);
   
     vprintf(s, ap); // warning
   }
   

   In this case Clang does not warn because the format string s and the corresponding arguments are annotated. If the arguments are incorrect, the caller of foo will receive a warning.

gnu_inline

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
gnu_inline	gnu::gnu_inline	gnu::gnu_inline				Yes

The gnu_inline changes the meaning of extern inline to use GNU inline semantics, meaning:

• If any declaration that is declared inline is not declared extern, then the inline keyword is just a hint. In particular, an out-of-line definition is still emitted for a function with external linkage, even if all call sites are inlined, unlike in C99 and C++ inline semantics.

• If all declarations that are declared inline are also declared extern, then the function body is present only for inlining and no out-of-line version is emitted.

Some important consequences: static inline emits an out-of-line version if needed, a plain inline definition emits an out-of-line version always, and an extern inline definition (in a header) followed by a (non-extern) inline declaration in a source file emits an out-of-line version of the function in that source file but provides the function body for inlining to all includers of the header.

Either __GNUC_GNU_INLINE__ (GNU inline semantics) or __GNUC_STDC_INLINE__ (C99 semantics) will be defined (they are mutually exclusive). If __GNUC_STDC_INLINE__ is defined, then the gnu_inline function attribute can be used to get GNU inline semantics on a per function basis. If __GNUC_GNU_INLINE__ is defined, then the translation unit is already being compiled with GNU inline semantics as the implied default. It is unspecified which macro is defined in a C++ compilation.

GNU inline semantics are the default behavior with -std=gnu89, -std=c89, -std=c94, or -fgnu89-inline.

guard

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
			guard

Code can indicate CFG checks are not wanted with the __declspec(guard(nocf)) attribute. This directs the compiler to not insert any CFG checks for the entire function. This approach is typically used only sparingly in specific situations where the programmer has manually inserted “CFG-equivalent” protection. The programmer knows that they are calling through some read-only function table whose address is obtained through read-only memory references and for which the index is masked to the function table limit. This approach may also be applied to small wrapper functions that are not inlined and that do nothing more than make a call through a function pointer. Since incorrect usage of this directive can compromise the security of CFG, the programmer must be very careful using the directive. Typically, this usage is limited to very small functions that only call one function.

Control Flow Guard documentation <https://docs.microsoft.com/en-us/windows/win32/secbp/pe-metadata>

ifunc

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
ifunc	gnu::ifunc	gnu::ifunc				Yes

__attribute__((ifunc("resolver"))) is used to mark that the address of a declaration should be resolved at runtime by calling a resolver function.

The symbol name of the resolver function is given in quotes. A function with this name (after mangling) must be defined in the current translation unit; it may be static. The resolver function should return a pointer.

The ifunc attribute may only be used on a function declaration. A function declaration with an ifunc attribute is considered to be a definition of the declared entity. The entity must not have weak linkage; for example, in C++, it cannot be applied to a declaration if a definition at that location would be considered inline.

Not all targets support this attribute. ELF target support depends on both the linker and runtime linker, and is available in at least lld 4.0 and later, binutils 2.20.1 and later, glibc v2.11.1 and later, and FreeBSD 9.1 and later. Non-ELF targets currently do not support this attribute.

import_module

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
import_module	clang::import_module	clang::import_module				Yes

Clang supports the __attribute__((import_module(<module_name>))) attribute for the WebAssembly target. This attribute may be attached to a function declaration, where it modifies how the symbol is to be imported within the WebAssembly linking environment.

WebAssembly imports use a two-level namespace scheme, consisting of a module name, which typically identifies a module from which to import, and a field name, which typically identifies a field from that module to import. By default, module names for C/C++ symbols are assigned automatically by the linker. This attribute can be used to override the default behavior, and request a specific module name be used instead.

import_name

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
import_name	clang::import_name	clang::import_name				Yes

Clang supports the __attribute__((import_name(<name>))) attribute for the WebAssembly target. This attribute may be attached to a function declaration, where it modifies how the symbol is to be imported within the WebAssembly linking environment.

WebAssembly imports use a two-level namespace scheme, consisting of a module name, which typically identifies a module from which to import, and a field name, which typically identifies a field from that module to import. By default, field names for C/C++ symbols are the same as their C/C++ symbol names. This attribute can be used to override the default behavior, and request a specific field name be used instead.

internal_linkage

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
internal_linkage	clang::internal_linkage	clang::internal_linkage				Yes

The internal_linkage attribute changes the linkage type of the declaration to internal. This is similar to C-style static, but can be used on classes and class methods. When applied to a class definition, this attribute affects all methods and static data members of that class. This can be used to contain the ABI of a C++ library by excluding unwanted class methods from the export tables.

interrupt (ARM)

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
interrupt	gnu::interrupt	gnu::interrupt

Clang supports the GNU style __attribute__((interrupt("TYPE"))) attribute on ARM targets. This attribute may be attached to a function definition and instructs the backend to generate appropriate function entry/exit code so that it can be used directly as an interrupt service routine.

The parameter passed to the interrupt attribute is optional, but if provided it must be a string literal with one of the following values: “IRQ”, “FIQ”, “SWI”, “ABORT”, “UNDEF”.

The semantics are as follows:

• If the function is AAPCS, Clang instructs the backend to realign the stack to 8 bytes on entry. This is a general requirement of the AAPCS at public interfaces, but may not hold when an exception is taken. Doing this allows other AAPCS functions to be called.

• If the CPU is M-class this is all that needs to be done since the architecture itself is designed in such a way that functions obeying the normal AAPCS ABI constraints are valid exception handlers.

• If the CPU is not M-class, the prologue and epilogue are modified to save all non-banked registers that are used, so that upon return the user-mode state will not be corrupted. Note that to avoid unnecessary overhead, only general-purpose (integer) registers are saved in this way. If VFP operations are needed, that state must be saved manually.

  Specifically, interrupt kinds other than “FIQ” will save all core registers except “lr” and “sp”. “FIQ” interrupts will save r0-r7.

• If the CPU is not M-class, the return instruction is changed to one of the canonical sequences permitted by the architecture for exception return. Where possible the function itself will make the necessary “lr” adjustments so that the “preferred return address” is selected.

  Unfortunately the compiler is unable to make this guarantee for an “UNDEF” handler, where the offset from “lr” to the preferred return address depends on the execution state of the code which generated the exception. In this case a sequence equivalent to “movs pc, lr” will be used.

interrupt (AVR)

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
interrupt	gnu::interrupt	gnu::interrupt				Yes

Clang supports the GNU style __attribute__((interrupt)) attribute on AVR targets. This attribute may be attached to a function definition and instructs the backend to generate appropriate function entry/exit code so that it can be used directly as an interrupt service routine.

On the AVR, the hardware globally disables interrupts when an interrupt is executed. The first instruction of an interrupt handler declared with this attribute is a SEI instruction to re-enable interrupts. See also the signal attribute that does not insert a SEI instruction.

interrupt (MIPS)

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
interrupt	gnu::interrupt	gnu::interrupt				Yes

Clang supports the GNU style __attribute__((interrupt("ARGUMENT"))) attribute on MIPS targets. This attribute may be attached to a function definition and instructs the backend to generate appropriate function entry/exit code so that it can be used directly as an interrupt service routine.

By default, the compiler will produce a function prologue and epilogue suitable for an interrupt service routine that handles an External Interrupt Controller (eic) generated interrupt. This behavior can be explicitly requested with the “eic” argument.

Otherwise, for use with vectored interrupt mode, the argument passed should be of the form “vector=LEVEL” where LEVEL is one of the following values: “sw0”, “sw1”, “hw0”, “hw1”, “hw2”, “hw3”, “hw4”, “hw5”. The compiler will then set the interrupt mask to the corresponding level which will mask all interrupts up to and including the argument.

The semantics are as follows:

• The prologue is modified so that the Exception Program Counter (EPC) and Status coprocessor registers are saved to the stack. The interrupt mask is set so that the function can only be interrupted by a higher priority interrupt. The epilogue will restore the previous values of EPC and Status.

• The prologue and epilogue are modified to save and restore all non-kernel registers as necessary.

• The FPU is disabled in the prologue, as the floating pointer registers are not spilled to the stack.

• The function return sequence is changed to use an exception return instruction.

• The parameter sets the interrupt mask for the function corresponding to the interrupt level specified. If no mask is specified the interrupt mask defaults to “eic”.

interrupt (RISCV)

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
interrupt	gnu::interrupt	gnu::interrupt				Yes

Clang supports the GNU style __attribute__((interrupt)) attribute on RISCV targets. This attribute may be attached to a function definition and instructs the backend to generate appropriate function entry/exit code so that it can be used directly as an interrupt service routine.

Permissible values for this parameter are user, supervisor, and machine. If there is no parameter, then it defaults to machine.

Repeated interrupt attribute on the same declaration will cause a warning to be emitted. In case of repeated declarations, the last one prevails.

Refer to: https://gcc.gnu.org/onlinedocs/gcc/RISC-V-Function-Attributes.html https://riscv.org/specifications/privileged-isa/ The RISC-V Instruction Set Manual Volume II: Privileged Architecture Version 1.10.

kernel

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
kernel						Yes

__attribute__((kernel)) is used to mark a kernel function in RenderScript.

In RenderScript, kernel functions are used to express data-parallel computations. The RenderScript runtime efficiently parallelizes kernel functions to run on computational resources such as multi-core CPUs and GPUs. See the RenderScript documentation for more information.

lifetimebound

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
lifetimebound	clang::lifetimebound

The lifetimebound attribute on a function parameter or implicit object parameter indicates that objects that are referred to by that parameter may also be referred to by the return value of the annotated function (or, for a parameter of a constructor, by the value of the constructed object). It is only supported in C++.

By default, a reference is considered to refer to its referenced object, a pointer is considered to refer to its pointee, a std::initializer_list<T> is considered to refer to its underlying array, and aggregates (arrays and simple structs) are considered to refer to all objects that their transitive subobjects refer to.

Clang warns if it is able to detect that an object or reference refers to another object with a shorter lifetime. For example, Clang will warn if a function returns a reference to a local variable, or if a reference is bound to a temporary object whose lifetime is not extended. By using the lifetimebound attribute, this determination can be extended to look through user-declared functions. For example:

// Returns m[key] if key is present, or default_value if not.
template<typename T, typename U>
const U &get_or_default(const std::map<T, U> &m [[clang::lifetimebound]],
                        const T &key, /* note, not lifetimebound */
                        const U &default_value [[clang::lifetimebound]]);

std::map<std::string, std::string> m;
// warning: temporary "bar"s that might be bound to local reference 'val'
// will be destroyed at the end of the full-expression
const std::string &val = get_or_default(m, "foo"s, "bar"s);

// No warning in this case.
std::string def_val = "bar"s;
const std::string &val = get_or_default(m, "foo"s, def_val);

The attribute can be applied to the implicit this parameter of a member function by writing the attribute after the function type:

struct string {
  // The returned pointer should not outlive ``*this``.
  const char *data() const [[clang::lifetimebound]];
};

This attribute is inspired by the C++ committee paper P0936R0, but does not affect whether temporary objects have their lifetimes extended.

long_call, far

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
long_call far	gnu::long_call gnu::far	gnu::long_call gnu::far				Yes

Clang supports the __attribute__((long_call)), __attribute__((far)), and __attribute__((near)) attributes on MIPS targets. These attributes may only be added to function declarations and change the code generated by the compiler when directly calling the function. The near attribute allows calls to the function to be made using the jal instruction, which requires the function to be located in the same naturally aligned 256MB segment as the caller. The long_call and far attributes are synonyms and require the use of a different call sequence that works regardless of the distance between the functions.

These attributes have no effect for position-independent code.

These attributes take priority over command line switches such as -mlong-calls and -mno-long-calls.

malloc

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
malloc	gnu::malloc	gnu::malloc	restrict			Yes

The malloc attribute indicates that the function acts like a system memory allocation function, returning a pointer to allocated storage disjoint from the storage for any other object accessible to the caller.

micromips

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
micromips	gnu::micromips	gnu::micromips				Yes

Clang supports the GNU style __attribute__((micromips)) and __attribute__((nomicromips)) attributes on MIPS targets. These attributes may be attached to a function definition and instructs the backend to generate or not to generate microMIPS code for that function.

These attributes override the -mmicromips and -mno-micromips options on the command line.

mig_server_routine

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
mig_server_routine	clang::mig_server_routine	clang::mig_server_routine				Yes

The Mach Interface Generator release-on-success convention dictates functions that follow it to only release arguments passed to them when they return “success” (a kern_return_t error code that indicates that no errors have occurred). Otherwise the release is performed by the MIG client that called the function. The annotation __attribute__((mig_server_routine)) is applied in order to specify which functions are expected to follow the convention. This allows the Static Analyzer to find bugs caused by violations of that convention. The attribute would normally appear on the forward declaration of the actual server routine in the MIG server header, but it may also be added to arbitrary functions that need to follow the same convention - for example, a user can add them to auxiliary functions called by the server routine that have their return value of type kern_return_t unconditionally returned from the routine. The attribute can be applied to C++ methods, and in this case it will be automatically applied to overrides if the method is virtual. The attribute can also be written using C++11 syntax: [[mig::server_routine]].

min_vector_width

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
min_vector_width	clang::min_vector_width	clang::min_vector_width				Yes

Clang supports the __attribute__((min_vector_width(width))) attribute. This attribute may be attached to a function and informs the backend that this function desires vectors of at least this width to be generated. Target-specific maximum vector widths still apply. This means even if you ask for something larger than the target supports, you will only get what the target supports. This attribute is meant to be a hint to control target heuristics that may generate narrower vectors than what the target hardware supports.

