-fbounds-safety: Enforcing bounds safety for C


-fbounds-safety is a C extension to enforce bounds safety to prevent out-of-bounds (OOB) memory accesses, which remain a major source of security vulnerabilities in C. -fbounds-safety aims to eliminate this class of bugs by turning OOB accesses into deterministic traps.

The -fbounds-safety extension offers bounds annotations that programmers can use to attach bounds to pointers. For example, programmers can add the __counted_by(N) annotation to parameter ptr, indicating that the pointer has N valid elements:

void foo(int *__counted_by(N) ptr, size_t N);

Using this bounds information, the compiler inserts bounds checks on every pointer dereference, ensuring that the program does not access memory outside the specified bounds. The compiler requires programmers to provide enough bounds information so that the accesses can be checked at either run time or compile time — and it rejects code if it cannot.

The most important contribution of -fbounds-safety is how it reduces the programmer’s annotation burden by reconciling bounds annotations at ABI boundaries with the use of implicit wide pointers (a.k.a. “fat” pointers) that carry bounds information on local variables without the need for annotations. We designed this model so that it preserves ABI compatibility with C while minimizing adoption effort.

The -fbounds-safety extension has been adopted on millions of lines of production C code and proven to work in a consumer operating system setting. The extension was designed to enable incremental adoption — a key requirement in real-world settings where modifying an entire project and its dependencies all at once is often not possible. It also addresses multiple of other practical challenges that have made existing approaches to safer C dialects difficult to adopt, offering these properties that make it widely adoptable in practice:

  • It is designed to preserve the Application Binary Interface (ABI).

  • It interoperates well with plain C code.

  • It can be adopted partially and incrementally while still providing safety benefits.

  • It is a conforming extension to C.

  • Consequently, source code that adopts the extension can continue to be compiled by toolchains that do not support the extension (CAVEAT: this still requires inclusion of a header file macro-defining bounds annotations to empty).

  • It has a relatively low adoption cost.

This document discusses the key designs of -fbounds-safety. The document is subject to be actively updated with a more detailed specification. The implementation plan can be found in Implementation plans for -fbounds-safety.

Programming Model


-fbounds-safety ensures that pointers are not used to access memory beyond their bounds by performing bounds checking. If a bounds check fails, the program will deterministically trap before out-of-bounds memory is accessed.

In our model, every pointer has an explicit or implicit bounds attribute that determines its bounds and ensures guaranteed bounds checking. Consider the example below where the __counted_by(count) annotation indicates that parameter p points to a buffer of integers containing count elements. An off-by-one error is present in the loop condition, leading to p[i] being out-of-bounds access during the loop’s final iteration. The compiler inserts a bounds check before p is dereferenced to ensure that the access remains within the specified bounds.

void fill_array_with_indices(int *__counted_by(count) p, unsigned count) {
   // off-by-one error (i < count)
   for (unsigned i = 0; i <= count; ++i) {
      // bounds check inserted:
      //   if (i >= count) trap();
      p[i] = i;

A bounds annotation defines an invariant for the pointer type, and the model ensures that this invariant remains true. In the example below, pointer p annotated with __counted_by(count) must always point to a memory buffer containing at least count elements of the pointee type. Changing the value of count, like in the example below, may violate this invariant and permit out-of-bounds access to the pointer. To avoid this, the compiler employs compile-time restrictions and emits run-time checks as necessary to ensure the new count value doesn’t exceed the actual length of the buffer. Section Maintaining correctness of bounds annotations provides more details about this programming model.

int g;

void foo(int *__counted_by(count) p, size_t count) {
   count++; // may violate the invariant of __counted_by
   count--; // may violate the invariant of __counted_by if count was 0.
   count = g; // may violate the invariant of __counted_by
              // depending on the value of `g`.

The requirement to annotate all pointers with explicit bounds information could present a significant adoption burden. To tackle this issue, the model incorporates the concept of a “wide pointer” (a.k.a. fat pointer) – a larger pointer that carries bounds information alongside the pointer value. Utilizing wide pointers can potentially reduce the adoption burden, as it contains bounds information internally and eliminates the need for explicit bounds annotations. However, wide pointers differ from standard C pointers in their data layout, which may result in incompatibilities with the application binary interface (ABI). Breaking the ABI complicates interoperability with external code that has not adopted the same programming model.

-fbounds-safety harmonizes the wide pointer and the bounds annotation approaches to reduce the adoption burden while maintaining the ABI. In this model, local variables of pointer type are implicitly treated as wide pointers, allowing them to carry bounds information without requiring explicit bounds annotations. Please note that this approach doesn’t apply to function parameters which are considered ABI-visible. As local variables are typically hidden from the ABI, this approach has a marginal impact on it. In addition, -fbounds-safety employs compile-time restrictions to prevent implicit wide pointers from silently breaking the ABI (see ABI implications of default bounds annotations). Pointers associated with any other variables, including function parameters, are treated as single object pointers (i.e., __single), ensuring that they always have the tightest bounds by default and offering a strong bounds safety guarantee.

