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Clang Compiler User’s Manual

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Clang Compiler User’s Manual


The Clang Compiler is an open-source compiler for the C family of programming languages, aiming to be the best in class implementation of these languages. Clang builds on the LLVM optimizer and code generator, allowing it to provide high-quality optimization and code generation support for many targets. For more general information, please see the Clang Web Site or the LLVM Web Site.

This document describes important notes about using Clang as a compiler for an end-user, documenting the supported features, command line options, etc. If you are interested in using Clang to build a tool that processes code, please see “Clang” CFE Internals Manual. If you are interested in the Clang Static Analyzer, please see its web page.

Clang is designed to support the C family of programming languages, which includes C, Objective-C, C++, and Objective-C++ as well as many dialects of those. For language-specific information, please see the corresponding language specific section:

In addition to these base languages and their dialects, Clang supports a broad variety of language extensions, which are documented in the corresponding language section. These extensions are provided to be compatible with the GCC, Microsoft, and other popular compilers as well as to improve functionality through Clang-specific features. The Clang driver and language features are intentionally designed to be as compatible with the GNU GCC compiler as reasonably possible, easing migration from GCC to Clang. In most cases, code “just works”. Clang also provides an alternative driver, clang-cl, that is designed to be compatible with the Visual C++ compiler, cl.exe.

In addition to language specific features, Clang has a variety of features that depend on what CPU architecture or operating system is being compiled for. Please see the Target-Specific Features and Limitations section for more details.

The rest of the introduction introduces some basic compiler terminology that is used throughout this manual and contains a basic introduction to using Clang as a command line compiler.


Front end, parser, backend, preprocessor, undefined behavior, diagnostic, optimizer

Basic Usage

Intro to how to use a C compiler for newbies.

compile + link compile then link debug info enabling optimizations picking a language to use, defaults to C11 by default. Autosenses based on extension. using a makefile

Command Line Options

This section is generally an index into other sections. It does not go into depth on the ones that are covered by other sections. However, the first part introduces the language selection and other high level options like -c, -g, etc.

Options to Control Error and Warning Messages


Turn warnings into errors.


Turn warning “foo” into an error.

Turn warning “foo” into an warning even if -Werror is specified.


Enable warning “foo”.


Disable warning “foo”.


Disable all diagnostics.


Enable all diagnostics.


Warn on language extensions.


Error on language extensions.


Enable warnings from system headers.


Stop emitting diagnostics after 123 errors have been produced. The default is 20, and the error limit can be disabled with -ferror-limit=0.


Only emit up to 123 template instantiation notes within the template instantiation backtrace for a single warning or error. The default is 10, and the limit can be disabled with -ftemplate-backtrace-limit=0.

Formatting of Diagnostics

Clang aims to produce beautiful diagnostics by default, particularly for new users that first come to Clang. However, different people have different preferences, and sometimes Clang is driven by another program that wants to parse simple and consistent output, not a person. For these cases, Clang provides a wide range of options to control the exact output format of the diagnostics that it generates.


Print column number in diagnostic.

This option, which defaults to on, controls whether or not Clang prints the column number of a diagnostic. For example, when this is enabled, Clang will print something like:

test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad

When this is disabled, Clang will print “test.c:28: warning...” with no column number.

The printed column numbers count bytes from the beginning of the line; take care if your source contains multibyte characters.


Print source file/line/column information in diagnostic.

This option, which defaults to on, controls whether or not Clang prints the filename, line number and column number of a diagnostic. For example, when this is enabled, Clang will print something like:

test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad

When this is disabled, Clang will not print the “test.c:28:8: ” part.


Print source line and ranges from source code in diagnostic. This option, which defaults to on, controls whether or not Clang prints the source line, source ranges, and caret when emitting a diagnostic. For example, when this is enabled, Clang will print something like:

test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad

This option, which defaults to on when a color-capable terminal is detected, controls whether or not Clang prints diagnostics in color.

When this option is enabled, Clang will use colors to highlight specific parts of the diagnostic, e.g.,

  test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
  #endif bad

When this is disabled, Clang will just print:

test.c:2:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad
Controls whether ANSI escape codes are used instead of the Windows Console API to output colored diagnostics. This option is only used on Windows and defaults to off.

Changes diagnostic output format to better match IDEs and command line tools.