This is currently used by the X86 target to allow some CPUs that support 512-bit vectors to be limited to using 256-bit vectors to avoid frequency penalties. This is currently enabled with the -prefer-vector-width=256 command line option. The min_vector_width attribute can be used to prevent the backend from trying to split vector operations to match the prefer-vector-width. All X86 vector intrinsics from x86intrin.h already set this attribute. Additionally, use of any of the X86-specific vector builtins will implicitly set this attribute on the calling function. The intent is that explicitly writing vector code using the X86 intrinsics will prevent prefer-vector-width from affecting the code.

no_builtin

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
no_builtin	clang::no_builtin	clang::no_builtin				Yes

Note

This attribute is not yet fully implemented, it is validated but has no effect on the generated code.

The __attribute__((no_builtin)) is similar to the -fno-builtin flag except it is specific to the body of a function. The attribute may also be applied to a virtual function but has no effect on the behavior of overriding functions in a derived class.

It accepts one or more strings corresponding to the specific names of the builtins to disable (e.g. “memcpy”, “memset”). If the attribute is used without parameters it will disable all buitins at once.

// The compiler is not allowed to add any builtin to foo's body.
void foo(char* data, size_t count) __attribute__((no_builtin)) {
  // The compiler is not allowed to convert the loop into
  // `__builtin_memset(data, 0xFE, count);`.
  for (size_t i = 0; i < count; ++i)
    data[i] = 0xFE;
}

// The compiler is not allowed to add the `memcpy` builtin to bar's body.
void bar(char* data, size_t count) __attribute__((no_builtin("memcpy"))) {
  // The compiler is allowed to convert the loop into
  // `__builtin_memset(data, 0xFE, count);` but cannot generate any
  // `__builtin_memcpy`
  for (size_t i = 0; i < count; ++i)
    data[i] = 0xFE;
}

no_caller_saved_registers

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
no_caller_saved_registers	gnu::no_caller_saved_registers	gnu::no_caller_saved_registers

Use this attribute to indicate that the specified function has no caller-saved registers. That is, all registers are callee-saved except for registers used for passing parameters to the function or returning parameters from the function. The compiler saves and restores any modified registers that were not used for passing or returning arguments to the function.

The user can call functions specified with the ‘no_caller_saved_registers’ attribute from an interrupt handler without saving and restoring all call-clobbered registers.

Note that ‘no_caller_saved_registers’ attribute is not a calling convention. In fact, it only overrides the decision of which registers should be saved by the caller, but not how the parameters are passed from the caller to the callee.

For example:

__attribute__ ((no_caller_saved_registers, fastcall))
void f (int arg1, int arg2) {
  ...
}

In this case parameters ‘arg1’ and ‘arg2’ will be passed in registers. In this case, on 32-bit x86 targets, the function ‘f’ will use ECX and EDX as register parameters. However, it will not assume any scratch registers and should save and restore any modified registers except for ECX and EDX.

no_profile_instrument_function

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
no_profile_instrument_function	gnu::no_profile_instrument_function	gnu::no_profile_instrument_function				Yes

Use the no_profile_instrument_function attribute on a function declaration to denote that the compiler should not instrument the function with profile-related instrumentation, such as via the -fprofile-generate / -fprofile-instr-generate / -fcs-profile-generate / -fprofile-arcs flags.

no_sanitize

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
no_sanitize	clang::no_sanitize	clang::no_sanitize				Yes

Use the no_sanitize attribute on a function or a global variable declaration to specify that a particular instrumentation or set of instrumentations should not be applied.

The attribute takes a list of string literals with the following accepted values: * all values accepted by -fno-sanitize=; * coverage, to disable SanitizerCoverage instrumentation.

For example, __attribute__((no_sanitize("address", "thread"))) specifies that AddressSanitizer and ThreadSanitizer should not be applied to the function or variable. Using __attribute__((no_sanitize("coverage"))) specifies that SanitizerCoverage should not be applied to the function.

See Controlling Code Generation for a full list of supported sanitizer flags.

no_sanitize_address, no_address_safety_analysis

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
no_address_safety_analysis no_sanitize_address no_sanitize_thread no_sanitize_memory	gnu::no_address_safety_analysis gnu::no_sanitize_address gnu::no_sanitize_thread clang::no_sanitize_memory	gnu::no_address_safety_analysis gnu::no_sanitize_address gnu::no_sanitize_thread clang::no_sanitize_memory				Yes

Use __attribute__((no_sanitize_address)) on a function or a global variable declaration to specify that address safety instrumentation (e.g. AddressSanitizer) should not be applied.

no_sanitize_memory

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
no_address_safety_analysis no_sanitize_address no_sanitize_thread no_sanitize_memory	gnu::no_address_safety_analysis gnu::no_sanitize_address gnu::no_sanitize_thread clang::no_sanitize_memory	gnu::no_address_safety_analysis gnu::no_sanitize_address gnu::no_sanitize_thread clang::no_sanitize_memory				Yes

Use __attribute__((no_sanitize_memory)) on a function declaration to specify that checks for uninitialized memory should not be inserted (e.g. by MemorySanitizer). The function may still be instrumented by the tool to avoid false positives in other places.

no_sanitize_thread

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
no_address_safety_analysis no_sanitize_address no_sanitize_thread no_sanitize_memory	gnu::no_address_safety_analysis gnu::no_sanitize_address gnu::no_sanitize_thread clang::no_sanitize_memory	gnu::no_address_safety_analysis gnu::no_sanitize_address gnu::no_sanitize_thread clang::no_sanitize_memory				Yes

Use __attribute__((no_sanitize_thread)) on a function declaration to specify that checks for data races on plain (non-atomic) memory accesses should not be inserted by ThreadSanitizer. The function is still instrumented by the tool to avoid false positives and provide meaningful stack traces.

no_speculative_load_hardening

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
no_speculative_load_hardening	clang::no_speculative_load_hardening	clang::no_speculative_load_hardening				Yes

This attribute can be applied to a function declaration in order to indicate

that Speculative Load Hardening is not needed for the function body. This can also be applied to a method in Objective C. This attribute will take precedence over the command line flag in the case where -mspeculative-load-hardening is specified.

Warning: This attribute may not prevent Speculative Load Hardening from being enabled for a function which inlines a function that has the ‘speculative_load_hardening’ attribute. This is intended to provide a maximally conservative model where the code that is marked with the ‘speculative_load_hardening’ attribute will always (even when inlined) be hardened. A user of this attribute may want to mark functions called by a function they do not want to be hardened with the ‘noinline’ attribute.

For example:

__attribute__((speculative_load_hardening))
int foo(int i) {
  return i;
}

// Note: bar() may still have speculative load hardening enabled if
// foo() is inlined into bar(). Mark foo() with __attribute__((noinline))
// to avoid this situation.
__attribute__((no_speculative_load_hardening))
int bar(int i) {
  return foo(i);
}

no_split_stack

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
no_split_stack	gnu::no_split_stack	gnu::no_split_stack				Yes

The no_split_stack attribute disables the emission of the split stack preamble for a particular function. It has no effect if -fsplit-stack is not specified.

no_stack_protector

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
no_stack_protector	clang::no_stack_protector	clang::no_stack_protector				Yes

Clang supports the __attribute__((no_stack_protector)) attribute which disables the stack protector on the specified function. This attribute is useful for selectively disabling the stack protector on some functions when building with -fstack-protector compiler option.

For example, it disables the stack protector for the function foo but function bar will still be built with the stack protector with the -fstack-protector option.

int __attribute__((no_stack_protector))
foo (int x); // stack protection will be disabled for foo.

int bar(int y); // bar can be built with the stack protector.

noalias

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
			noalias

The noalias attribute indicates that the only memory accesses inside function are loads and stores from objects pointed to by its pointer-typed arguments, with arbitrary offsets.

nocf_check

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
nocf_check	gnu::nocf_check	gnu::nocf_check				Yes

Jump Oriented Programming attacks rely on tampering with addresses used by indirect call / jmp, e.g. redirect control-flow to non-programmer intended bytes in the binary. X86 Supports Indirect Branch Tracking (IBT) as part of Control-Flow Enforcement Technology (CET). IBT instruments ENDBR instructions used to specify valid targets of indirect call / jmp. The nocf_check attribute has two roles: 1. Appertains to a function - do not add ENDBR instruction at the beginning of the function. 2. Appertains to a function pointer - do not track the target function of this pointer (by adding nocf_check prefix to the indirect-call instruction).

nodiscard, warn_unused_result

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
warn_unused_result	nodiscard clang::warn_unused_result gnu::warn_unused_result	nodiscard gnu::warn_unused_result				Yes

Clang supports the ability to diagnose when the results of a function call expression are discarded under suspicious circumstances. A diagnostic is generated when a function or its return type is marked with [[nodiscard]] (or __attribute__((warn_unused_result))) and the function call appears as a potentially-evaluated discarded-value expression that is not explicitly cast to void.

A string literal may optionally be provided to the attribute, which will be reproduced in any resulting diagnostics. Redeclarations using different forms of the attribute (with or without the string literal or with different string literal contents) are allowed. If there are redeclarations of the entity with differing string literals, it is unspecified which one will be used by Clang in any resulting diagnostics.

Additionally, discarded temporaries resulting from a call to a constructor marked with [[nodiscard]] or a constructor of a type marked [[nodiscard]] will also diagnose. This also applies to type conversions that use the annotated [[nodiscard]] constructor or result in an annotated type.

noduplicate

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
noduplicate	clang::noduplicate	clang::noduplicate				Yes

The noduplicate attribute can be placed on function declarations to control whether function calls to this function can be duplicated or not as a result of optimizations. This is required for the implementation of functions with certain special requirements, like the OpenCL “barrier” function, that might need to be run concurrently by all the threads that are executing in lockstep on the hardware. For example this attribute applied on the function “nodupfunc” in the code below avoids that:

void nodupfunc() __attribute__((noduplicate));
// Setting it as a C++11 attribute is also valid
// void nodupfunc() [[clang::noduplicate]];
void foo();
void bar();

nodupfunc();
if (a > n) {
  foo();
} else {
  bar();
}

gets possibly modified by some optimizations into code similar to this:

if (a > n) {
  nodupfunc();
  foo();
} else {
  nodupfunc();
  bar();
}

where the call to “nodupfunc” is duplicated and sunk into the two branches of the condition.

nomicromips

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
nomicromips	gnu::nomicromips	gnu::nomicromips				Yes

Clang supports the GNU style __attribute__((micromips)) and __attribute__((nomicromips)) attributes on MIPS targets. These attributes may be attached to a function definition and instructs the backend to generate or not to generate microMIPS code for that function.

These attributes override the -mmicromips and -mno-micromips options on the command line.

noreturn

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
	noreturn					Yes

A function declared as [[noreturn]] shall not return to its caller. The compiler will generate a diagnostic for a function declared as [[noreturn]] that appears to be capable of returning to its caller.

not_tail_called

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
not_tail_called	clang::not_tail_called	clang::not_tail_called				Yes

The not_tail_called attribute prevents tail-call optimization on statically bound calls. Objective-c methods, and functions marked as always_inline cannot be marked as not_tail_called.

For example, it prevents tail-call optimization in the following case:

int __attribute__((not_tail_called)) foo1(int);

int foo2(int a) {
  return foo1(a); // No tail-call optimization on direct calls.
}

However, it doesn’t prevent tail-call optimization in this case:

int __attribute__((not_tail_called)) foo1(int);

int foo2(int a) {
  int (*fn)(int) = &foo1;

  // not_tail_called has no effect on an indirect call even if the call can
  // be resolved at compile time.
  return (*fn)(a);
}

Generally, marking an overriding virtual function as not_tail_called is not useful, because this attribute is a property of the static type. Calls made through a pointer or reference to the base class type will respect the not_tail_called attribute of the base class’s member function, regardless of the runtime destination of the call:

struct Foo { virtual void f(); };
struct Bar : Foo {
  [[clang::not_tail_called]] void f() override;
};
void callera(Bar& bar) {
  Foo& foo = bar;
  // not_tail_called has no effect on here, even though the
  // underlying method is f from Bar.
  foo.f();
  bar.f(); // No tail-call optimization on here.
}

nothrow

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
nothrow	gnu::nothrow	gnu::nothrow	nothrow			Yes

Clang supports the GNU style __attribute__((nothrow)) and Microsoft style __declspec(nothrow) attribute as an equivalent of noexcept on function declarations. This attribute informs the compiler that the annotated function does not throw an exception. This prevents exception-unwinding. This attribute is particularly useful on functions in the C Standard Library that are guaranteed to not throw an exception.

ns_consumed

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
ns_consumed	clang::ns_consumed	clang::ns_consumed				Yes

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

ns_consumes_self

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
ns_consumes_self	clang::ns_consumes_self	clang::ns_consumes_self				Yes

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

ns_returns_autoreleased

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
ns_returns_autoreleased	clang::ns_returns_autoreleased	clang::ns_returns_autoreleased

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

ns_returns_not_retained

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
ns_returns_not_retained	clang::ns_returns_not_retained	clang::ns_returns_not_retained

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

ns_returns_retained

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
ns_returns_retained	clang::ns_returns_retained	clang::ns_returns_retained

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

objc_method_family

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
objc_method_family	clang::objc_method_family	clang::objc_method_family				Yes

Many methods in Objective-C have conventional meanings determined by their selectors. It is sometimes useful to be able to mark a method as having a particular conventional meaning despite not having the right selector, or as not having the conventional meaning that its selector would suggest. For these use cases, we provide an attribute to specifically describe the “method family” that a method belongs to.