By implementing default bounds annotations based on ABI visibility, a considerable portion of C code can operate without modifications within this programming model, reducing the adoption burden.

The rest of the section will discuss individual bounds annotations and the programming model in more detail.

Bounds annotations

Annotation for pointers to a single object

The C language allows pointer arithmetic on arbitrary pointers and this has been a source of many bounds safety issues. In practice, many pointers are merely pointing to a single object and incrementing or decrementing such a pointer immediately makes the pointer go out-of-bounds. To prevent this unsafety, -fbounds-safety provides the annotation __single that causes pointer arithmetic on annotated pointers to be a compile time error.

  • __single : indicates that the pointer is either pointing to a single object or null. Hence, pointers with __single do not permit pointer arithmetic nor being subscripted with a non-zero index. Dereferencing a __single pointer is allowed but it requires a null check. Upper and lower bounds checks are not required because the __single pointer should point to a valid object unless it’s null.

__single is the default annotation for ABI-visible pointers. This gives strong security guarantees in that these pointers cannot be incremented or decremented unless they have an explicit, overriding bounds annotation that can be used to verify the safety of the operation. The compiler issues an error when a __single pointer is utilized for pointer arithmetic or array access, as these operations would immediately cause the pointer to exceed its bounds. Consequently, this prompts programmers to provide sufficient bounds information to pointers. In the following example, the pointer on parameter p is single-by-default, and is employed for array access. As a result, the compiler generates an error suggesting to add __counted_by to the pointer.

void fill_array_with_indices(int *p, unsigned count) {
   for (unsigned i = 0; i < count; ++i) {
      p[i] = i; // error

External bounds annotations

“External” bounds annotations provide a way to express a relationship between a pointer variable and another variable (or expression) containing the bounds information of the pointer. In the following example, __counted_by(count) annotation expresses the bounds of parameter p using another parameter count. This model works naturally with many C interfaces and structs because the bounds of a pointer is often available adjacent to the pointer itself, e.g., at another parameter of the same function prototype, or at another field of the same struct declaration.

void fill_array_with_indices(int *__counted_by(count) p, size_t count) {
   // off-by-one error
   for (size_t i = 0; i <= count; ++i)
      p[i] = i;

External bounds annotations include __counted_by, __sized_by, and __ended_by. These annotations do not change the pointer representation, meaning they do not have ABI implications.

  • __counted_by(N) : The pointer points to memory that contains N elements of pointee type. N is an expression of integer type which can be a simple reference to declaration, a constant including calls to constant functions, or an arithmetic expression that does not have side effect. The __counted_by annotation cannot apply to pointers to incomplete types or types without size such as void *. Instead, __sized_by can be used to describe the byte count.

  • __sized_by(N) : The pointer points to memory that contains N bytes. Just like the argument of __counted_by, N is an expression of integer type which can be a constant, a simple reference to a declaration, or an arithmetic expression that does not have side effects. This is mainly used for pointers to incomplete types or types without size such as void *.

  • __ended_by(P) : The pointer has the upper bound of value P, which is one past the last element of the pointer. In other words, this annotation describes a range that starts with the pointer that has this annotation and ends with P which is the argument of the annotation. P itself may be annotated with __ended_by(Q). In this case, the end of the range extends to the pointer Q. This is used for “iterator” support in C where you’re iterating from one pointer value to another until a final pointer value is reached (and the final pointer value is not dereferencable).

Accessing a pointer outside the specified bounds causes a run-time trap or a compile-time error. Also, the model maintains correctness of bounds annotations when the pointer and/or the related value containing the bounds information are updated or passed as arguments. This is done by compile-time restrictions or run-time checks (see Maintaining correctness of bounds annotations for more detail). For instance, initializing buf with null while assigning non-zero value to count, as shown in the following example, would violate the __counted_by annotation because a null pointer does not point to any valid memory location. To avoid this, the compiler produces either a compile-time error or run-time trap.

void null_with_count_10(int *__counted_by(count) buf, unsigned count) {
   buf = 0;
   // This is not allowed as it creates a null pointer with non-zero length
   count = 10;

However, there are use cases where a pointer is either a null pointer or is pointing to memory of the specified size. To support this idiom, -fbounds-safety provides *_or_null variants, __counted_by_or_null(N), __sized_by_or_null(N), and __ended_by_or_null(P). Accessing a pointer with any of these bounds annotations will require an extra null check to avoid a null pointer dereference.