This option controls the output format of the filename, line number, and column printed in diagnostic messages. The options, and their affect on formatting a simple conversion diagnostic, follow:

clang (default)
t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int'
t.c(3,11) : warning: conversion specifies type 'char *' but the argument has type 'int'
t.c +3:11: warning: conversion specifies type 'char *' but the argument has type 'int'

Enable [-Woption] information in diagnostic line.

This option, which defaults to on, controls whether or not Clang prints the associated warning group option name when outputting a warning diagnostic. For example, in this output:

test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad

Passing -fno-diagnostics-show-option will prevent Clang from printing the [-Wextra-tokens] information in the diagnostic. This information tells you the flag needed to enable or disable the diagnostic, either from the command line or through #pragma GCC diagnostic.


Enable printing category information in diagnostic line.

This option, which defaults to “none”, controls whether or not Clang prints the category associated with a diagnostic when emitting it. Each diagnostic may or many not have an associated category, if it has one, it is listed in the diagnostic categorization field of the diagnostic line (in the []’s).

For example, a format string warning will produce these three renditions based on the setting of this option:

t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat]
t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat,1]
t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat,Format String]

This category can be used by clients that want to group diagnostics by category, so it should be a high level category. We want dozens of these, not hundreds or thousands of them.


Enable “FixIt” information in the diagnostics output.

This option, which defaults to on, controls whether or not Clang prints the information on how to fix a specific diagnostic underneath it when it knows. For example, in this output:

test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad

Passing -fno-diagnostics-fixit-info will prevent Clang from printing the “//” line at the end of the message. This information is useful for users who may not understand what is wrong, but can be confusing for machine parsing.


Print machine parsable information about source ranges. This option makes Clang print information about source ranges in a machine parsable format after the file/line/column number information. The information is a simple sequence of brace enclosed ranges, where each range lists the start and end line/column locations. For example, in this output:

exprs.c:47:15:{47:8-47:14}{47:17-47:24}: error: invalid operands to binary expression ('int *' and '_Complex float')
   P = (P-42) + Gamma*4;
       ~~~~~~ ^ ~~~~~~~

The {}’s are generated by -fdiagnostics-print-source-range-info.

The printed column numbers count bytes from the beginning of the line; take care if your source contains multibyte characters.


Print Fix-Its in a machine parseable form.

This option makes Clang print available Fix-Its in a machine parseable format at the end of diagnostics. The following example illustrates the format:


The range printed is a half-open range, so in this example the characters at column 25 up to but not including column 29 on line 7 in t.cpp should be replaced with the string “Gamma”. Either the range or the replacement string may be empty (representing strict insertions and strict erasures, respectively). Both the file name and the insertion string escape backslash (as “\\”), tabs (as “\t”), newlines (as “\n”), double quotes(as “\””) and non-printable characters (as octal “\xxx”).

The printed column numbers count bytes from the beginning of the line; take care if your source contains multibyte characters.


Turns off elision in template type printing.

The default for template type printing is to elide as many template arguments as possible, removing those which are the same in both template types, leaving only the differences. Adding this flag will print all the template arguments. If supported by the terminal, highlighting will still appear on differing arguments.

Default: note: candidate function not viable: no known conversion from 'vector<map<[...], map<float, [...]>>>' to 'vector<map<[...], map<double, [...]>>>' for 1st argument;

-fno-elide-type: note: candidate function not viable: no known conversion from 'vector<map<int, map<float, int>>>' to 'vector<map<int, map<double, int>>>' for 1st argument;

Template type diffing prints a text tree.

For diffing large templated types, this option will cause Clang to display the templates as an indented text tree, one argument per line, with differences marked inline. This is compatible with -fno-elide-type.

Default: note: candidate function not viable: no known conversion from 'vector<map<[...], map<float, [...]>>>' to 'vector<map<[...], map<double, [...]>>>' for 1st argument;

With -fdiagnostics-show-template-tree: note: candidate function not viable: no known conversion for 1st argument;
        [float != double],

Individual Warning Groups

TODO: Generate this from tblgen. Define one anchor per warning group.


Warn about excess tokens at the end of a preprocessor directive.

This option, which defaults to on, enables warnings about extra tokens at the end of preprocessor directives. For example:

test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad

These extra tokens are not strictly conforming, and are usually best handled by commenting them out.