Usage: __attribute__((objc_method_family(X))), where X is one of none, alloc, copy, init, mutableCopy, or new. This attribute can only be placed at the end of a method declaration:

- (NSString *)initMyStringValue __attribute__((objc_method_family(none)));

Users who do not wish to change the conventional meaning of a method, and who merely want to document its non-standard retain and release semantics, should use the retaining behavior attributes (ns_returns_retained, ns_returns_not_retained, etc).

Query for this feature with __has_attribute(objc_method_family).

objc_requires_super

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
objc_requires_super	clang::objc_requires_super	clang::objc_requires_super				Yes

Some Objective-C classes allow a subclass to override a particular method in a parent class but expect that the overriding method also calls the overridden method in the parent class. For these cases, we provide an attribute to designate that a method requires a “call to super” in the overriding method in the subclass.

Usage: __attribute__((objc_requires_super)). This attribute can only be placed at the end of a method declaration:

- (void)foo __attribute__((objc_requires_super));

This attribute can only be applied the method declarations within a class, and not a protocol. Currently this attribute does not enforce any placement of where the call occurs in the overriding method (such as in the case of -dealloc where the call must appear at the end). It checks only that it exists.

Note that on both OS X and iOS that the Foundation framework provides a convenience macro NS_REQUIRES_SUPER that provides syntactic sugar for this attribute:

- (void)foo NS_REQUIRES_SUPER;

This macro is conditionally defined depending on the compiler’s support for this attribute. If the compiler does not support the attribute the macro expands to nothing.

Operationally, when a method has this annotation the compiler will warn if the implementation of an override in a subclass does not call super. For example:

warning: method possibly missing a [super AnnotMeth] call
- (void) AnnotMeth{};
                   ^

optnone

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
optnone	clang::optnone	clang::optnone				Yes

The optnone attribute suppresses essentially all optimizations on a function or method, regardless of the optimization level applied to the compilation unit as a whole. This is particularly useful when you need to debug a particular function, but it is infeasible to build the entire application without optimization. Avoiding optimization on the specified function can improve the quality of the debugging information for that function.

This attribute is incompatible with the always_inline and minsize attributes.

os_consumed

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
os_consumed	clang::os_consumed	clang::os_consumed				Yes

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

os_consumes_this

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
os_consumes_this	clang::os_consumes_this	clang::os_consumes_this

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

os_returns_not_retained

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
os_returns_not_retained	clang::os_returns_not_retained	clang::os_returns_not_retained				Yes

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

os_returns_retained

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
os_returns_retained	clang::os_returns_retained	clang::os_returns_retained				Yes

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

os_returns_retained_on_non_zero

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
os_returns_retained_on_non_zero	clang::os_returns_retained_on_non_zero	clang::os_returns_retained_on_non_zero				Yes

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

os_returns_retained_on_zero

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
os_returns_retained_on_zero	clang::os_returns_retained_on_zero	clang::os_returns_retained_on_zero				Yes

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

overloadable

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
overloadable	clang::overloadable	clang::overloadable				Yes

Clang provides support for C++ function overloading in C. Function overloading in C is introduced using the overloadable attribute. For example, one might provide several overloaded versions of a tgsin function that invokes the appropriate standard function computing the sine of a value with float, double, or long double precision:

#include <math.h>
float __attribute__((overloadable)) tgsin(float x) { return sinf(x); }
double __attribute__((overloadable)) tgsin(double x) { return sin(x); }
long double __attribute__((overloadable)) tgsin(long double x) { return sinl(x); }

Given these declarations, one can call tgsin with a float value to receive a float result, with a double to receive a double result, etc. Function overloading in C follows the rules of C++ function overloading to pick the best overload given the call arguments, with a few C-specific semantics:

• Conversion from float or double to long double is ranked as a floating-point promotion (per C99) rather than as a floating-point conversion (as in C++).

• A conversion from a pointer of type T* to a pointer of type U* is considered a pointer conversion (with conversion rank) if T and U are compatible types.

• A conversion from type T to a value of type U is permitted if T and U are compatible types. This conversion is given “conversion” rank.

• If no viable candidates are otherwise available, we allow a conversion from a pointer of type T* to a pointer of type U*, where T and U are incompatible. This conversion is ranked below all other types of conversions. Please note: U lacking qualifiers that are present on T is sufficient for T and U to be incompatible.

The declaration of overloadable functions is restricted to function declarations and definitions. If a function is marked with the overloadable attribute, then all declarations and definitions of functions with that name, except for at most one (see the note below about unmarked overloads), must have the overloadable attribute. In addition, redeclarations of a function with the overloadable attribute must have the overloadable attribute, and redeclarations of a function without the overloadable attribute must not have the overloadable attribute. e.g.,

int f(int) __attribute__((overloadable));
float f(float); // error: declaration of "f" must have the "overloadable" attribute
int f(int); // error: redeclaration of "f" must have the "overloadable" attribute

int g(int) __attribute__((overloadable));
int g(int) { } // error: redeclaration of "g" must also have the "overloadable" attribute

int h(int);
int h(int) __attribute__((overloadable)); // error: declaration of "h" must not
                                          // have the "overloadable" attribute

Functions marked overloadable must have prototypes. Therefore, the following code is ill-formed:

int h() __attribute__((overloadable)); // error: h does not have a prototype

However, overloadable functions are allowed to use a ellipsis even if there are no named parameters (as is permitted in C++). This feature is particularly useful when combined with the unavailable attribute:

void honeypot(...) __attribute__((overloadable, unavailable)); // calling me is an error

Functions declared with the overloadable attribute have their names mangled according to the same rules as C++ function names. For example, the three tgsin functions in our motivating example get the mangled names _Z5tgsinf, _Z5tgsind, and _Z5tgsine, respectively. There are two caveats to this use of name mangling:

• Future versions of Clang may change the name mangling of functions overloaded in C, so you should not depend on an specific mangling. To be completely safe, we strongly urge the use of static inline with overloadable functions.

• The overloadable attribute has almost no meaning when used in C++, because names will already be mangled and functions are already overloadable. However, when an overloadable function occurs within an extern "C" linkage specification, it’s name will be mangled in the same way as it would in C.

For the purpose of backwards compatibility, at most one function with the same name as other overloadable functions may omit the overloadable attribute. In this case, the function without the overloadable attribute will not have its name mangled.

For example:

// Notes with mangled names assume Itanium mangling.
int f(int);
int f(double) __attribute__((overloadable));
void foo() {
  f(5); // Emits a call to f (not _Z1fi, as it would with an overload that
        // was marked with overloadable).
  f(1.0); // Emits a call to _Z1fd.
}

Support for unmarked overloads is not present in some versions of clang. You may query for it using __has_extension(overloadable_unmarked).

Query for this attribute with __has_attribute(overloadable).

patchable_function_entry

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
patchable_function_entry	gnu::patchable_function_entry	gnu::patchable_function_entry				Yes

__attribute__((patchable_function_entry(N,M))) is used to generate M NOPs before the function entry and N-M NOPs after the function entry. This attribute takes precedence over the command line option -fpatchable-function-entry=N,M. M defaults to 0 if omitted.

This attribute is only supported on aarch64/aarch64-be/riscv32/riscv64/i386/x86-64 targets.

preserve_access_index

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
preserve_access_index	clang::preserve_access_index	clang::preserve_access_index				Yes

Clang supports the __attribute__((preserve_access_index)) attribute for the BPF target. This attribute may be attached to a struct or union declaration, where if -g is specified, it enables preserving struct or union member access debuginfo indices of this struct or union, similar to clang __builtin_preserve_access_index().

reinitializes

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
reinitializes	clang::reinitializes

The reinitializes attribute can be applied to a non-static, non-const C++ member function to indicate that this member function reinitializes the entire object to a known state, independent of the previous state of the object.

This attribute can be interpreted by static analyzers that warn about uses of an object that has been left in an indeterminate state by a move operation. If a member function marked with the reinitializes attribute is called on a moved-from object, the analyzer can conclude that the object is no longer in an indeterminate state.

A typical example where this attribute would be used is on functions that clear a container class:

template <class T>
class Container {
public:
  ...
  [[clang::reinitializes]] void Clear();
  ...
};

release_capability, release_shared_capability

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
release_capability release_shared_capability release_generic_capability unlock_function	clang::release_capability clang::release_shared_capability clang::release_generic_capability clang::unlock_function

Marks a function as releasing a capability.

retain

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
retain	gnu::retain	gnu::retain

This attribute, when attached to a function or variable definition, prevents section garbage collection in the linker. It does not prevent other discard mechanisms, such as archive member selection, and COMDAT group resolution.

If the compiler does not emit the definition, e.g. because it was not used in the translation unit or the compiler was able to eliminate all of the uses, this attribute has no effect. This attribute is typically combined with the used attribute to force the definition to be emitted and preserved into the final linked image.

This attribute is only necessary on ELF targets; other targets prevent section garbage collection by the linker when using the used attribute alone. Using the attributes together should result in consistent behavior across targets.

This attribute requires the linker to support the SHF_GNU_RETAIN extension. This support is available in GNU ld and gold as of binutils 2.36, as well as in ld.lld 13.

short_call, near

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
short_call near	gnu::short_call gnu::near	gnu::short_call gnu::near				Yes

Clang supports the __attribute__((long_call)), __attribute__((far)), __attribute__((short__call)), and __attribute__((near)) attributes on MIPS targets. These attributes may only be added to function declarations and change the code generated by the compiler when directly calling the function. The short_call and near attributes are synonyms and allow calls to the function to be made using the jal instruction, which requires the function to be located in the same naturally aligned 256MB segment as the caller. The long_call and far attributes are synonyms and require the use of a different call sequence that works regardless of the distance between the functions.

These attributes have no effect for position-independent code.

These attributes take priority over command line switches such as -mlong-calls and -mno-long-calls.

signal

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
signal	gnu::signal	gnu::signal				Yes

Clang supports the GNU style __attribute__((signal)) attribute on AVR targets. This attribute may be attached to a function definition and instructs the backend to generate appropriate function entry/exit code so that it can be used directly as an interrupt service routine.

Interrupt handler functions defined with the signal attribute do not re-enable interrupts.

speculative_load_hardening

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
speculative_load_hardening	clang::speculative_load_hardening	clang::speculative_load_hardening				Yes

This attribute can be applied to a function declaration in order to indicate

that Speculative Load Hardening should be enabled for the function body. This can also be applied to a method in Objective C. This attribute will take precedence over the command line flag in the case where -mno-speculative-load-hardening is specified.

Speculative Load Hardening is a best-effort mitigation against information leak attacks that make use of control flow miss-speculation - specifically miss-speculation of whether a branch is taken or not. Typically vulnerabilities enabling such attacks are classified as “Spectre variant #1”. Notably, this does not attempt to mitigate against miss-speculation of branch target, classified as “Spectre variant #2” vulnerabilities.

When inlining, the attribute is sticky. Inlining a function that carries this attribute will cause the caller to gain the attribute. This is intended to provide a maximally conservative model where the code in a function annotated with this attribute will always (even after inlining) end up hardened.

sycl_kernel

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
sycl_kernel	clang::sycl_kernel	clang::sycl_kernel

The sycl_kernel attribute specifies that a function template will be used to outline device code and to generate an OpenCL kernel. Here is a code example of the SYCL program, which demonstrates the compiler’s outlining job:

int foo(int x) { return ++x; }

using namespace cl::sycl;
queue Q;
buffer<int, 1> a(range<1>{1024});
Q.submit([&](handler& cgh) {
  auto A = a.get_access<access::mode::write>(cgh);
  cgh.parallel_for<init_a>(range<1>{1024}, [=](id<1> index) {
    A[index] = index[0] + foo(42);
  });
}

A C++ function object passed to the parallel_for is called a “SYCL kernel”. A SYCL kernel defines the entry point to the “device part” of the code. The compiler will emit all symbols accessible from a “kernel”. In this code example, the compiler will emit “foo” function. More details about the compilation of functions for the device part can be found in the SYCL 1.2.1 specification Section 6.4. To show to the compiler entry point to the “device part” of the code, the SYCL runtime can use the sycl_kernel attribute in the following way:

namespace cl {
namespace sycl {
class handler {
  template <typename KernelName, typename KernelType/*, ...*/>
  __attribute__((sycl_kernel)) void sycl_kernel_function(KernelType KernelFuncObj) {
    // ...
    KernelFuncObj();
  }

  template <typename KernelName, typename KernelType, int Dims>
  void parallel_for(range<Dims> NumWorkItems, KernelType KernelFunc) {
#ifdef __SYCL_DEVICE_ONLY__
    sycl_kernel_function<KernelName, KernelType, Dims>(KernelFunc);
#else
    // Host implementation
#endif
  }
};
} // namespace sycl
} // namespace cl

The compiler will also generate an OpenCL kernel using the function marked with the sycl_kernel attribute. Here is the list of SYCL device compiler expectations with regard to the function marked with the sycl_kernel attribute:

• The function must be a template with at least two type template parameters. The compiler generates an OpenCL kernel and uses the first template parameter as a unique name for the generated OpenCL kernel. The host application uses this unique name to invoke the OpenCL kernel generated for the SYCL kernel specialized by this name and second template parameter KernelType (which might be an unnamed function object type).

• The function must have at least one parameter. The first parameter is required to be a function object type (named or unnamed i.e. lambda). The compiler uses function object type fields to generate OpenCL kernel parameters.

• The function must return void. The compiler reuses the body of marked functions to generate the OpenCL kernel body, and the OpenCL kernel must return void.

The SYCL kernel in the previous code sample meets these expectations.

target

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
target	gnu::target	gnu::target				Yes

Clang supports the GNU style __attribute__((target("OPTIONS"))) attribute. This attribute may be attached to a function definition and instructs the backend to use different code generation options than were passed on the command line.

The current set of options correspond to the existing “subtarget features” for the target with or without a “-mno-” in front corresponding to the absence of the feature, as well as arch="CPU" which will change the default “CPU” for the function.