Internal bounds annotations

A wide pointer (sometimes known as a “fat” pointer) is a pointer that carries additional bounds information internally (as part of its data). The bounds require additional storage space making wide pointers larger than normal pointers, hence the name “wide pointer”. The memory layout of a wide pointer is equivalent to a struct with the pointer, upper bound, and (optionally) lower bound as its fields as shown below.

struct wide_pointer_datalayout {
   void* pointer; // Address used for dereferences and pointer arithmetic
   void* upper_bound; // Points one past the highest address that can be
                      // accessed
   void* lower_bound; // (Optional) Points to lowest address that can be
                      // accessed

Even with this representational change, wide pointers act syntactically as normal pointers to allow standard pointer operations, such as pointer dereference (*p), array subscript (p[i]), member access (p->), and pointer arithmetic, with some restrictions on bounds-unsafe uses.

-fbounds-safety has a set of “internal” bounds annotations to turn pointers into wide pointers. These are __bidi_indexable and __indexable. When a pointer has either of these annotations, the compiler changes the pointer to the corresponding wide pointer. This means these annotations will break the ABI and will not be compatible with plain C, and thus they should generally not be used in ABI surfaces.

  • __bidi_indexable : A pointer with this annotation becomes a wide pointer to carry the upper bound and the lower bound, the layout of which is equivalent to struct { T *ptr; T *upper_bound; T *lower_bound; };. As the name indicates, pointers with this annotation are “bidirectionally indexable”, meaning that they can be indexed with either a negative or a positive offset and the pointers can be incremented or decremented using pointer arithmetic. A __bidi_indexable pointer is allowed to hold an out-of-bounds pointer value. While creating an OOB pointer is undefined behavior in C, -fbounds-safety makes it well-defined behavior. That is, pointer arithmetic overflow with __bidi_indexable is defined as equivalent of two’s complement integer computation, and at the LLVM IR level this means getelementptr won’t get inbounds keyword. Accessing memory using the OOB pointer is prevented via a run-time bounds check.

  • __indexable : A pointer with this annotation becomes a wide pointer carrying the upper bound (but no explicit lower bound), the layout of which is equivalent to struct { T *ptr; T *upper_bound; };. Since __indexable pointers do not have a separate lower bound, the pointer value itself acts as the lower bound. An __indexable pointer can only be incremented or indexed in the positive direction. Indexing it in the negative direction will trigger a compile-time error. Otherwise, the compiler inserts a run-time check to ensure pointer arithmetic doesn’t make the pointer smaller than the original __indexable pointer (Note that __indexable doesn’t have a lower bound so the pointer value is effectively the lower bound). As pointer arithmetic overflow will make the pointer smaller than the original pointer, it will cause a trap at runtime. Similar to __bidi_indexable, an __indexable pointer is allowed to have a pointer value above the upper bound and creating such a pointer is well-defined behavior. Dereferencing such a pointer, however, will cause a run-time trap.

  • __bidi_indexable offers the best flexibility out of all the pointer annotations in this model, as __bidi_indexable pointers can be used for any pointer operation. However, this comes with the largest code size and memory cost out of the available pointer annotations in this model. In some cases, use of the __bidi_indexable annotation may be duplicating bounds information that exists elsewhere in the program. In such cases, using external bounds annotations may be a better choice.

__bidi_indexable is the default annotation for non-ABI visible pointers, such as local pointer variables — that is, if the programmer does not specify another bounds annotation, a local pointer variable is implicitly __bidi_indexable. Since __bidi_indexable pointers automatically carry bounds information and have no restrictions on kinds of pointer operations that can be used with these pointers, most code inside a function works as is without modification. In the example below, int *buf doesn’t require manual annotation as it’s implicitly int *__bidi_indexable buf, carrying the bounds information passed from the return value of malloc, which is necessary to insert bounds checking for buf[i].

void *__sized_by(size) malloc(size_t size);

int *__counted_by(n) get_array_with_0_to_n_1(size_t n) {
   int *buf = malloc(sizeof(int) * n);
   for (size_t i = 0; i < n; ++i)
      buf[i] = i;
   return buf;

Annotations for sentinel-delimited arrays

A C string is an array of characters. The null terminator — the first null character (’0’) element in the array — marks the end of the string. -fbounds-safety provides __null_terminated to annotate C strings and the generalized form __terminated_by(T) to annotate pointers and arrays with an end marked by a sentinel value. The model prevents dereferencing a __terminated_by pointer beyond its end. Calculating the location of the end (i.e., the address of the sentinel value), requires reading the entire array in memory and would have some performance costs. To avoid an unintended performance hit, the model puts some restrictions on how these pointers can be used. __terminated_by pointers cannot be indexed and can only be incremented one element at a time. To allow these operations, the pointers must be explicitly converted to __indexable pointers using the intrinsic function __unsafe_terminated_by_to_indexable(P, T) (or __unsafe_null_terminated_to_indexable(P)) which converts the __terminated_by pointer P to an __indexable pointer.