Warn about unqualified uses of a member template whose name resolves to another template at the location of the use.

This option, which defaults to on, enables a warning in the following code:

template<typename T> struct set{};
template<typename T> struct trait { typedef const T& type; };
struct Value {
  template<typename T> void set(typename trait<T>::type value) {}
void foo() {
  Value v;

C++ [basic.lookup.classref] requires this to be an error, but, because it’s hard to work around, Clang downgrades it to a warning as an extension.


Warn about an unusable copy constructor when binding a reference to a temporary.

This option enables warnings about binding a reference to a temporary when the temporary doesn’t have a usable copy constructor. For example:

struct NonCopyable {
  NonCopyable(const NonCopyable&);
void foo(const NonCopyable&);
void bar() {
  foo(NonCopyable());  // Disallowed in C++98; allowed in C++11.
struct NonCopyable2 {
void foo(const NonCopyable2&);
void bar() {
  foo(NonCopyable2());  // Disallowed in C++98; allowed in C++11.

Note that if NonCopyable2::NonCopyable2() has a default argument whose instantiation produces a compile error, that error will still be a hard error in C++98 mode even if this warning is turned off.

Options to Control Clang Crash Diagnostics

As unbelievable as it may sound, Clang does crash from time to time. Generally, this only occurs to those living on the bleeding edge. Clang goes to great lengths to assist you in filing a bug report. Specifically, Clang generates preprocessed source file(s) and associated run script(s) upon a crash. These files should be attached to a bug report to ease reproducibility of the failure. Below are the command line options to control the crash diagnostics.


Disable auto-generation of preprocessed source files during a clang crash.

The -fno-crash-diagnostics flag can be helpful for speeding the process of generating a delta reduced test case.

Options to Emit Optimization Reports

Optimization reports trace, at a high-level, all the major decisions done by compiler transformations. For instance, when the inliner decides to inline function foo() into bar(), or the loop unroller decides to unroll a loop N times, or the vectorizer decides to vectorize a loop body.

Clang offers a family of flags which the optimizers can use to emit a diagnostic in three cases:

  1. When the pass makes a transformation (-Rpass).
  2. When the pass fails to make a transformation (-Rpass-missed).
  3. When the pass determines whether or not to make a transformation (-Rpass-analysis).

NOTE: Although the discussion below focuses on -Rpass, the exact same options apply to -Rpass-missed and -Rpass-analysis.

Since there are dozens of passes inside the compiler, each of these flags take a regular expression that identifies the name of the pass which should emit the associated diagnostic. For example, to get a report from the inliner, compile the code with:

$ clang -O2 -Rpass=inline -o code remark: foo inlined into bar [-Rpass=inline]
int bar(int j) { return foo(j, j - 2); }

Note that remarks from the inliner are identified with [-Rpass=inline]. To request a report from every optimization pass, you should use -Rpass=.* (in fact, you can use any valid POSIX regular expression). However, do not expect a report from every transformation made by the compiler. Optimization remarks do not really make sense outside of the major transformations (e.g., inlining, vectorization, loop optimizations) and not every optimization pass supports this feature.

Current limitations

  1. Optimization remarks that refer to function names will display the mangled name of the function. Since these remarks are emitted by the back end of the compiler, it does not know anything about the input language, nor its mangling rules.
  2. Some source locations are not displayed correctly. The front end has a more detailed source location tracking than the locations included in the debug info (e.g., the front end can locate code inside macro expansions). However, the locations used by -Rpass are translated from debug annotations. That translation can be lossy, which results in some remarks having no location information.

Language and Target-Independent Features

Controlling Errors and Warnings

Clang provides a number of ways to control which code constructs cause it to emit errors and warning messages, and how they are displayed to the console.