For X86, the attribute also allows tune="CPU" to optimize the generated code for the given CPU without changing the available instructions.

For AArch64, the attribute also allows the “branch-protection=<args>” option, where the permissible arguments and their effect on code generation are the same as for the command-line option -mbranch-protection.

Example “subtarget features” from the x86 backend include: “mmx”, “sse”, “sse4.2”, “avx”, “xop” and largely correspond to the machine specific options handled by the front end.

Additionally, this attribute supports function multiversioning for ELF based x86/x86-64 targets, which can be used to create multiple implementations of the same function that will be resolved at runtime based on the priority of their target attribute strings. A function is considered a multiversioned function if either two declarations of the function have different target attribute strings, or if it has a target attribute string of default. For example:

__attribute__((target("arch=atom")))
void foo() {} // will be called on 'atom' processors.
__attribute__((target("default")))
void foo() {} // will be called on any other processors.

All multiversioned functions must contain a default (fallback) implementation, otherwise usages of the function are considered invalid. Additionally, a function may not become multiversioned after its first use.

target_clones

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
target_clones	gnu::target_clones	gnu::target_clones				Yes

Clang supports the target_clones("OPTIONS") attribute. This attribute may be attached to a function declaration and causes function multiversioning, where multiple versions of the function will be emitted with different code generation options. Additionally, these versions will be resolved at runtime based on the priority of their attribute options. All target_clone functions are considered multiversioned functions.

All multiversioned functions must contain a default (fallback) implementation, otherwise usages of the function are considered invalid. Additionally, a function may not become multiversioned after its first use.

The options to target_clones can either be a target-specific architecture (specified as arch=CPU), or one of a list of subtarget features.

Example “subtarget features” from the x86 backend include: “mmx”, “sse”, “sse4.2”, “avx”, “xop” and largely correspond to the machine specific options handled by the front end.

The versions can either be listed as a comma-separated sequence of string literals or as a single string literal containing a comma-separated list of versions. For compatibility with GCC, the two formats can be mixed. For example, the following will emit 4 versions of the function:

__attribute__((target_clones("arch=atom,avx2","arch=ivybridge","default")))
void foo() {}

try_acquire_capability, try_acquire_shared_capability

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
try_acquire_capability try_acquire_shared_capability	clang::try_acquire_capability clang::try_acquire_shared_capability

Marks a function that attempts to acquire a capability. This function may fail to actually acquire the capability; they accept a Boolean value determining whether acquiring the capability means success (true), or failing to acquire the capability means success (false).

used

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
used	gnu::used	gnu::used

This attribute, when attached to a function or variable definition, indicates that there may be references to the entity which are not apparent in the source code. For example, it may be referenced from inline asm, or it may be found through a dynamic symbol or section lookup.

The compiler must emit the definition even if it appears to be unused, and it must not apply optimizations which depend on fully understanding how the entity is used.

Whether this attribute has any effect on the linker depends on the target and the linker. Most linkers support the feature of section garbage collection (--gc-sections), also known as “dead stripping” (ld64 -dead_strip) or discarding unreferenced sections (link.exe /OPT:REF). On COFF and Mach-O targets (Windows and Apple platforms), the used attribute prevents symbols from being removed by linker section GC. On ELF targets, it has no effect on its own, and the linker may remove the definition if it is not otherwise referenced. This linker GC can be avoided by also adding the retain attribute. Note that retain requires special support from the linker; see that attribute’s documentation for further information.

xray_always_instrument, xray_never_instrument, xray_log_args

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
xray_always_instrument xray_never_instrument	clang::xray_always_instrument clang::xray_never_instrument	clang::xray_always_instrument clang::xray_never_instrument				Yes

__attribute__((xray_always_instrument)) or [[clang::xray_always_instrument]] is used to mark member functions (in C++), methods (in Objective C), and free functions (in C, C++, and Objective C) to be instrumented with XRay. This will cause the function to always have space at the beginning and exit points to allow for runtime patching.

Conversely, __attribute__((xray_never_instrument)) or [[clang::xray_never_instrument]] will inhibit the insertion of these instrumentation points.

If a function has neither of these attributes, they become subject to the XRay heuristics used to determine whether a function should be instrumented or otherwise.

__attribute__((xray_log_args(N))) or [[clang::xray_log_args(N)]] is used to preserve N function arguments for the logging function. Currently, only N==1 is supported.

xray_always_instrument, xray_never_instrument, xray_log_args

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
xray_log_args	clang::xray_log_args	clang::xray_log_args				Yes

__attribute__((xray_always_instrument)) or [[clang::xray_always_instrument]] is used to mark member functions (in C++), methods (in Objective C), and free functions (in C, C++, and Objective C) to be instrumented with XRay. This will cause the function to always have space at the beginning and exit points to allow for runtime patching.

Conversely, __attribute__((xray_never_instrument)) or [[clang::xray_never_instrument]] will inhibit the insertion of these instrumentation points.

If a function has neither of these attributes, they become subject to the XRay heuristics used to determine whether a function should be instrumented or otherwise.

__attribute__((xray_log_args(N))) or [[clang::xray_log_args(N)]] is used to preserve N function arguments for the logging function. Currently, only N==1 is supported.

Handle Attributes

Handles are a way to identify resources like files, sockets, and processes. They are more opaque than pointers and widely used in system programming. They have similar risks such as never releasing a resource associated with a handle, attempting to use a handle that was already released, or trying to release a handle twice. Using the annotations below it is possible to make the ownership of the handles clear: whose responsibility is to release them. They can also aid static analysis tools to find bugs.

acquire_handle

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
acquire_handle	clang::acquire_handle	clang::acquire_handle				Yes

If this annotation is on a function or a function type it is assumed to return a new handle. In case this annotation is on an output parameter, the function is assumed to fill the corresponding argument with a new handle. The attribute requires a string literal argument which used to identify the handle with later uses of use_handle or release_handle.

// Output arguments from Zircon.
zx_status_t zx_socket_create(uint32_t options,
                             zx_handle_t __attribute__((acquire_handle("zircon"))) * out0,
                             zx_handle_t* out1 [[clang::acquire_handle("zircon")]]);


// Returned handle.
[[clang::acquire_handle("tag")]] int open(const char *path, int oflag, ... );
int open(const char *path, int oflag, ... ) __attribute__((acquire_handle("tag")));

release_handle

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
release_handle	clang::release_handle	clang::release_handle				Yes

If a function parameter is annotated with release_handle(tag) it is assumed to close the handle. It is also assumed to require an open handle to work with. The attribute requires a string literal argument to identify the handle being released.

zx_status_t zx_handle_close(zx_handle_t handle [[clang::release_handle("tag")]]);

use_handle

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
use_handle	clang::use_handle	clang::use_handle				Yes

A function taking a handle by value might close the handle. If a function parameter is annotated with use_handle(tag) it is assumed to not to change the state of the handle. It is also assumed to require an open handle to work with. The attribute requires a string literal argument to identify the handle being used.

zx_status_t zx_port_wait(zx_handle_t handle [[clang::use_handle("zircon")]],
                         zx_time_t deadline,
                         zx_port_packet_t* packet);

Nullability Attributes

Whether a particular pointer may be “null” is an important concern when working with pointers in the C family of languages. The various nullability attributes indicate whether a particular pointer can be null or not, which makes APIs more expressive and can help static analysis tools identify bugs involving null pointers. Clang supports several kinds of nullability attributes: the nonnull and returns_nonnull attributes indicate which function or method parameters and result types can never be null, while nullability type qualifiers indicate which pointer types can be null (_Nullable) or cannot be null (_Nonnull).

The nullability (type) qualifiers express whether a value of a given pointer type can be null (the _Nullable qualifier), doesn’t have a defined meaning for null (the _Nonnull qualifier), or for which the purpose of null is unclear (the _Null_unspecified qualifier). Because nullability qualifiers are expressed within the type system, they are more general than the nonnull and returns_nonnull attributes, allowing one to express (for example) a nullable pointer to an array of nonnull pointers. Nullability qualifiers are written to the right of the pointer to which they apply. For example:

// No meaningful result when 'ptr' is null (here, it happens to be undefined behavior).
int fetch(int * _Nonnull ptr) { return *ptr; }

// 'ptr' may be null.
int fetch_or_zero(int * _Nullable ptr) {
  return ptr ? *ptr : 0;
}

// A nullable pointer to non-null pointers to const characters.
const char *join_strings(const char * _Nonnull * _Nullable strings, unsigned n);

In Objective-C, there is an alternate spelling for the nullability qualifiers that can be used in Objective-C methods and properties using context-sensitive, non-underscored keywords. For example:

@interface NSView : NSResponder
  - (nullable NSView *)ancestorSharedWithView:(nonnull NSView *)aView;
  @property (assign, nullable) NSView *superview;
  @property (readonly, nonnull) NSArray *subviews;
@end

_Nonnull

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
				_Nonnull

The _Nonnull nullability qualifier indicates that null is not a meaningful value for a value of the _Nonnull pointer type. For example, given a declaration such as:

int fetch(int * _Nonnull ptr);

a caller of fetch should not provide a null value, and the compiler will produce a warning if it sees a literal null value passed to fetch. Note that, unlike the declaration attribute nonnull, the presence of _Nonnull does not imply that passing null is undefined behavior: fetch is free to consider null undefined behavior or (perhaps for backward-compatibility reasons) defensively handle null.

_Null_unspecified

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
				_Null_unspecified

The _Null_unspecified nullability qualifier indicates that neither the _Nonnull nor _Nullable qualifiers make sense for a particular pointer type. It is used primarily to indicate that the role of null with specific pointers in a nullability-annotated header is unclear, e.g., due to overly-complex implementations or historical factors with a long-lived API.

_Nullable

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
				_Nullable

The _Nullable nullability qualifier indicates that a value of the _Nullable pointer type can be null. For example, given:

int fetch_or_zero(int * _Nullable ptr);

a caller of fetch_or_zero can provide null.

_Nullable_result

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
				_Nullable_result

The _Nullable_result nullability qualifier means that a value of the _Nullable_result pointer can be nil, just like _Nullable. Where this attribute differs from _Nullable is when it’s used on a parameter to a completion handler in a Swift async method. For instance, here:

-(void)fetchSomeDataWithID:(int)identifier
         completionHandler:(void (^)(Data *_Nullable_result result, NSError *error))completionHandler;

This method asynchronously calls completionHandler when the data is available, or calls it with an error. _Nullable_result indicates to the Swift importer that this is the uncommon case where result can get nil even if no error has occured, and will therefore import it as a Swift optional type. Otherwise, if result was annotated with _Nullable, the Swift importer will assume that result will always be non-nil unless an error occured.

nonnull

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
nonnull	gnu::nonnull	gnu::nonnull

The nonnull attribute indicates that some function parameters must not be null, and can be used in several different ways. It’s original usage (from GCC) is as a function (or Objective-C method) attribute that specifies which parameters of the function are nonnull in a comma-separated list. For example:

extern void * my_memcpy (void *dest, const void *src, size_t len)
                __attribute__((nonnull (1, 2)));

Here, the nonnull attribute indicates that parameters 1 and 2 cannot have a null value. Omitting the parenthesized list of parameter indices means that all parameters of pointer type cannot be null:

extern void * my_memcpy (void *dest, const void *src, size_t len)
                __attribute__((nonnull));

Clang also allows the nonnull attribute to be placed directly on a function (or Objective-C method) parameter, eliminating the need to specify the parameter index ahead of type. For example:

extern void * my_memcpy (void *dest __attribute__((nonnull)),
                         const void *src __attribute__((nonnull)), size_t len);

Note that the nonnull attribute indicates that passing null to a non-null parameter is undefined behavior, which the optimizer may take advantage of to, e.g., remove null checks. The _Nonnull type qualifier indicates that a pointer cannot be null in a more general manner (because it is part of the type system) and does not imply undefined behavior, making it more widely applicable.

returns_nonnull

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
returns_nonnull	gnu::returns_nonnull	gnu::returns_nonnull				Yes

The returns_nonnull attribute indicates that a particular function (or Objective-C method) always returns a non-null pointer. For example, a particular system malloc might be defined to terminate a process when memory is not available rather than returning a null pointer:

extern void * malloc (size_t size) __attribute__((returns_nonnull));

The returns_nonnull attribute implies that returning a null pointer is undefined behavior, which the optimizer may take advantage of. The _Nonnull type qualifier indicates that a pointer cannot be null in a more general manner (because it is part of the type system) and does not imply undefined behavior, making it more widely applicable

OpenCL Address Spaces

The address space qualifier may be used to specify the region of memory that is used to allocate the object. OpenCL supports the following address spaces: __generic(generic), __global(global), __local(local), __private(private), __constant(constant).

__constant int c = ...;

__generic int* foo(global int* g) {
  __local int* l;
  private int p;
  ...
  return l;
}

More details can be found in the OpenCL C language Spec v2.0, Section 6.5.

[[clang::opencl_global_device]], [[clang::opencl_global_host]]

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
opencl_global_device	clang::opencl_global_device	clang::opencl_global_device

The global_device and global_host address space attributes specify that an object is allocated in global memory on the device/host. It helps to distinguish USM (Unified Shared Memory) pointers that access global device memory from those that access global host memory. These new address spaces are a subset of the __global/opencl_global address space, the full address space set model for OpenCL 2.0 with the extension looks as follows:

generic->global->host

->device

->private

->local

constant

As global_device and global_host are a subset of __global/opencl_global address spaces it is allowed to convert global_device and global_host address spaces to __global/opencl_global address spaces (following ISO/IEC TR 18037 5.1.3 “Address space nesting and rules for pointers”).