  • __null_terminated : The pointer or array is terminated by NULL or 0. Modifying the terminator or incrementing the pointer beyond it is prevented at run time.

  • __terminated_by(T) : The pointer or array is terminated by T which is a constant expression. Accessing or incrementing the pointer beyond the terminator is not allowed. This is a generalization of __null_terminated which is defined as __terminated_by(0).

Annotation for interoperating with bounds-unsafe code

A pointer with the __unsafe_indexable annotation behaves the same as a plain C pointer. That is, the pointer does not have any bounds information and pointer operations are not checked.

__unsafe_indexable can be used to mark pointers from system headers or pointers from code that has not adopted -fbounds safety. This enables interoperation between code using -fbounds-safety and code that does not.

Default pointer types

ABI visibility and default annotations

Requiring -fbounds-safety adopters to add bounds annotations to all pointers in the codebase would be a significant adoption burden. To avoid this and to secure all pointers by default, -fbounds-safety applies default bounds annotations to pointer types. Default annotations apply to pointer types of declarations

-fbounds-safety applies default bounds annotations to pointer types used in declarations. The default annotations are determined by the ABI visibility of the pointer. A pointer type is ABI-visible if changing its size or representation affects the ABI. For instance, changing the size of a type used in a function parameter will affect the ABI and thus pointers used in function parameters are ABI-visible pointers. On the other hand, changing the types of local variables won’t have such ABI implications. Hence, -fbounds-safety considers the outermost pointer types of local variables as non-ABI visible. The rest of the pointers such as nested pointer types, pointer types of global variables, struct fields, and function prototypes are considered ABI-visible.

All ABI-visible pointers are treated as __single by default unless annotated otherwise. This default both preserves ABI and makes these pointers safe by default. This behavior can be controlled with macros, i.e., __ptrcheck_abi_assume_*ATTR*(), to set the default annotation for ABI-visible pointers to be either __single, __bidi_indexable, __indexable, or __unsafe_indexable. For instance, __ptrcheck_abi_assume_unsafe_indexable() will make all ABI-visible pointers be __unsafe_indexable. Non-ABI visible pointers — the outermost pointer types of local variables — are __bidi_indexable by default, so that these pointers have the bounds information necessary to perform bounds checks without the need for a manual annotation. All const char pointers or any typedefs equivalent to const char pointers are __null_terminated by default. This means that char8_t is unsigned char so const char8_t * won’t be __null_terminated by default. Similarly, const wchar_t * won’t be __null_terminated by default unless the platform defines it as typedef char wchar_t. Please note, however, that the programmers can still explicitly use __null_terminated in any other pointers, e.g., char8_t *__null_terminated, wchar_t *__null_terminated, int *__null_terminated, etc. if they should be treated as __null_terminated. The same applies to other annotations. In system headers, the default pointer attribute for ABI-visible pointers is set to __unsafe_indexable by default.

The __ptrcheck_abi_assume_*ATTR*() macros are defined as pragmas in the toolchain header (See Portability with toolchains that do not support the extension for more details about the toolchain header):

#define __ptrcheck_abi_assume_single()

_Pragma(“clang abi_ptr_attr set(single)”)

#define __ptrcheck_abi_assume_indexable()

_Pragma(“clang abi_ptr_attr set(indexable)”)

#define __ptrcheck_abi_assume_bidi_indexable()

_Pragma(“clang abi_ptr_attr set(bidi_indexable)”)

#define __ptrcheck_abi_assume_unsafe_indexable()

_Pragma(“clang abi_ptr_attr set(unsafe_indexable)”)

ABI implications of default bounds annotations

Although simply modifying types of a local variable doesn’t normally impact the ABI, taking the address of such a modified type could create a pointer type that has an ABI mismatch. Looking at the following example, int *local is implicitly int *__bidi_indexable and thus the type of &local is a pointer to int *__bidi_indexable. On the other hand, in void foo(int **), the parameter type is a pointer to int *__single (i.e., void foo(int *__single *__single)) (or a pointer to int *__unsafe_indexable if it’s from a system header). The compiler reports an error for casts between pointers whose elements have incompatible pointer attributes. This way, -fbounds-safety prevents pointers that are implicitly __bidi_indexable from silently escaping thereby breaking the ABI.

void foo(int **);

void bar(void) {
   int *local = 0;
   // error: passing 'int *__bidi_indexable*__bidi_indexable' to parameter of
   // incompatible nested pointer type 'int *__single*__single'

A local variable may still be exposed to the ABI if typeof() takes the type of local variable to define an interface as shown in the following example.