Controlling How Clang Displays Diagnostics

When Clang emits a diagnostic, it includes rich information in the output, and gives you fine-grain control over which information is printed. Clang has the ability to print this information, and these are the options that control it:

  1. A file/line/column indicator that shows exactly where the diagnostic occurs in your code [-fshow-column, -fshow-source-location].
  2. A categorization of the diagnostic as a note, warning, error, or fatal error.
  3. A text string that describes what the problem is.
  4. An option that indicates how to control the diagnostic (for diagnostics that support it) [-fdiagnostics-show-option].
  5. A high-level category for the diagnostic for clients that want to group diagnostics by class (for diagnostics that support it) [-fdiagnostics-show-category].
  6. The line of source code that the issue occurs on, along with a caret and ranges that indicate the important locations [-fcaret-diagnostics].
  7. “FixIt” information, which is a concise explanation of how to fix the problem (when Clang is certain it knows) [-fdiagnostics-fixit-info].
  8. A machine-parsable representation of the ranges involved (off by default) [-fdiagnostics-print-source-range-info].

For more information please see Formatting of Diagnostics.

Diagnostic Mappings

All diagnostics are mapped into one of these 5 classes:

  • Ignored
  • Note
  • Remark
  • Warning
  • Error
  • Fatal

Diagnostic Categories

Though not shown by default, diagnostics may each be associated with a high-level category. This category is intended to make it possible to triage builds that produce a large number of errors or warnings in a grouped way.

Categories are not shown by default, but they can be turned on with the -fdiagnostics-show-category option. When set to “name”, the category is printed textually in the diagnostic output. When it is set to “id”, a category number is printed. The mapping of category names to category id’s can be obtained by running ‘clang   --print-diagnostic-categories‘.

Controlling Diagnostics via Command Line Flags

TODO: -W flags, -pedantic, etc

Controlling Diagnostics via Pragmas

Clang can also control what diagnostics are enabled through the use of pragmas in the source code. This is useful for turning off specific warnings in a section of source code. Clang supports GCC’s pragma for compatibility with existing source code, as well as several extensions.

The pragma may control any warning that can be used from the command line. Warnings may be set to ignored, warning, error, or fatal. The following example code will tell Clang or GCC to ignore the -Wall warnings:

#pragma GCC diagnostic ignored "-Wall"

In addition to all of the functionality provided by GCC’s pragma, Clang also allows you to push and pop the current warning state. This is particularly useful when writing a header file that will be compiled by other people, because you don’t know what warning flags they build with.

In the below example -Wmultichar is ignored for only a single line of code, after which the diagnostics return to whatever state had previously existed.

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wmultichar"

char b = 'df'; // no warning.

#pragma clang diagnostic pop

The push and pop pragmas will save and restore the full diagnostic state of the compiler, regardless of how it was set. That means that it is possible to use push and pop around GCC compatible diagnostics and Clang will push and pop them appropriately, while GCC will ignore the pushes and pops as unknown pragmas. It should be noted that while Clang supports the GCC pragma, Clang and GCC do not support the exact same set of warnings, so even when using GCC compatible #pragmas there is no guarantee that they will have identical behaviour on both compilers.

In addition to controlling warnings and errors generated by the compiler, it is possible to generate custom warning and error messages through the following pragmas:

// The following will produce warning messages
#pragma message "some diagnostic message"
#pragma GCC warning "TODO: replace deprecated feature"

// The following will produce an error message
#pragma GCC error "Not supported"

These pragmas operate similarly to the #warning and #error preprocessor directives, except that they may also be embedded into preprocessor macros via the C99 _Pragma operator, for example:

#define STR(X) #X
#define DEFER(M,...) M(__VA_ARGS__)
#define CUSTOM_ERROR(X) _Pragma(STR(GCC error(X " at line " DEFER(STR,__LINE__))))

CUSTOM_ERROR("Feature not available");

Controlling Diagnostics in System Headers

Warnings are suppressed when they occur in system headers. By default, an included file is treated as a system header if it is found in an include path specified by -isystem, but this can be overridden in several ways.

The system_header pragma can be used to mark the current file as being a system header. No warnings will be produced from the location of the pragma onwards within the same file.

char a = 'xy'; // warning

#pragma clang system_header

char b = 'ab'; // no warning

The --system-header-prefix= and --no-system-header-prefix= command-line arguments can be used to override whether subsets of an include path are treated as system headers. When the name in a #include directive is found within a header search path and starts with a system prefix, the header is treated as a system header. The last prefix on the command-line which matches the specified header name takes precedence. For instance:

$ clang -Ifoo -isystem bar --system-header-prefix=x/ \

Here, #include "x/a.h" is treated as including a system header, even if the header is found in foo, and #include "x/y/b.h" is treated as not including a system header, even if the header is found in bar.