[[clang::opencl_global_device]], [[clang::opencl_global_host]]

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
opencl_global_host	clang::opencl_global_host	clang::opencl_global_host

The global_device and global_host address space attributes specify that an object is allocated in global memory on the device/host. It helps to distinguish USM (Unified Shared Memory) pointers that access global device memory from those that access global host memory. These new address spaces are a subset of the __global/opencl_global address space, the full address space set model for OpenCL 2.0 with the extension looks as follows:

generic->global->host

->device

->private

->local

constant

As global_device and global_host are a subset of __global/opencl_global address spaces it is allowed to convert global_device and global_host address spaces to __global/opencl_global address spaces (following ISO/IEC TR 18037 5.1.3 “Address space nesting and rules for pointers”).

__constant, constant, [[clang::opencl_constant]]

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
opencl_constant	clang::opencl_constant	clang::opencl_constant		__constant constant

The constant address space attribute signals that an object is located in a constant (non-modifiable) memory region. It is available to all work items. Any type can be annotated with the constant address space attribute. Objects with the constant address space qualifier can be declared in any scope and must have an initializer.

__generic, generic, [[clang::opencl_generic]]

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
opencl_generic	clang::opencl_generic	clang::opencl_generic		__generic generic

The generic address space attribute is only available with OpenCL v2.0 and later. It can be used with pointer types. Variables in global and local scope and function parameters in non-kernel functions can have the generic address space type attribute. It is intended to be a placeholder for any other address space except for ‘__constant’ in OpenCL code which can be used with multiple address spaces.

__global, global, [[clang::opencl_global]]

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
opencl_global	clang::opencl_global	clang::opencl_global		__global global

The global address space attribute specifies that an object is allocated in global memory, which is accessible by all work items. The content stored in this memory area persists between kernel executions. Pointer types to the global address space are allowed as function parameters or local variables. Starting with OpenCL v2.0, the global address space can be used with global (program scope) variables and static local variable as well.

__local, local, [[clang::opencl_local]]

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
opencl_local	clang::opencl_local	clang::opencl_local		__local local

The local address space specifies that an object is allocated in the local (work group) memory area, which is accessible to all work items in the same work group. The content stored in this memory region is not accessible after the kernel execution ends. In a kernel function scope, any variable can be in the local address space. In other scopes, only pointer types to the local address space are allowed. Local address space variables cannot have an initializer.

__private, private, [[clang::opencl_private]]

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
opencl_private	clang::opencl_private	clang::opencl_private		__private private

The private address space specifies that an object is allocated in the private (work item) memory. Other work items cannot access the same memory area and its content is destroyed after work item execution ends. Local variables can be declared in the private address space. Function arguments are always in the private address space. Kernel function arguments of a pointer or an array type cannot point to the private address space.

Statement Attributes

#pragma clang loop

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
					clang loop unroll nounroll unroll_and_jam nounroll_and_jam

The #pragma clang loop directive allows loop optimization hints to be specified for the subsequent loop. The directive allows pipelining to be disabled, or vectorization, vector predication, interleaving, and unrolling to be enabled or disabled. Vector width, vector predication, interleave count, unrolling count, and the initiation interval for pipelining can be explicitly specified. See language extensions for details.

#pragma unroll, #pragma nounroll

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
					clang loop unroll nounroll unroll_and_jam nounroll_and_jam

Loop unrolling optimization hints can be specified with #pragma unroll and #pragma nounroll. The pragma is placed immediately before a for, while, do-while, or c++11 range-based for loop. GCC’s loop unrolling hints #pragma GCC unroll and #pragma GCC nounroll are also supported and have identical semantics to #pragma unroll and #pragma nounroll.

Specifying #pragma unroll without a parameter directs the loop unroller to attempt to fully unroll the loop if the trip count is known at compile time and attempt to partially unroll the loop if the trip count is not known at compile time:

#pragma unroll
for (...) {
  ...
}

Specifying the optional parameter, #pragma unroll _value_, directs the unroller to unroll the loop _value_ times. The parameter may optionally be enclosed in parentheses:

#pragma unroll 16
for (...) {
  ...
}

#pragma unroll(16)
for (...) {
  ...
}

Specifying #pragma nounroll indicates that the loop should not be unrolled:

#pragma nounroll
for (...) {
  ...
}

#pragma unroll and #pragma unroll _value_ have identical semantics to #pragma clang loop unroll(full) and #pragma clang loop unroll_count(_value_) respectively. #pragma nounroll is equivalent to #pragma clang loop unroll(disable). See language extensions for further details including limitations of the unroll hints.

__read_only, __write_only, __read_write (read_only, write_only, read_write)

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
				__read_only read_only __write_only write_only __read_write read_write

The access qualifiers must be used with image object arguments or pipe arguments to declare if they are being read or written by a kernel or function.

The read_only/__read_only, write_only/__write_only and read_write/__read_write names are reserved for use as access qualifiers and shall not be used otherwise.

kernel void
foo (read_only image2d_t imageA,
     write_only image2d_t imageB) {
  ...
}

In the above example imageA is a read-only 2D image object, and imageB is a write-only 2D image object.

The read_write (or __read_write) qualifier can not be used with pipe.

More details can be found in the OpenCL C language Spec v2.0, Section 6.6.

fallthrough

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
fallthrough	fallthrough clang::fallthrough gnu::fallthrough	fallthrough gnu::fallthrough

The fallthrough (or clang::fallthrough) attribute is used to annotate intentional fall-through between switch labels. It can only be applied to a null statement placed at a point of execution between any statement and the next switch label. It is common to mark these places with a specific comment, but this attribute is meant to replace comments with a more strict annotation, which can be checked by the compiler. This attribute doesn’t change semantics of the code and can be used wherever an intended fall-through occurs. It is designed to mimic control-flow statements like break;, so it can be placed in most places where break; can, but only if there are no statements on the execution path between it and the next switch label.

By default, Clang does not warn on unannotated fallthrough from one switch case to another. Diagnostics on fallthrough without a corresponding annotation can be enabled with the -Wimplicit-fallthrough argument.

Here is an example:

// compile with -Wimplicit-fallthrough
switch (n) {
case 22:
case 33:  // no warning: no statements between case labels
  f();
case 44:  // warning: unannotated fall-through
  g();
  [[clang::fallthrough]];
case 55:  // no warning
  if (x) {
    h();
    break;
  }
  else {
    i();
    [[clang::fallthrough]];
  }
case 66:  // no warning
  p();
  [[clang::fallthrough]]; // warning: fallthrough annotation does not
                          //          directly precede case label
  q();
case 77:  // warning: unannotated fall-through
  r();
}

intel_reqd_sub_group_size

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
intel_reqd_sub_group_size						Yes

The optional attribute intel_reqd_sub_group_size can be used to indicate that the kernel must be compiled and executed with the specified subgroup size. When this attribute is present, get_max_sub_group_size() is guaranteed to return the specified integer value. This is important for the correctness of many subgroup algorithms, and in some cases may be used by the compiler to generate more optimal code. See cl_intel_required_subgroup_size <https://www.khronos.org/registry/OpenCL/extensions/intel/cl_intel_required_subgroup_size.txt> for details.

likely and unlikely

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
	likely	clang::likely

The likely and unlikely attributes are used as compiler hints. The attributes are used to aid the compiler to determine which branch is likely or unlikely to be taken. This is done by marking the branch substatement with one of the two attributes.

It isn’t allowed to annotate a single statement with both likely and unlikely. Annotating the true and false branch of an if statement with the same likelihood attribute will result in a diagnostic and the attributes are ignored on both branches.

In a switch statement it’s allowed to annotate multiple case labels or the default label with the same likelihood attribute. This makes * all labels without an attribute have a neutral likelihood, * all labels marked [[likely]] have an equally positive likelihood, and * all labels marked [[unlikely]] have an equally negative likelihood. The neutral likelihood is the more likely of path execution than the negative likelihood. The positive likelihood is the more likely of path of execution than the neutral likelihood.

These attributes have no effect on the generated code when using PGO (Profile-Guided Optimization) or at optimization level 0.

In Clang, the attributes will be ignored if they’re not placed on * the case or default label of a switch statement, * or on the substatement of an if or else statement, * or on the substatement of an for or while statement. The C++ Standard recommends to honor them on every statement in the path of execution, but that can be confusing:

if (b) {
  [[unlikely]] --b; // In the path of execution,
                    // this branch is considered unlikely.
}

if (b) {
  --b;
  if(b)
    return;
  [[unlikely]] --b; // Not in the path of execution,
}                   // the branch has no likelihood information.

if (b) {
  --b;
  foo(b);
  // Whether or not the next statement is in the path of execution depends
  // on the declaration of foo():
  // In the path of execution: void foo(int);
  // Not in the path of execution: [[noreturn]] void foo(int);
  // This means the likelihood of the branch depends on the declaration
  // of foo().
  [[unlikely]] --b;
}

Below are some example usages of the likelihood attributes and their effects:

if (b) [[likely]] { // Placement on the first statement in the branch.
  // The compiler will optimize to execute the code here.
} else {
}

if (b)
  [[unlikely]] b++; // Placement on the first statement in the branch.
else {
  // The compiler will optimize to execute the code here.
}

if (b) {
  [[unlikely]] b++; // Placement on the second statement in the branch.
}                   // The attribute will be ignored.

if (b) [[likely]] {
  [[unlikely]] b++; // No contradiction since the second attribute
}                   // is ignored.

if (b)
  ;
else [[likely]] {
  // The compiler will optimize to execute the code here.
}

if (b)
  ;
else
  // The compiler will optimize to execute the next statement.
  [[likely]] b = f();

if (b) [[likely]]; // Both branches are likely. A diagnostic is issued
else [[likely]];   // and the attributes are ignored.

if (b)
  [[likely]] int i = 5; // Issues a diagnostic since the attribute
                        // isn't allowed on a declaration.

switch (i) {
  [[likely]] case 1:    // This value is likely
    ...
    break;

  [[unlikely]] case 2:  // This value is unlikely
    ...
    [[fallthrough]];

  case 3:               // No likelihood attribute
    ...
    [[likely]] break;   // No effect

  case 4: [[likely]] {  // attribute on substatement has no effect
    ...
    break;
    }

  [[unlikely]] default: // All other values are unlikely
    ...
    break;
}

switch (i) {
  [[likely]] case 0:    // This value and code path is likely
    ...
    [[fallthrough]];

  case 1:               // No likelihood attribute, code path is neutral
    break;              // falling through has no effect on the likelihood

  case 2:               // No likelihood attribute, code path is neutral
    [[fallthrough]];

  [[unlikely]] default: // This value and code path are both unlikely
    break;
}

for(int i = 0; i != size; ++i) [[likely]] {
  ...               // The loop is the likely path of execution
}

for(const auto &E : Elements) [[likely]] {
  ...               // The loop is the likely path of execution
}

while(i != size) [[unlikely]] {
  ...               // The loop is the unlikely path of execution
}                   // The generated code will optimize to skip the loop body

while(true) [[unlikely]] {
  ...               // The attribute has no effect
}                   // Clang elides the comparison and generates an infinite
                    // loop

likely and unlikely

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
	unlikely	clang::unlikely

The likely and unlikely attributes are used as compiler hints. The attributes are used to aid the compiler to determine which branch is likely or unlikely to be taken. This is done by marking the branch substatement with one of the two attributes.

It isn’t allowed to annotate a single statement with both likely and unlikely. Annotating the true and false branch of an if statement with the same likelihood attribute will result in a diagnostic and the attributes are ignored on both branches.

In a switch statement it’s allowed to annotate multiple case labels or the default label with the same likelihood attribute. This makes * all labels without an attribute have a neutral likelihood, * all labels marked [[likely]] have an equally positive likelihood, and * all labels marked [[unlikely]] have an equally negative likelihood. The neutral likelihood is the more likely of path execution than the negative likelihood. The positive likelihood is the more likely of path of execution than the neutral likelihood.

These attributes have no effect on the generated code when using PGO (Profile-Guided Optimization) or at optimization level 0.

In Clang, the attributes will be ignored if they’re not placed on * the case or default label of a switch statement, * or on the substatement of an if or else statement, * or on the substatement of an for or while statement. The C++ Standard recommends to honor them on every statement in the path of execution, but that can be confusing:

if (b) {
  [[unlikely]] --b; // In the path of execution,
                    // this branch is considered unlikely.
}

if (b) {
  --b;
  if(b)
    return;
  [[unlikely]] --b; // Not in the path of execution,
}                   // the branch has no likelihood information.

if (b) {
  --b;
  foo(b);
  // Whether or not the next statement is in the path of execution depends
  // on the declaration of foo():
  // In the path of execution: void foo(int);
  // Not in the path of execution: [[noreturn]] void foo(int);
  // This means the likelihood of the branch depends on the declaration
  // of foo().
  [[unlikely]] --b;
}

Below are some example usages of the likelihood attributes and their effects:

if (b) [[likely]] { // Placement on the first statement in the branch.
  // The compiler will optimize to execute the code here.
} else {
}

if (b)
  [[unlikely]] b++; // Placement on the first statement in the branch.
else {
  // The compiler will optimize to execute the code here.
}

if (b) {
  [[unlikely]] b++; // Placement on the second statement in the branch.
}                   // The attribute will be ignored.

if (b) [[likely]] {
  [[unlikely]] b++; // No contradiction since the second attribute
}                   // is ignored.

if (b)
  ;
else [[likely]] {
  // The compiler will optimize to execute the code here.
}

if (b)
  ;
else
  // The compiler will optimize to execute the next statement.
  [[likely]] b = f();

if (b) [[likely]]; // Both branches are likely. A diagnostic is issued
else [[likely]];   // and the attributes are ignored.

if (b)
  [[likely]] int i = 5; // Issues a diagnostic since the attribute
                        // isn't allowed on a declaration.

switch (i) {
  [[likely]] case 1:    // This value is likely
    ...
    break;

  [[unlikely]] case 2:  // This value is unlikely
    ...
    [[fallthrough]];

  case 3:               // No likelihood attribute
    ...
    [[likely]] break;   // No effect

  case 4: [[likely]] {  // attribute on substatement has no effect
    ...
    break;
    }

  [[unlikely]] default: // All other values are unlikely
    ...
    break;
}

switch (i) {
  [[likely]] case 0:    // This value and code path is likely
    ...
    [[fallthrough]];

  case 1:               // No likelihood attribute, code path is neutral
    break;              // falling through has no effect on the likelihood

  case 2:               // No likelihood attribute, code path is neutral
    [[fallthrough]];

  [[unlikely]] default: // This value and code path are both unlikely
    break;
}

for(int i = 0; i != size; ++i) [[likely]] {
  ...               // The loop is the likely path of execution
}

for(const auto &E : Elements) [[likely]] {
  ...               // The loop is the likely path of execution
}

while(i != size) [[unlikely]] {
  ...               // The loop is the unlikely path of execution
}                   // The generated code will optimize to skip the loop body

while(true) [[unlikely]] {
  ...               // The attribute has no effect
}                   // Clang elides the comparison and generates an infinite
                    // loop

musttail

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
musttail	clang::musttail	clang::musttail

If a return statement is marked musttail, this indicates that the compiler must generate a tail call for the program to be correct, even when optimizations are disabled. This guarantees that the call will not cause unbounded stack growth if it is part of a recursive cycle in the call graph.