// bar.c
void bar(int *) { ... }

// foo.c
void foo(void) {
   int *p; // implicitly `int *__bidi_indexable p`
   extern void bar(typeof(p)); // creates an interface of type
                               // `void bar(int *__bidi_indexable)`

Doing this may break the ABI if the parameter is not __bidi_indexable at the definition of function bar() which is likely the case because parameters are __single by default without an explicit annotation.

In order to avoid an implicitly wide pointer from silently breaking the ABI, the compiler reports a warning when typeof() is used on an implicit wide pointer at any ABI visible context (e.g., function prototype, struct definition, etc.).

Default pointer types in typeof()

When typeof() takes an expression, it respects the bounds annotation on the expression type, including the bounds annotation is implcit. For example, the global variable g in the following code is implicitly __single so typeof(g) gets char *__single. The similar is true for the parameter p, so typeof(p) returns void *__single. The local variable l is implicitly __bidi_indexable, so typeof(l) becomes int *__bidi_indexable.

char *g; // typeof(g) == char *__single

void foo(void *p) {
   // typeof(p) == void *__single

   int *l; // typeof(l) == int *__bidi_indexable

When the type of expression has an “external” bounds annotation, e.g., __sized_by, __counted_by, etc., the compiler may report an error on typeof if the annotation creates a dependency with another declaration or variable. For example, the compiler reports an error on typeof(p1) shown in the following code because allowing it can potentially create another type dependent on the parameter size in a different context (Please note that an external bounds annotation on a parameter may only refer to another parameter of the same function). On the other hand, typeof(p2) works resulting in int *__counted_by(10), since it doesn’t depend on any other declaration.

void foo(int *__counted_by(size) p1, size_t size) {
   // typeof(p1) == int *__counted_by(size)
   // -> a compiler error as it tries to create another type
   // dependent on `size`.

   int *__counted_by(10) p2; // typeof(p2) == int *__counted_by(10)
                             // -> no error


When typeof() takes a type name, the compiler doesn’t apply an implicit bounds annotation on the named pointer types. For example, typeof(int*) returns int * without any bounds annotation. A bounds annotation may be added after the fact depending on the context. In the following example, typeof(int *) returns int * so it’s equivalent as the local variable is declared as int *l, so it eventually becomes implicitly __bidi_indexable.

void foo(void) {
   typeof(int *) l; // `int *__bidi_indexable` (same as `int *l`)

The programmers can still explicitly add a bounds annotation on the types named inside typeof, e.g., typeof(int *__bidi_indexable), which evaluates to int *__bidi_indexable.

Default pointer types in sizeof()

When sizeof() takes a type name, the compiler doesn’t apply an implicit bounds annotation on the named pointer types. This means if a bounds annotation is not specified, the evaluated pointer type is treated identically to a plain C pointer type. Therefore, sizeof(int*) remains the same with or without -fbounds-safety. That said, programmers can explicitly add attribute to the types, e.g., sizeof(int *__bidi_indexable), in which case the sizeof evaluates to the size of type int *__bidi_indexable (the value equivalent to 3 * sizeof(int*)).

When sizeof() takes an expression, i.e., sizeof(expr, it behaves as sizeof(typeof(expr)), except that sizeof(expr) does not report an error with expr that has a type with an external bounds annotation dependent on another declaration, whereas typeof() on the same expression would be an error as described in Default pointer types in typeof(). The following example describes this behavior.

void foo(int *__counted_by(size) p, size_t size) {
   // sizeof(p) == sizeof(int *__counted_by(size)) == sizeof(int *)
   // typeof(p): error

Default pointer types in alignof()

alignof() only takes a type name as the argument and it doesn’t take an expression. Similar to sizeof() and typeof, the compiler doesn’t apply an implicit bounds annotation on the pointer types named inside alignof(). Therefore, alignof(T *) remains the same with or without -fbounds-safety, evaluating into the alignment of the raw pointer T *. The programmers can explicitly add a bounds annotation to the types, e.g., alignof(int *__bidi_indexable), which returns the alignment of int *__bidi_indexable. A bounds annotation including an internal bounds annotation (i.e., __indexable and __bidi_indexable) doesn’t affect the alignment of the original pointer. Therefore, alignof(int *__bidi_indexable) is equal to alignof(int *).

Default pointer types used in C-style casts

A pointer type used in a C-style cast (e.g., (int *)src) inherits the same pointer attribute in the type of src. For instance, if the type of src is T *__single (with T being an arbitrary C type), (int *)src will be int *__single. The reasoning behind this behavior is so that a C-style cast doesn’t introduce any unexpected side effects caused by an implicit cast of bounds attribute.