A #include directive which finds a file relative to the current directory is treated as including a system header if the including file is treated as a system header.

Enabling All Diagnostics

In addition to the traditional -W flags, one can enable all diagnostics by passing -Weverything. This works as expected with -Werror, and also includes the warnings from -pedantic.

Note that when combined with -w (which disables all warnings), that flag wins.

Controlling Static Analyzer Diagnostics

While not strictly part of the compiler, the diagnostics from Clang’s static analyzer can also be influenced by the user via changes to the source code. See the available annotations and the analyzer’s FAQ page for more information.

Precompiled Headers

Precompiled headers are a general approach employed by many compilers to reduce compilation time. The underlying motivation of the approach is that it is common for the same (and often large) header files to be included by multiple source files. Consequently, compile times can often be greatly improved by caching some of the (redundant) work done by a compiler to process headers. Precompiled header files, which represent one of many ways to implement this optimization, are literally files that represent an on-disk cache that contains the vital information necessary to reduce some of the work needed to process a corresponding header file. While details of precompiled headers vary between compilers, precompiled headers have been shown to be highly effective at speeding up program compilation on systems with very large system headers (e.g., Mac OS X).

Generating a PCH File

To generate a PCH file using Clang, one invokes Clang with the -x <language>-header option. This mirrors the interface in GCC for generating PCH files:

$ gcc -x c-header test.h -o test.h.gch
$ clang -x c-header test.h -o test.h.pch

Using a PCH File

A PCH file can then be used as a prefix header when a -include option is passed to clang:

$ clang -include test.h test.c -o test

The clang driver will first check if a PCH file for test.h is available; if so, the contents of test.h (and the files it includes) will be processed from the PCH file. Otherwise, Clang falls back to directly processing the content of test.h. This mirrors the behavior of GCC.


Clang does not automatically use PCH files for headers that are directly included within a source file. For example:

$ clang -x c-header test.h -o test.h.pch
$ cat test.c
#include "test.h"
$ clang test.c -o test

In this example, clang will not automatically use the PCH file for test.h since test.h was included directly in the source file and not specified on the command line using -include.

Relocatable PCH Files

It is sometimes necessary to build a precompiled header from headers that are not yet in their final, installed locations. For example, one might build a precompiled header within the build tree that is then meant to be installed alongside the headers. Clang permits the creation of “relocatable” precompiled headers, which are built with a given path (into the build directory) and can later be used from an installed location.

To build a relocatable precompiled header, place your headers into a subdirectory whose structure mimics the installed location. For example, if you want to build a precompiled header for the header mylib.h that will be installed into /usr/include, create a subdirectory build/usr/include and place the header mylib.h into that subdirectory. If mylib.h depends on other headers, then they can be stored within build/usr/include in a way that mimics the installed location.

Building a relocatable precompiled header requires two additional arguments. First, pass the --relocatable-pch flag to indicate that the resulting PCH file should be relocatable. Second, pass -isysroot /path/to/build, which makes all includes for your library relative to the build directory. For example:

# clang -x c-header --relocatable-pch -isysroot /path/to/build /path/to/build/mylib.h mylib.h.pch

When loading the relocatable PCH file, the various headers used in the PCH file are found from the system header root. For example, mylib.h can be found in /usr/include/mylib.h. If the headers are installed in some other system root, the -isysroot option can be used provide a different system root from which the headers will be based. For example, -isysroot /Developer/SDKs/MacOSX10.4u.sdk will look for mylib.h in /Developer/SDKs/MacOSX10.4u.sdk/usr/include/mylib.h.

Relocatable precompiled headers are intended to be used in a limited number of cases where the compilation environment is tightly controlled and the precompiled header cannot be generated after headers have been installed.

Controlling Code Generation

Clang provides a number of ways to control code generation. The options are listed below.


Turn on runtime checks for various forms of undefined or suspicious behavior.

This option controls whether Clang adds runtime checks for various forms of undefined or suspicious behavior, and is disabled by default. If a check fails, a diagnostic message is produced at runtime explaining the problem. The main checks are:

  • -fsanitize=address: AddressSanitizer, a memory error detector.

  • -fsanitize=integer: Enables checks for undefined or suspicious integer behavior.
  • -fsanitize=thread: ThreadSanitizer, a data race detector.