If the callee is a virtual function that is implemented by a thunk, there is no guarantee in general that the thunk tail-calls the implementation of the virtual function, so such a call in a recursive cycle can still result in unbounded stack growth.

clang::musttail can only be applied to a return statement whose value is the result of a function call (even functions returning void must use return, although no value is returned). The target function must have the same number of arguments as the caller. The types of the return value and all arguments must be similar according to C++ rules (differing only in cv qualifiers or array size), including the implicit “this” argument, if any. Any variables in scope, including all arguments to the function and the return value must be trivially destructible. The calling convention of the caller and callee must match, and they must not be variadic functions or have old style K&R C function declarations.

nomerge

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
nomerge	clang::nomerge	clang::nomerge				Yes

If a statement is marked nomerge and contains call expressions, those call expressions inside the statement will not be merged during optimization. This attribute can be used to prevent the optimizer from obscuring the source location of certain calls. For example, it will prevent tail merging otherwise identical code sequences that raise an exception or terminate the program. Tail merging normally reduces the precision of source location information, making stack traces less useful for debugging. This attribute gives the user control over the tradeoff between code size and debug information precision.

nomerge attribute can also be used as function attribute to prevent all calls to the specified function from merging. It has no effect on indirect calls.

opencl_unroll_hint

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
opencl_unroll_hint

The opencl_unroll_hint attribute qualifier can be used to specify that a loop (for, while and do loops) can be unrolled. This attribute qualifier can be used to specify full unrolling or partial unrolling by a specified amount. This is a compiler hint and the compiler may ignore this directive. See OpenCL v2.0 s6.11.5 for details.

suppress

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
	gsl::suppress

The [[gsl::suppress]] attribute suppresses specific clang-tidy diagnostics for rules of the C++ Core Guidelines in a portable way. The attribute can be attached to declarations, statements, and at namespace scope.

[[gsl::suppress("Rh-public")]]
void f_() {
  int *p;
  [[gsl::suppress("type")]] {
    p = reinterpret_cast<int*>(7);
  }
}
namespace N {
  [[clang::suppress("type", "bounds")]];
  ...
}

sycl_special_class

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
sycl_special_class	clang::sycl_special_class	clang::sycl_special_class				Yes

SYCL defines some special classes (accessor, sampler, and stream) which require specific handling during the generation of the SPIR entry point. The __attribute__((sycl_special_class)) attribute is used in SYCL headers to indicate that a class or a struct needs a specific handling when it is passed from host to device. Special classes will have a mandatory __init method and an optional __finalize method (the __finalize method is used only with the stream type). Kernel parameters types are extract from the __init method parameters. The kernel function arguments list is derived from the arguments of the __init method. The arguments of the __init method are copied into the kernel function argument list and the __init and __finalize methods are called at the beginning and the end of the kernel, respectively. The __init and __finalize methods must be defined inside the special class. Please note that this is an attribute that is used as an internal implementation detail and not intended to be used by external users.

The syntax of the attribute is as follows:

class __attribute__((sycl_special_class)) accessor {};
class [[clang::sycl_special_class]] accessor {};

This is a code example that illustrates the use of the attribute:

class __attribute__((sycl_special_class)) SpecialType {
  int F1;
  int F2;
  void __init(int f1) {
    F1 = f1;
    F2 = f1;
  }
  void __finalize() {}
public:
  SpecialType() = default;
  int getF2() const { return F2; }
};

int main () {
  SpecialType T;
  cgh.single_task([=] {
    T.getF2();
  });
}

This would trigger the following kernel entry point in the AST:

void __sycl_kernel(int f1) {
  SpecialType T;
  T.__init(f1);
  ...
  T.__finalize()
}

Type Attributes

__ptr32

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
				__ptr32

The __ptr32 qualifier represents a native pointer on a 32-bit system. On a 64-bit system, a pointer with __ptr32 is extended to a 64-bit pointer. The __sptr and __uptr qualifiers can be used to specify whether the pointer is sign extended or zero extended. This qualifier is enabled under -fms-extensions.

__ptr64

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
				__ptr64

The __ptr64 qualifier represents a native pointer on a 64-bit system. On a 32-bit system, a __ptr64 pointer is truncated to a 32-bit pointer. This qualifier is enabled under -fms-extensions.

__sptr

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
				__sptr

The __sptr qualifier specifies that a 32-bit pointer should be sign extended when converted to a 64-bit pointer.

__uptr

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
				__uptr

The __uptr qualifier specifies that a 32-bit pointer should be zero extended when converted to a 64-bit pointer.

align_value

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
align_value						Yes

The align_value attribute can be added to the typedef of a pointer type or the declaration of a variable of pointer or reference type. It specifies that the pointer will point to, or the reference will bind to, only objects with at least the provided alignment. This alignment value must be some positive power of 2.

typedef double * aligned_double_ptr __attribute__((align_value(64)));
void foo(double & x  __attribute__((align_value(128)),
         aligned_double_ptr y) { ... }

If the pointer value does not have the specified alignment at runtime, the behavior of the program is undefined.

arm_sve_vector_bits

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
arm_sve_vector_bits

The arm_sve_vector_bits(N) attribute is defined by the Arm C Language Extensions (ACLE) for SVE. It is used to define fixed-length (VLST) variants of sizeless types (VLAT).

For example:

#include <arm_sve.h>

#if __ARM_FEATURE_SVE_BITS==512
typedef svint32_t fixed_svint32_t __attribute__((arm_sve_vector_bits(512)));
#endif

Creates a type fixed_svint32_t that is a fixed-length variant of svint32_t that contains exactly 512-bits. Unlike svint32_t, this type can be used in globals, structs, unions, and arrays, all of which are unsupported for sizeless types.

The attribute can be attached to a single SVE vector (such as svint32_t) or to the SVE predicate type svbool_t, this excludes tuple types such as svint32x4_t. The behavior of the attribute is undefined unless N==__ARM_FEATURE_SVE_BITS, the implementation defined feature macro that is enabled under the -msve-vector-bits flag.

For more information See Arm C Language Extensions for SVE for more information.

btf_type_tag

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
btf_type_tag	clang::btf_type_tag	clang::btf_type_tag

Clang supports the __attribute__((btf_type_tag("ARGUMENT"))) attribute for all targets. It only has effect when -g is specified on the command line and is currently silently ignored when not applied to a pointer type (note: this scenario may be diagnosed in the future).

The ARGUMENT string will be preserved in IR and emitted to DWARF for the types used in variable declarations, function declarations, or typedef declarations.

For BPF targets, the ARGUMENT string will also be emitted to .BTF ELF section.

clang_arm_mve_strict_polymorphism

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
__clang_arm_mve_strict_polymorphism	clang::__clang_arm_mve_strict_polymorphism	clang::__clang_arm_mve_strict_polymorphism

This attribute is used in the implementation of the ACLE intrinsics for the Arm MVE instruction set. It is used to define the vector types used by the MVE intrinsics.

Its effect is to modify the behavior of a vector type with respect to function overloading. If a candidate function for overload resolution has a parameter type with this attribute, then the selection of that candidate function will be disallowed if the actual argument can only be converted via a lax vector conversion. The aim is to prevent spurious ambiguity in ARM MVE polymorphic intrinsics.

void overloaded(uint16x8_t vector, uint16_t scalar);
void overloaded(int32x4_t vector, int32_t scalar);
uint16x8_t myVector;
uint16_t myScalar;

// myScalar is promoted to int32_t as a side effect of the addition,
// so if lax vector conversions are considered for myVector, then
// the two overloads are equally good (one argument conversion
// each). But if the vector has the __clang_arm_mve_strict_polymorphism
// attribute, only the uint16x8_t,uint16_t overload will match.
overloaded(myVector, myScalar + 1);

However, this attribute does not prohibit lax vector conversions in contexts other than overloading.

uint16x8_t function();

// This is still permitted with lax vector conversion enabled, even
// if the vector types have __clang_arm_mve_strict_polymorphism
int32x4_t result = function();

cmse_nonsecure_call

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
cmse_nonsecure_call

This attribute declares a non-secure function type. When compiling for secure state, a call to such a function would switch from secure to non-secure state. All non-secure function calls must happen only through a function pointer, and a non-secure function type should only be used as a base type of a pointer. See ARMv8-M Security Extensions: Requirements on Development Tools - Engineering Specification Documentation for more information.

device_builtin_surface_type

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
device_builtin_surface_type			__device_builtin_surface_type__			Yes

The device_builtin_surface_type attribute can be applied to a class template when declaring the surface reference. A surface reference variable could be accessed on the host side and, on the device side, might be translated into an internal surface object, which is established through surface bind and unbind runtime APIs.

device_builtin_texture_type

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
device_builtin_texture_type			__device_builtin_texture_type__			Yes

The device_builtin_texture_type attribute can be applied to a class template when declaring the texture reference. A texture reference variable could be accessed on the host side and, on the device side, might be translated into an internal texture object, which is established through texture bind and unbind runtime APIs.

noderef

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
noderef	clang::noderef	clang::noderef

The noderef attribute causes clang to diagnose dereferences of annotated pointer types. This is ideally used with pointers that point to special memory which cannot be read from or written to, but allowing for the pointer to be used in pointer arithmetic. The following are examples of valid expressions where dereferences are diagnosed:

int __attribute__((noderef)) *p;
int x = *p;  // warning

int __attribute__((noderef)) **p2;
x = **p2;  // warning

int * __attribute__((noderef)) *p3;
p = *p3;  // warning

struct S {
  int a;
};
struct S __attribute__((noderef)) *s;
x = s->a;    // warning
x = (*s).a;  // warning

Not all dereferences may diagnose a warning if the value directed by the pointer may not be accessed. The following are examples of valid expressions where may not be diagnosed:

int *q;
int __attribute__((noderef)) *p;
q = &*p;
q = *&p;

struct S {
  int a;
};
struct S __attribute__((noderef)) *s;
p = &s->a;
p = &(*s).a;

noderef is currently only supported for pointers and arrays and not usable for references or Objective-C object pointers.

objc_class_stub

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
objc_class_stub	clang::objc_class_stub	clang::objc_class_stub				Yes

This attribute specifies that the Objective-C class to which it applies is instantiated at runtime.

Unlike __attribute__((objc_runtime_visible)), a class having this attribute still has a “class stub” that is visible to the linker. This allows categories to be defined. Static message sends with the class as a receiver use a special access pattern to ensure the class is lazily instantiated from the class stub.

Classes annotated with this attribute cannot be subclassed and cannot have implementations defined for them. This attribute is intended for use in Swift-generated headers for classes defined in Swift.

Adding or removing this attribute to a class is an ABI-breaking change.

Type Safety Checking

Clang supports additional attributes to enable checking type safety properties that can’t be enforced by the C type system. To see warnings produced by these checks, ensure that -Wtype-safety is enabled. Use cases include:

• MPI library implementations, where these attributes enable checking that the buffer type matches the passed MPI_Datatype;

• for HDF5 library there is a similar use case to MPI;

• checking types of variadic functions’ arguments for functions like fcntl() and ioctl().

You can detect support for these attributes with __has_attribute(). For example:

#if defined(__has_attribute)
#  if __has_attribute(argument_with_type_tag) && \
      __has_attribute(pointer_with_type_tag) && \
      __has_attribute(type_tag_for_datatype)
#    define ATTR_MPI_PWT(buffer_idx, type_idx) __attribute__((pointer_with_type_tag(mpi,buffer_idx,type_idx)))
/* ... other macros ... */
#  endif
#endif

#if !defined(ATTR_MPI_PWT)
# define ATTR_MPI_PWT(buffer_idx, type_idx)
#endif

int MPI_Send(void *buf, int count, MPI_Datatype datatype /*, other args omitted */)
    ATTR_MPI_PWT(1,3);

argument_with_type_tag

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
argument_with_type_tag pointer_with_type_tag	clang::argument_with_type_tag clang::pointer_with_type_tag	clang::argument_with_type_tag clang::pointer_with_type_tag

Use __attribute__((argument_with_type_tag(arg_kind, arg_idx, type_tag_idx))) on a function declaration to specify that the function accepts a type tag that determines the type of some other argument.