Pointer casts can have explicit bounds annotations. For instance, (int *__bidi_indexable)src casts to int *__bidi_indexable as long as src has a bounds annotation that can implicitly convert to __bidi_indexable. If src has type int *__single, it can implicitly convert to int *__bidi_indexable which then will have the upper bound pointing to one past the first element. However, if src has type int *__unsafe_indexable, the explicit cast (int *__bidi_indexable)src will cause an error because __unsafe_indexable cannot cast to __bidi_indexable as __unsafe_indexable doesn’t have bounds information. Cast rules describes in more detail what kinds of casts are allowed between pointers with different bounds annotations.

Default pointer types in typedef

Pointer types in typedefs do not have implicit default bounds annotations. Instead, the bounds annotation is determined when the typedef is used. The following example shows that no pointer annotation is specified in the typedef pint_t while each instance of typedef’ed pointer gets its bounds annotation based on the context in which the type is used.

typedef int * pint_t; // int *

pint_t glob; // int *__single glob;

void foo(void) {
   pint_t local; // int *__bidi_indexable local;

Pointer types in a typedef can still have explicit annotations, e.g., typedef int *__single, in which case the bounds annotation __single will apply to every use of the typedef.

Array to pointer promotion to secure arrays (including VLAs)

Arrays on function prototypes

In C, arrays on function prototypes are promoted (or “decayed”) to a pointer to its first element (e.g., &arr[0]). In -fbounds-safety, arrays are also decayed to pointers, but with the addition of an implicit bounds annotation, which includes variable-length arrays (VLAs). As shown in the following example, arrays on function prototypes are decalyed to corresponding __counted_by pointers.

// Function prototype: void foo(int n, int *__counted_by(n) arr);
void foo(int n, int arr[n]);

// Function prototype: void bar(int *__counted_by(10) arr);
void bar(int arr[10]);

This means the array parameters are treated as __counted_by pointers within the function and callers of the function also see them as the corresponding __counted_by pointers.

Incomplete arrays on function prototypes will cause a compiler error unless it has __counted_by annotation in its bracket.

void f1(int n, int arr[]); // error

void f3(int n, int arr[__counted_by(n)]); // ok

void f2(int n, int arr[n]); // ok, decays to int *__counted_by(n)

void f4(int n, int *__counted_by(n) arr); // ok

void f5(int n, int *arr); // ok, but decays to int *__single,
                          // and cannot be used for pointer arithmetic

Array references

In C, similar to arrays on the function prototypes, a reference to array is automatically promoted (or “decayed”) to a pointer to its first element (e.g., &arr[0]).

In -fbounds-safety, array references are promoted to __bidi_indexable pointers which contain the upper and lower bounds of the array, with the equivalent of &arr[0] serving as the lower bound and &arr[array_size] (or one past the last element) serving as the upper bound. This applies to all types of arrays including constant-length arrays, variable-length arrays (VLAs), and flexible array members annotated with __counted_by.

In the following example, reference to vla promotes to int *__bidi_indexable, with &vla[n] as the upper bound and &vla[0] as the lower bound. Then, it’s copied to int *p, which is implicitly int *__bidi_indexable p. Please note that value of n used to create the upper bound is 10, not 100, in this case because 10 is the actual length of vla, the value of n at the time when the array is being allocated.

void foo(void) {
   int n = 10;
   int vla[n];
   n = 100;
   int *p = vla; // { .ptr: &vla[0], .upper: &vla[10], .lower: &vla[0] }
                 // it's `&vla[10]` because the value of `n` was 10 at the
                 // time when the array is actually allocated.
   // ...

By promoting array references to __bidi_indexable, all array accesses are bounds checked in -fbounds-safety, just as __bidi_indexable pointers are.

Maintaining correctness of bounds annotations

-fbounds-safety maintains correctness of bounds annotations by performing additional checks when a pointer object and/or its related value containing the bounds information is updated.

For example, __single expresses an invariant that the pointer must either point to a single valid object or be a null pointer. To maintain this invariant, the compiler inserts checks when initializing a __single pointer, as shown in the following example:

void foo(void *__sized_by(size) vp, size_t size) {
   // Inserted check:
   // if ((int*)upper_bound(vp) - (int*)vp < sizeof(int) && !!vp) trap();
   int *__single ip = (int *)vp;

Additionally, an explicit bounds annotation such as int *__counted_by(count) buf defines a relationship between two variables, buf and count: namely, that buf has count number of elements available. This relationship must hold even after any of these related variables are updated. To this end, the model requires that assignments to buf and count must be side by side, with no side effects between them. This prevents buf and count from temporarily falling out of sync due to updates happening at a distance.

The example below shows a function alloc_buf that initializes a struct that members that use the __counted_by annotation. The compiler allows these assignments because sbuf->buf and sbuf->count are updated side by side without any side effects in between the assignments.