  • -fsanitize=memory: MemorySanitizer, an experimental detector of uninitialized reads. Not ready for widespread use.

  • -fsanitize=undefined: Fast and compatible undefined behavior checker. Enables the undefined behavior checks that have small runtime cost and no impact on address space layout or ABI. This includes all of the checks listed below other than unsigned-integer-overflow.

  • -fsanitize=undefined-trap: This includes all sanitizers included by -fsanitize=undefined, except those that require runtime support. This group of sanitizers is intended to be used in conjunction with the -fsanitize-undefined-trap-on-error flag. This includes all of the checks listed below other than unsigned-integer-overflow and vptr.
  • -fsanitize=dataflow: DataFlowSanitizer, a general data flow analysis.

The following more fine-grained checks are also available:

  • -fsanitize=alignment: Use of a misaligned pointer or creation of a misaligned reference.
  • -fsanitize=bool: Load of a bool value which is neither true nor false.
  • -fsanitize=bounds: Out of bounds array indexing, in cases where the array bound can be statically determined.
  • -fsanitize=enum: Load of a value of an enumerated type which is not in the range of representable values for that enumerated type.
  • -fsanitize=float-cast-overflow: Conversion to, from, or between floating-point types which would overflow the destination.
  • -fsanitize=float-divide-by-zero: Floating point division by zero.
  • -fsanitize=function: Indirect call of a function through a function pointer of the wrong type (Linux, C++ and x86/x86_64 only).
  • -fsanitize=integer-divide-by-zero: Integer division by zero.
  • -fsanitize=nonnull-attribute: Passing null pointer as a function parameter which is declared to never be null.
  • -fsanitize=null: Use of a null pointer or creation of a null reference.
  • -fsanitize=object-size: An attempt to use bytes which the optimizer can determine are not part of the object being accessed. The sizes of objects are determined using __builtin_object_size, and consequently may be able to detect more problems at higher optimization levels.
  • -fsanitize=return: In C++, reaching the end of a value-returning function without returning a value.
  • -fsanitize=returns-nonnull-attribute: Returning null pointer from a function which is declared to never return null.
  • -fsanitize=shift: Shift operators where the amount shifted is greater or equal to the promoted bit-width of the left hand side or less than zero, or where the left hand side is negative. For a signed left shift, also checks for signed overflow in C, and for unsigned overflow in C++.
  • -fsanitize=signed-integer-overflow: Signed integer overflow, including all the checks added by -ftrapv, and checking for overflow in signed division (INT_MIN / -1).
  • -fsanitize=unreachable: If control flow reaches __builtin_unreachable.
  • -fsanitize=unsigned-integer-overflow: Unsigned integer overflows.
  • -fsanitize=vla-bound: A variable-length array whose bound does not evaluate to a positive value.
  • -fsanitize=vptr: Use of an object whose vptr indicates that it is of the wrong dynamic type, or that its lifetime has not begun or has ended. Incompatible with -fno-rtti.

You can turn off or modify checks for certain source files, functions or even variables by providing a special file:

  • -fsanitize-blacklist=/path/to/blacklist/file: disable or modify sanitizer checks for objects listed in the file. See Sanitizer special case list for file format description.
  • -fno-sanitize-blacklist: don’t use blacklist file, if it was specified earlier in the command line.

Extra features of MemorySanitizer (require explicit -fsanitize=memory):

  • -fsanitize-memory-track-origins[=level]: Enables origin tracking in MemorySanitizer. Adds a second section to MemorySanitizer reports pointing to the heap or stack allocation the uninitialized bits came from. Slows down execution by additional 1.5x-2x.

    Possible values for level are 0 (off), 1 (default), 2. Level 2 adds more sections to MemorySanitizer reports describing the order of memory stores the uninitialized value went through. Beware, this mode may use a lot of extra memory.

Extra features of UndefinedBehaviorSanitizer:

  • -fsanitize-undefined-trap-on-error: Causes traps to be emitted rather than calls to runtime libraries when a problem is detected. This option is intended for use in cases where the sanitizer runtime cannot be used (for instance, when building libc or a kernel module). This is only compatible with the sanitizers in the undefined-trap group.