This attribute is primarily useful for checking arguments of variadic functions (pointer_with_type_tag can be used in most non-variadic cases).

In the attribute prototype above:

• arg_kind is an identifier that should be used when annotating all applicable type tags.

• arg_idx provides the position of a function argument. The expected type of this function argument will be determined by the function argument specified by type_tag_idx. In the code example below, “3” means that the type of the function’s third argument will be determined by type_tag_idx.

• type_tag_idx provides the position of a function argument. This function argument will be a type tag. The type tag will determine the expected type of the argument specified by arg_idx. In the code example below, “2” means that the type tag associated with the function’s second argument should agree with the type of the argument specified by arg_idx.

For example:

int fcntl(int fd, int cmd, ...)
    __attribute__(( argument_with_type_tag(fcntl,3,2) ));
// The function's second argument will be a type tag; this type tag will
// determine the expected type of the function's third argument.

pointer_with_type_tag

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
argument_with_type_tag pointer_with_type_tag	clang::argument_with_type_tag clang::pointer_with_type_tag	clang::argument_with_type_tag clang::pointer_with_type_tag

Use __attribute__((pointer_with_type_tag(ptr_kind, ptr_idx, type_tag_idx))) on a function declaration to specify that the function accepts a type tag that determines the pointee type of some other pointer argument.

In the attribute prototype above:

• ptr_kind is an identifier that should be used when annotating all applicable type tags.

• ptr_idx provides the position of a function argument; this function argument will have a pointer type. The expected pointee type of this pointer type will be determined by the function argument specified by type_tag_idx. In the code example below, “1” means that the pointee type of the function’s first argument will be determined by type_tag_idx.

• type_tag_idx provides the position of a function argument; this function argument will be a type tag. The type tag will determine the expected pointee type of the pointer argument specified by ptr_idx. In the code example below, “3” means that the type tag associated with the function’s third argument should agree with the pointee type of the pointer argument specified by ptr_idx.

For example:

typedef int MPI_Datatype;
int MPI_Send(void *buf, int count, MPI_Datatype datatype /*, other args omitted */)
    __attribute__(( pointer_with_type_tag(mpi,1,3) ));
// The function's 3rd argument will be a type tag; this type tag will
// determine the expected pointee type of the function's 1st argument.

type_tag_for_datatype

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
type_tag_for_datatype	clang::type_tag_for_datatype	clang::type_tag_for_datatype

When declaring a variable, use __attribute__((type_tag_for_datatype(kind, type))) to create a type tag that is tied to the type argument given to the attribute.

In the attribute prototype above:

• kind is an identifier that should be used when annotating all applicable type tags.

• type indicates the name of the type.

Clang supports annotating type tags of two forms.

• Type tag that is a reference to a declared identifier. Use __attribute__((type_tag_for_datatype(kind, type))) when declaring that identifier:

  typedef int MPI_Datatype;
  extern struct mpi_datatype mpi_datatype_int
      __attribute__(( type_tag_for_datatype(mpi,int) ));
  #define MPI_INT ((MPI_Datatype) &mpi_datatype_int)
  // &mpi_datatype_int is a type tag. It is tied to type "int".
  

• Type tag that is an integral literal. Declare a static const variable with an initializer value and attach __attribute__((type_tag_for_datatype(kind, type))) on that declaration:

  typedef int MPI_Datatype;
  static const MPI_Datatype mpi_datatype_int
      __attribute__(( type_tag_for_datatype(mpi,int) )) = 42;
  #define MPI_INT ((MPI_Datatype) 42)
  // The number 42 is a type tag. It is tied to type "int".
  

The type_tag_for_datatype attribute also accepts an optional third argument that determines how the type of the function argument specified by either arg_idx or ptr_idx is compared against the type associated with the type tag. (Recall that for the argument_with_type_tag attribute, the type of the function argument specified by arg_idx is compared against the type associated with the type tag. Also recall that for the pointer_with_type_tag attribute, the pointee type of the function argument specified by ptr_idx is compared against the type associated with the type tag.) There are two supported values for this optional third argument:

• layout_compatible will cause types to be compared according to layout-compatibility rules (In C++11 [class.mem] p 17, 18, see the layout-compatibility rules for two standard-layout struct types and for two standard-layout union types). This is useful when creating a type tag associated with a struct or union type. For example:

  /* In mpi.h */
  typedef int MPI_Datatype;
  struct internal_mpi_double_int { double d; int i; };
  extern struct mpi_datatype mpi_datatype_double_int
      __attribute__(( type_tag_for_datatype(mpi,
                      struct internal_mpi_double_int, layout_compatible) ));
  
  #define MPI_DOUBLE_INT ((MPI_Datatype) &mpi_datatype_double_int)
  
  int MPI_Send(void *buf, int count, MPI_Datatype datatype, ...)
      __attribute__(( pointer_with_type_tag(mpi,1,3) ));
  
  /* In user code */
  struct my_pair { double a; int b; };
  struct my_pair *buffer;
  MPI_Send(buffer, 1, MPI_DOUBLE_INT /*, ... */); // no warning because the
                                                   // layout of my_pair is
                                                   // compatible with that of
                                                   // internal_mpi_double_int
  
  struct my_int_pair { int a; int b; }
  struct my_int_pair *buffer2;
  MPI_Send(buffer2, 1, MPI_DOUBLE_INT /*, ... */); // warning because the
                                                    // layout of my_int_pair
                                                    // does not match that of
                                                    // internal_mpi_double_int
  

• must_be_null specifies that the function argument specified by either arg_idx (for the argument_with_type_tag attribute) or ptr_idx (for the pointer_with_type_tag attribute) should be a null pointer constant. The second argument to the type_tag_for_datatype attribute is ignored. For example:

  /* In mpi.h */
  typedef int MPI_Datatype;
  extern struct mpi_datatype mpi_datatype_null
      __attribute__(( type_tag_for_datatype(mpi, void, must_be_null) ));
  
  #define MPI_DATATYPE_NULL ((MPI_Datatype) &mpi_datatype_null)
  int MPI_Send(void *buf, int count, MPI_Datatype datatype, ...)
      __attribute__(( pointer_with_type_tag(mpi,1,3) ));
  
  /* In user code */
  struct my_pair { double a; int b; };
  struct my_pair *buffer;
  MPI_Send(buffer, 1, MPI_DATATYPE_NULL /*, ... */); // warning: MPI_DATATYPE_NULL
                                                      // was specified but buffer
                                                      // is not a null pointer
  

Variable Attributes

always_destroy

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
always_destroy	clang::always_destroy					Yes

The always_destroy attribute specifies that a variable with static or thread storage duration should have its exit-time destructor run. This attribute is the default unless clang was invoked with -fno-c++-static-destructors.

called_once

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
called_once	clang::called_once	clang::called_once				Yes

The called_once attribute specifies that the annotated function or method parameter is invoked exactly once on all execution paths. It only applies to parameters with function-like types, i.e. function pointers or blocks. This concept is particularly useful for asynchronous programs.

Clang implements a check for called_once parameters, -Wcalled-once-parameter. It is on by default and finds the following violations:

• Parameter is not called at all.

• Parameter is called more than once.

• Parameter is not called on one of the execution paths.

In the latter case, Clang pinpoints the path where parameter is not invoked by showing the control-flow statement where the path diverges.

void fooWithCallback(void (^callback)(void) __attribute__((called_once))) {
  if (somePredicate()) {
    ...
    callback();
  } esle {
    callback(); // OK: callback is called on every path
  }
}

void barWithCallback(void (^callback)(void) __attribute__((called_once))) {
  if (somePredicate()) {
    ...
    callback(); // note: previous call is here
  }
  callback(); // warning: callback is called twice
}

void foobarWithCallback(void (^callback)(void) __attribute__((called_once))) {
  if (somePredicate()) {  // warning: callback is not called when condition is false
    ...
    callback();
  }
}

This attribute is useful for API developers who want to double-check if they implemented their method correctly.

dllexport

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
dllexport	gnu::dllexport	gnu::dllexport	dllexport			Yes

The __declspec(dllexport) attribute declares a variable, function, or Objective-C interface to be exported from the module. It is available under the -fdeclspec flag for compatibility with various compilers. The primary use is for COFF object files which explicitly specify what interfaces are available for external use. See the dllexport documentation on MSDN for more information.

dllimport

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
dllimport	gnu::dllimport	gnu::dllimport	dllimport			Yes

The __declspec(dllimport) attribute declares a variable, function, or Objective-C interface to be imported from an external module. It is available under the -fdeclspec flag for compatibility with various compilers. The primary use is for COFF object files which explicitly specify what interfaces are imported from external modules. See the dllimport documentation on MSDN for more information.

Note that a dllimport function may still be inlined, if its definition is available and it doesn’t reference any non-dllimport functions or global variables.

init_priority

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
init_priority	gnu::init_priority					Yes

In C++, the order in which global variables are initialized across translation units is unspecified, unlike the ordering within a single translation unit. The init_priority attribute allows you to specify a relative ordering for the initialization of objects declared at namespace scope in C++. The priority is given as an integer constant expression between 101 and 65535 (inclusive). Priorities outside of that range are reserved for use by the implementation. A lower value indicates a higher priority of initialization. Note that only the relative ordering of values is important. For example:

struct SomeType { SomeType(); };
__attribute__((init_priority(200))) SomeType Obj1;
__attribute__((init_priority(101))) SomeType Obj2;

Obj2 will be initialized before Obj1 despite the usual order of initialization being the opposite.

This attribute is only supported for C++ and Objective-C++ and is ignored in other language modes. Currently, this attribute is not implemented on z/OS.

init_seg

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
					init_seg

The attribute applied by pragma init_seg() controls the section into which global initialization function pointers are emitted. It is only available with -fms-extensions. Typically, this function pointer is emitted into .CRT$XCU on Windows. The user can change the order of initialization by using a different section name with the same .CRT$XC prefix and a suffix that sorts lexicographically before or after the standard .CRT$XCU sections. See the init_seg documentation on MSDN for more information.

leaf

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
leaf	gnu::leaf	gnu::leaf				Yes

The leaf attribute is used as a compiler hint to improve dataflow analysis in library functions. Functions marked with the leaf attribute are not allowed to jump back into the caller’s translation unit, whether through invoking a callback function, an external function call, use of longjmp, or other means. Therefore, they cannot use or modify any data that does not escape the caller function’s compilation unit.

For more information see gcc documentation <https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html>

loader_uninitialized

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
loader_uninitialized	clang::loader_uninitialized	clang::loader_uninitialized				Yes

The loader_uninitialized attribute can be placed on global variables to indicate that the variable does not need to be zero initialized by the loader. On most targets, zero-initialization does not incur any additional cost. For example, most general purpose operating systems deliberately ensure that all memory is properly initialized in order to avoid leaking privileged information from the kernel or other programs. However, some targets do not make this guarantee, and on these targets, avoiding an unnecessary zero-initialization can have a significant impact on load times and/or code size.

A declaration with this attribute is a non-tentative definition just as if it provided an initializer. Variables with this attribute are considered to be uninitialized in the same sense as a local variable, and the programs must write to them before reading from them. If the variable’s type is a C++ class type with a non-trivial default constructor, or an array thereof, this attribute only suppresses the static zero-initialization of the variable, not the dynamic initialization provided by executing the default constructor.

maybe_unused, unused

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
unused	maybe_unused gnu::unused	gnu::unused maybe_unused

When passing the -Wunused flag to Clang, entities that are unused by the program may be diagnosed. The [[maybe_unused]] (or __attribute__((unused))) attribute can be used to silence such diagnostics when the entity cannot be removed. For instance, a local variable may exist solely for use in an assert() statement, which makes the local variable unused when NDEBUG is defined.

The attribute may be applied to the declaration of a class, a typedef, a variable, a function or method, a function parameter, an enumeration, an enumerator, a non-static data member, or a label.

no_destroy

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
no_destroy	clang::no_destroy					Yes

The no_destroy attribute specifies that a variable with static or thread storage duration shouldn’t have its exit-time destructor run. Annotating every static and thread duration variable with this attribute is equivalent to invoking clang with -fno-c++-static-destructors.

If a variable is declared with this attribute, clang doesn’t access check or generate the type’s destructor. If you have a type that you only want to be annotated with no_destroy, you can therefore declare the destructor private:

struct only_no_destroy {
  only_no_destroy();
private:
  ~only_no_destroy();
};

[[clang::no_destroy]] only_no_destroy global; // fine!

Note that destructors are still required for subobjects of aggregates annotated with this attribute. This is because previously constructed subobjects need to be destroyed if an exception gets thrown before the initialization of the complete object is complete. For instance:

void f() {
  try {
    [[clang::no_destroy]]
    static only_no_destroy array[10]; // error, only_no_destroy has a private destructor.
  } catch (...) {
    // Handle the error
  }
}

Here, if the construction of array[9] fails with an exception, array[0..8] will be destroyed, so the element’s destructor needs to be accessible.

nodebug

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
nodebug	gnu::nodebug	gnu::nodebug				Yes

The nodebug attribute allows you to suppress debugging information for a function or method, for a variable that is not a parameter or a non-static data member, or for a typedef or using declaration.

noescape

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
noescape	clang::noescape	clang::noescape				Yes

noescape placed on a function parameter of a pointer type is used to inform the compiler that the pointer cannot escape: that is, no reference to the object the pointer points to that is derived from the parameter value will survive after the function returns. Users are responsible for making sure parameters annotated with noescape do not actually escape. Calling free() on such a parameter does not constitute an escape.