Furthermore, the compiler inserts additional run-time checks to ensure the new buf has at least as many elements as the new count indicates as shown in the transformed pseudo code of function alloc_buf() in the example below.

typedef struct {
   int *__counted_by(count) buf;
   size_t count;
} sized_buf_t;

void alloc_buf(sized_buf_t *sbuf, sized_t nelems) {
   sbuf->buf = (int *)malloc(sizeof(int) * nelems);
   sbuf->count = nelems;

// Transformed pseudo code:
void alloc_buf(sized_buf_t *sbuf, sized_t nelems) {
   // Materialize RHS values:
   int *tmp_ptr = (int *)malloc(sizeof(int) * nelems);
   int tmp_count = nelems;
   // Inserted check:
   //   - checks to ensure that `lower <= tmp_ptr <= upper`
   //   - if (upper(tmp_ptr) - tmp_ptr < tmp_count) trap();
   sbuf->buf = tmp_ptr;
   sbuf->count = tmp_count;

Whether the compiler can optimize such run-time checks depends on how the upper bound of the pointer is derived. If the source pointer has __sized_by, __counted_by, or a variant of such, the compiler assumes that the upper bound calculation doesn’t overflow, e.g., ptr + size (where the type of ptr is void *__sized_by(size)), because when the __sized_by pointer is initialized, -fbounds-safety inserts run-time checks to ensure that ptr + size doesn’t overflow and that size >= 0.

Assuming the upper bound calculation doesn’t overflow, the compiler can simplify the trap condition upper(tmp_ptr) - tmp_ptr < tmp_count to size < tmp_count so if both size and tmp_count values are known at compile time such that 0 <= tmp_count <= size, the optimizer can remove the check.

ptr + size may still overflow if the __sized_by pointer is created from code that doesn’t enable -fbounds-safety, which is undefined behavior.

In the previous code example with the transformed alloc_buf(), the upper bound of tmp_ptr is derived from void *__sized_by_or_null(size), which is the return type of malloc(). Hence, the pointer arithmetic doesn’t overflow or tmp_ptr is null. Therefore, if nelems was given as a compile-time constant, the compiler could remove the checks.

Cast rules

-fbounds-safety does not enforce overall type safety and bounds invariants can still be violated by incorrect casts in some cases. That said, -fbounds-safety prevents type conversions that change bounds attributes in a way to violate the bounds invariant of the destination’s pointer annotation. Type conversions that change bounds attributes may be allowed if it does not violate the invariant of the destination or that can be verified at run time. Here are some of the important cast rules.

Two pointers that have different bounds annotations on their nested pointer types are incompatible and cannot implicitly cast to each other. For example, T *__single *__single cannot be converted to T *__bidi_indexable *__single. Such a conversion between incompatible nested bounds annotations can be allowed using an explicit cast (e.g., C-style cast). Hereafter, the rules only apply to the top pointer types. __unsafe_indexable cannot be converted to any other safe pointer types (__single, __bidi_indexable, __counted_by, etc) using a cast. The extension provides builtins to force this conversion, __unsafe_forge_bidi_indexable(type, pointer, char_count) to convert pointer to a __bidi_indexable pointer of type with char_count bytes available and __unsafe_forge_single(type, pointer) to convert pointer to a single pointer of type type. The following examples show the usage of these functions. Function example_forge_bidi() gets an external buffer from an unsafe library by calling get_buf() which returns void *__unsafe_indexable. Under the type rules, this cannot be directly assigned to void *buf (implicitly void *__bidi_indexable). Thus, __unsafe_forge_bidi_indexable is used to manually create a __bidi_indexable from the unsafe buffer.

// unsafe_library.h
void *__unsafe_indexable get_buf(void);
size_t get_buf_size(void);

// my_source1.c (enables -fbounds-safety)
#include "unsafe_library.h"
void example_forge_bidi(void) {
   void *buf =
     __unsafe_forge_bidi_indexable(void *, get_buf(), get_buf_size());
   // ...

// my_source2.c (enables -fbounds-safety)
#include <stdio.h>
void example_forge_single(void) {
   FILE *fp = __unsafe_forge_single(FILE *, fopen("mypath", "rb"));
   // ...
  • Function example_forge_single takes a file handle by calling fopen defined in system header stdio.h. Assuming stdio.h did not adopt -fbounds-safety, the return type of fopen would implicitly be FILE *__unsafe_indexable and thus it cannot be directly assigned to FILE *fp in the bounds-safe source. To allow this operation, __unsafe_forge_single is used to create a __single from the return value of fopen.

  • Similar to __unsafe_indexable, any non-pointer type (including int, intptr_t, uintptr_t, etc.) cannot be converted to any safe pointer type because these don’t have bounds information. __unsafe_forge_single or __unsafe_forge_bidi_indexable must be used to force the conversion.

  • Any safe pointer types can cast to __unsafe_indexable because it doesn’t have any invariant to maintain.

  • __single casts to __bidi_indexable if the pointee type has a known size. After the conversion, the resulting __bidi_indexable has the size of a single object of the pointee type of __single. __single cannot cast to __bidi_indexable if the pointee type is incomplete or sizeless. For example, void *__single cannot convert to void *__bidi_indexable because void is an incomplete type and thus the compiler cannot correctly determine the upper bound of a single void pointer.

  • Similarly, __single can cast to __indexable if the pointee type has a known size. The resulting __indexable has the size of a single object of the pointee type.

  • __single casts to __counted_by(E) only if E is 0 or 1.

  • __single can cast to __single including when they have different pointee types as long as it is allowed in the underlying C standard. -fbounds-safety doesn’t guarantee type safety.

  • __bidi_indexable and __indexable can cast to __single. The compiler may insert run-time checks to ensure the pointer has at least a single element or is a null pointer.

  • __bidi_indexable casts to __indexable if the pointer does not have an underflow. The compiler may insert run-time checks to ensure the pointer is not below the lower bound.

  • __indexable casts to __bidi_indexable. The resulting __bidi_indexable gets the lower bound same as the pointer value.

  • A type conversion may involve both a bitcast and a bounds annotation cast. For example, casting from int *__bidi_indexable to char *__single involve a bitcast (int * to char *) and a bounds annotation cast (__bidi_indexable to __single). In this case, the compiler performs the bitcast and then converts the bounds annotation. This means, int *__bidi_indexable will be converted to char *__bidi_indexable and then to char *__single.

  • __terminated_by(T) cannot cast to any safe pointer type without the same __terminated_by(T) attribute. To perform the cast, programmers can use an intrinsic function such as __unsafe_terminated_by_to_indexable(P) to force the conversion.

  • __terminated_by(T) can cast to __unsafe_indexable.

  • Any type without __terminated_by(T) cannot cast to __terminated_by(T) without explicitly using an intrinsic function to allow it.

    • __unsafe_terminated_by_from_indexable(T, PTR [, PTR_TO_TERM]) casts any safe pointer PTR to a __terminated_by(T) pointer. PTR_TO_TERM is an optional argument where the programmer can provide the exact location of the terminator. With this argument, the function can skip reading the entire array in order to locate the end of the pointer (or the upper bound). Providing an incorrect PTR_TO_TERM causes a run-time trap.

    • __unsafe_forge_terminated_by(T, P, E) creates T __terminated_by(E) pointer given any pointer P. Tmust be a pointer type.

Portability with toolchains that do not support the extension

The language model is designed so that it doesn’t alter the semantics of the original C program, other than introducing deterministic traps where otherwise the behavior is undefined and/or unsafe. Clang provides a toolchain header (ptrcheck.h) that macro-defines the annotations as type attributes when -fbounds-safety is enabled and defines them to empty when the extension is disabled. Thus, the code adopting -fbounds-safety can compile with toolchains that do not support this extension, by including the header or adding macros to define the annotations to empty. For example, the toolchain not supporting this extension may not have a header defining __counted_by, so the code using __counted_by must define it as nothing or include a header that has the define.

#if defined(__has_feature) && __has_feature(bounds_safety)
#define __counted_by(T) __attribute__((__counted_by__(T)))
// ... other bounds annotations
#else #define __counted_by(T) // defined as nothing
// ... other bounds annotations

// expands to `void foo(int * ptr, size_t count);`
// when extension is not enabled or not available
void foo(int *__counted_by(count) ptr, size_t count);

Other potential applications of bounds annotations

The bounds annotations provided by the -fbounds-safety programming model have potential use cases beyond the language extension itself. For example, static and dynamic analysis tools could use the bounds information to improve diagnostics for out-of-bounds accesses, even if -fbounds-safety is not used. The bounds annotations could be used to improve C interoperability with bounds-safe languages, providing a better mapping to bounds-safe types in the safe language interface. The bounds annotations can also serve as documentation specifying the relationship between declarations.


-fbounds-safety aims to bring the bounds safety guarantee to the C language, and it does not guarantee other types of memory safety properties. Consequently, it may not prevent some of the secondary bounds safety violations caused by other types of safety violations such as type confusion. For instance, -fbounds-safety does not perform type-safety checks on conversions between __single` pointers of different pointee types (e.g., char *__singlevoid *__singleint *__single) beyond what the foundation languages (C/C++) already offer.

-fbounds-safety heavily relies on run-time checks to keep the bounds safety and the soundness of the type system. This may incur significant code size overhead in unoptimized builds and leaving some of the adoption mistakes to be caught only at run time. This is not a fundamental limitation, however, because incrementally adding necessary static analysis will allow us to catch issues early on and remove unnecessary bounds checks in unoptimized builds.