For example:

int *gp;

void nonescapingFunc(__attribute__((noescape)) int *p) {
  *p += 100; // OK.
}

void escapingFunc(__attribute__((noescape)) int *p) {
  gp = p; // Not OK.
}

Additionally, when the parameter is a block pointer <https://clang.llvm.org/docs/BlockLanguageSpec.html>, the same restriction applies to copies of the block. For example:

typedef void (^BlockTy)();
BlockTy g0, g1;

void nonescapingFunc(__attribute__((noescape)) BlockTy block) {
  block(); // OK.
}

void escapingFunc(__attribute__((noescape)) BlockTy block) {
  g0 = block; // Not OK.
  g1 = Block_copy(block); // Not OK either.
}

nosvm

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
nosvm						Yes

OpenCL 2.0 supports the optional __attribute__((nosvm)) qualifier for pointer variable. It informs the compiler that the pointer does not refer to a shared virtual memory region. See OpenCL v2.0 s6.7.2 for details.

Since it is not widely used and has been removed from OpenCL 2.1, it is ignored by Clang.

objc_externally_retained

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
objc_externally_retained	clang::objc_externally_retained	clang::objc_externally_retained				Yes

The objc_externally_retained attribute can be applied to strong local variables, functions, methods, or blocks to opt into externally-retained semantics.

When applied to the definition of a function, method, or block, every parameter of the function with implicit strong retainable object pointer type is considered externally-retained, and becomes const. By explicitly annotating a parameter with __strong, you can opt back into the default non-externally-retained behavior for that parameter. For instance, first_param is externally-retained below, but not second_param:

__attribute__((objc_externally_retained))
void f(NSArray *first_param, __strong NSArray *second_param) {
  // ...
}

Likewise, when applied to a strong local variable, that variable becomes const and is considered externally-retained.

When compiled without -fobjc-arc, this attribute is ignored.

pass_object_size, pass_dynamic_object_size

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
pass_object_size pass_dynamic_object_size	clang::pass_object_size clang::pass_dynamic_object_size	clang::pass_object_size clang::pass_dynamic_object_size				Yes

Note

The mangling of functions with parameters that are annotated with pass_object_size is subject to change. You can get around this by using __asm__("foo") to explicitly name your functions, thus preserving your ABI; also, non-overloadable C functions with pass_object_size are not mangled.

The pass_object_size(Type) attribute can be placed on function parameters to instruct clang to call __builtin_object_size(param, Type) at each callsite of said function, and implicitly pass the result of this call in as an invisible argument of type size_t directly after the parameter annotated with pass_object_size. Clang will also replace any calls to __builtin_object_size(param, Type) in the function by said implicit parameter.

Example usage:

int bzero1(char *const p __attribute__((pass_object_size(0))))
    __attribute__((noinline)) {
  int i = 0;
  for (/**/; i < (int)__builtin_object_size(p, 0); ++i) {
    p[i] = 0;
  }
  return i;
}

int main() {
  char chars[100];
  int n = bzero1(&chars[0]);
  assert(n == sizeof(chars));
  return 0;
}

If successfully evaluating __builtin_object_size(param, Type) at the callsite is not possible, then the “failed” value is passed in. So, using the definition of bzero1 from above, the following code would exit cleanly:

int main2(int argc, char *argv[]) {
  int n = bzero1(argv);
  assert(n == -1);
  return 0;
}

pass_object_size plays a part in overload resolution. If two overload candidates are otherwise equally good, then the overload with one or more parameters with pass_object_size is preferred. This implies that the choice between two identical overloads both with pass_object_size on one or more parameters will always be ambiguous; for this reason, having two such overloads is illegal. For example:

#define PS(N) __attribute__((pass_object_size(N)))
// OK
void Foo(char *a, char *b); // Overload A
// OK -- overload A has no parameters with pass_object_size.
void Foo(char *a PS(0), char *b PS(0)); // Overload B
// Error -- Same signature (sans pass_object_size) as overload B, and both
// overloads have one or more parameters with the pass_object_size attribute.
void Foo(void *a PS(0), void *b);

// OK
void Bar(void *a PS(0)); // Overload C
// OK
void Bar(char *c PS(1)); // Overload D

void main() {
  char known[10], *unknown;
  Foo(unknown, unknown); // Calls overload B
  Foo(known, unknown); // Calls overload B
  Foo(unknown, known); // Calls overload B
  Foo(known, known); // Calls overload B

  Bar(known); // Calls overload D
  Bar(unknown); // Calls overload D
}

Currently, pass_object_size is a bit restricted in terms of its usage:

• Only one use of pass_object_size is allowed per parameter.

• It is an error to take the address of a function with pass_object_size on any of its parameters. If you wish to do this, you can create an overload without pass_object_size on any parameters.

• It is an error to apply the pass_object_size attribute to parameters that are not pointers. Additionally, any parameter that pass_object_size is applied to must be marked const at its function’s definition.

Clang also supports the pass_dynamic_object_size attribute, which behaves identically to pass_object_size, but evaluates a call to __builtin_dynamic_object_size at the callee instead of __builtin_object_size. __builtin_dynamic_object_size provides some extra runtime checks when the object size can’t be determined at compile-time. You can read more about __builtin_dynamic_object_size here.

require_constant_initialization, constinit (C++20)

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
require_constant_initialization	clang::require_constant_initialization			constinit		Yes

This attribute specifies that the variable to which it is attached is intended to have a constant initializer according to the rules of [basic.start.static]. The variable is required to have static or thread storage duration. If the initialization of the variable is not a constant initializer an error will be produced. This attribute may only be used in C++; the constinit spelling is only accepted in C++20 onwards.

Note that in C++03 strict constant expression checking is not done. Instead the attribute reports if Clang can emit the variable as a constant, even if it’s not technically a ‘constant initializer’. This behavior is non-portable.

Static storage duration variables with constant initializers avoid hard-to-find bugs caused by the indeterminate order of dynamic initialization. They can also be safely used during dynamic initialization across translation units.

This attribute acts as a compile time assertion that the requirements for constant initialization have been met. Since these requirements change between dialects and have subtle pitfalls it’s important to fail fast instead of silently falling back on dynamic initialization.

The first use of the attribute on a variable must be part of, or precede, the initializing declaration of the variable. C++20 requires the constinit spelling of the attribute to be present on the initializing declaration if it is used anywhere. The other spellings can be specified on a forward declaration and omitted on a later initializing declaration.

// -std=c++14
#define SAFE_STATIC [[clang::require_constant_initialization]]
struct T {
  constexpr T(int) {}
  ~T(); // non-trivial
};
SAFE_STATIC T x = {42}; // Initialization OK. Doesn't check destructor.
SAFE_STATIC T y = 42; // error: variable does not have a constant initializer
// copy initialization is not a constant expression on a non-literal type.

section, __declspec(allocate)

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
section	gnu::section	gnu::section	allocate			Yes

The section attribute allows you to specify a specific section a global variable or function should be in after translation.

standalone_debug

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
standalone_debug	clang::standalone_debug					Yes

The standalone_debug attribute causes debug info to be emitted for a record type regardless of the debug info optimizations that are enabled with -fno-standalone-debug. This attribute only has an effect when debug info optimizations are enabled (e.g. with -fno-standalone-debug), and is C++-only.

swift_async_context

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_async_context	clang::swift_async_context	clang::swift_async_context				Yes

The swift_async_context attribute marks a parameter of a swiftasynccall function as having the special asynchronous context-parameter ABI treatment.

If the function is not swiftasynccall, this attribute only generates extended frame information.

A context parameter must have pointer or reference type.

swift_context

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_context	clang::swift_context	clang::swift_context				Yes

The swift_context attribute marks a parameter of a swiftcall or swiftasynccall function as having the special context-parameter ABI treatment.

This treatment generally passes the context value in a special register which is normally callee-preserved.

A swift_context parameter must either be the last parameter or must be followed by a swift_error_result parameter (which itself must always be the last parameter).

A context parameter must have pointer or reference type.

swift_error_result

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_error_result	clang::swift_error_result	clang::swift_error_result				Yes

The swift_error_result attribute marks a parameter of a swiftcall function as having the special error-result ABI treatment.

This treatment generally passes the underlying error value in and out of the function through a special register which is normally callee-preserved. This is modeled in C by pretending that the register is addressable memory:

• The caller appears to pass the address of a variable of pointer type. The current value of this variable is copied into the register before the call; if the call returns normally, the value is copied back into the variable.

• The callee appears to receive the address of a variable. This address is actually a hidden location in its own stack, initialized with the value of the register upon entry. When the function returns normally, the value in that hidden location is written back to the register.

A swift_error_result parameter must be the last parameter, and it must be preceded by a swift_context parameter.

A swift_error_result parameter must have type T** or T*& for some type T. Note that no qualifiers are permitted on the intermediate level.

It is undefined behavior if the caller does not pass a pointer or reference to a valid object.

The standard convention is that the error value itself (that is, the value stored in the apparent argument) will be null upon function entry, but this is not enforced by the ABI.

swift_indirect_result

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swift_indirect_result	clang::swift_indirect_result	clang::swift_indirect_result				Yes

The swift_indirect_result attribute marks a parameter of a swiftcall or swiftasynccall function as having the special indirect-result ABI treatment.

This treatment gives the parameter the target’s normal indirect-result ABI treatment, which may involve passing it differently from an ordinary parameter. However, only the first indirect result will receive this treatment. Furthermore, low-level lowering may decide that a direct result must be returned indirectly; if so, this will take priority over the swift_indirect_result parameters.

A swift_indirect_result parameter must either be the first parameter or follow another swift_indirect_result parameter.

A swift_indirect_result parameter must have type T* or T& for some object type T. If T is a complete type at the point of definition of a function, it is undefined behavior if the argument value does not point to storage of adequate size and alignment for a value of type T.

Making indirect results explicit in the signature allows C functions to directly construct objects into them without relying on language optimizations like C++’s named return value optimization (NRVO).

swiftasynccall

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swiftasynccall	clang::swiftasynccall	clang::swiftasynccall

The swiftasynccall attribute indicates that a function is compatible with the low-level conventions of Swift async functions, provided it declares the right formal arguments.

In most respects, this is similar to the swiftcall attribute, except for the following:

• A parameter may be marked swift_async_context, swift_context or swift_indirect_result (with the same restrictions on parameter ordering as swiftcall) but the parameter attribute swift_error_result is not permitted.

• A swiftasynccall function must have return type void.

• Within a swiftasynccall function, a call to a swiftasynccall function that is the immediate operand of a return statement is guaranteed to be performed as a tail call. This syntax is allowed even in C as an extension (a call to a void-returning function cannot be a return operand in standard C). If something in the calling function would semantically be performed after a guaranteed tail call, such as the non-trivial destruction of a local variable or temporary, then the program is ill-formed.

swiftcall

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
swiftcall	clang::swiftcall	clang::swiftcall

The swiftcall attribute indicates that a function should be called using the Swift calling convention for a function or function pointer.

The lowering for the Swift calling convention, as described by the Swift ABI documentation, occurs in multiple phases. The first, “high-level” phase breaks down the formal parameters and results into innately direct and indirect components, adds implicit parameters for the generic signature, and assigns the context and error ABI treatments to parameters where applicable. The second phase breaks down the direct parameters and results from the first phase and assigns them to registers or the stack. The swiftcall convention only handles this second phase of lowering; the C function type must accurately reflect the results of the first phase, as follows:

• Results classified as indirect by high-level lowering should be represented as parameters with the swift_indirect_result attribute.

• Results classified as direct by high-level lowering should be represented as follows:

  • First, remove any empty direct results.

  • If there are no direct results, the C result type should be void.

  • If there is one direct result, the C result type should be a type with the exact layout of that result type.

  • If there are a multiple direct results, the C result type should be a struct type with the exact layout of a tuple of those results.

• Parameters classified as indirect by high-level lowering should be represented as parameters of pointer type.

• Parameters classified as direct by high-level lowering should be omitted if they are empty types; otherwise, they should be represented as a parameter type with a layout exactly matching the layout of the Swift parameter type.

• The context parameter, if present, should be represented as a trailing parameter with the swift_context attribute.

• The error result parameter, if present, should be represented as a trailing parameter (always following a context parameter) with the swift_error_result attribute.

swiftcall does not support variadic arguments or unprototyped functions.

The parameter ABI treatment attributes are aspects of the function type. A function type which applies an ABI treatment attribute to a parameter is a different type from an otherwise-identical function type that does not. A single parameter may not have multiple ABI treatment attributes.

Support for this feature is target-dependent, although it should be supported on every target that Swift supports. Query for this support with __has_attribute(swiftcall). This implies support for the swift_context, swift_error_result, and swift_indirect_result attributes.

thread

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
			thread

The __declspec(thread) attribute declares a variable with thread local storage. It is available under the -fms-extensions flag for MSVC compatibility. See the documentation for __declspec(thread) on MSDN.

In Clang, __declspec(thread) is generally equivalent in functionality to the GNU __thread keyword. The variable must not have a destructor and must have a constant initializer, if any. The attribute only applies to variables declared with static storage duration, such as globals, class static data members, and static locals.

tls_model

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
tls_model	gnu::tls_model	gnu::tls_model				Yes

The tls_model attribute allows you to specify which thread-local storage model to use. It accepts the following strings:

• global-dynamic

• local-dynamic

• initial-exec

• local-exec

TLS models are mutually exclusive.

uninitialized

Supported Syntaxes

GNU	C++11	C2x	__declspec	Keyword	#pragma	#pragma clang attribute
uninitialized	clang::uninitialized					Yes

The command-line parameter -ftrivial-auto-var-init=* can be used to initialize trivial automatic stack variables. By default, trivial automatic stack variables are uninitialized. This attribute is used to override the command-line parameter, forcing variables to remain uninitialized. It has no semantic meaning in that using uninitialized values is undefined behavior, it rather documents the programmer’s intent.