The -fsanitize= argument must also be provided when linking, in order to link to the appropriate runtime library. When using -fsanitize=vptr (or a group that includes it, such as -fsanitize=undefined) with a C++ program, the link must be performed by clang++, not clang, in order to link against the C++-specific parts of the runtime library.

It is not possible to combine more than one of the -fsanitize=address, -fsanitize=thread, and -fsanitize=memory checkers in the same program. The -fsanitize=undefined checks can be combined with other sanitizers.


Controls which checks enabled by -fsanitize= flag are non-fatal. If the check is fatal, program will halt after the first error of this kind is detected and error report is printed.

By default, non-fatal checks are those enabled by UndefinedBehaviorSanitizer, except for -fsanitize=return and -fsanitize=unreachable. Some sanitizers (e.g. AddressSanitizer) may not support recovery, and always crash the program after the issue is detected.


Don’t assume that the C++’s new operator is sane.

This option tells the compiler to do not assume that C++’s global new operator will always return a pointer that does not alias any other pointer when the function returns.


Instruct code generator to emit a function call to the specified function name for __builtin_trap().

LLVM code generator translates __builtin_trap() to a trap instruction if it is supported by the target ISA. Otherwise, the builtin is translated into a call to abort. If this option is set, then the code generator will always lower the builtin to a call to the specified function regardless of whether the target ISA has a trap instruction. This option is useful for environments (e.g. deeply embedded) where a trap cannot be properly handled, or when some custom behavior is desired.


Select which TLS model to use.

Valid values are: global-dynamic, local-dynamic, initial-exec and local-exec. The default value is global-dynamic. The compiler may use a different model if the selected model is not supported by the target, or if a more efficient model can be used. The TLS model can be overridden per variable using the tls_model attribute.


Select the ARM modes (arm or thumb) that support hardware division instructions.

Valid values are: arm, thumb and arm,thumb. This option is used to indicate which mode (arm or thumb) supports hardware division instructions. This only applies to the ARM architecture.


Enable or disable CRC instructions.

This option is used to indicate whether CRC instructions are to be generated. This only applies to the ARM architecture.

CRC instructions are enabled by default on ARMv8.


Generate code which only uses the general purpose registers.

This option restricts the generated code to use general registers only. This only applies to the AArch64 architecture.


Instruct the code generator to not enforce a higher alignment than the given number (of bytes) when accessing memory via an opaque pointer or reference. This cap is ignored when directly accessing a variable or when the pointee type has an explicit “aligned” attribute.

The value should usually be determined by the properties of the system allocator. Some builtin types, especially vector types, have very high natural alignments; when working with values of those types, Clang usually wants to use instructions that take advantage of that alignment. However, many system allocators do not promise to return memory that is more than 8-byte or 16-byte-aligned. Use this option to limit the alignment that the compiler can assume for an arbitrary pointer, which may point onto the heap.

This option does not affect the ABI alignment of types; the layout of structs and unions and the value returned by the alignof operator remain the same.

This option can be overridden on a case-by-case basis by putting an explicit “aligned” alignment on a struct, union, or typedef. For example:

#include <immintrin.h>
// Make an aligned typedef of the AVX-512 16-int vector type.
typedef __v16si __aligned_v16si __attribute__((aligned(64)));

void initialize_vector(__aligned_v16si *v) {
  // The compiler may assume that ‘v’ is 64-byte aligned, regardless of the
  // value of -fmax-unknown-pointer-align.

Profile Guided Optimization

Profile information enables better optimization. For example, knowing that a branch is taken very frequently helps the compiler make better decisions when ordering basic blocks. Knowing that a function foo is called more frequently than another function bar helps the inliner.

Clang supports profile guided optimization with two different kinds of profiling. A sampling profiler can generate a profile with very low runtime overhead, or you can build an instrumented version of the code that collects more detailed profile information. Both kinds of profiles can provide execution counts for instructions in the code and information on branches taken and function invocation.

Regardless of which kind of profiling you use, be careful to collect profiles by running your code with inputs that are representative of the typical behavior. Code that is not exercised in the profile will be optimized as if it is unimportant, and the compiler may make poor optimization choices for code that is disproportionately used while profiling.

Using Sampling Profilers

Sampling profilers are used to collect runtime information, such as hardware counters, while your application executes. They are typically very efficient and do not incur a large runtime overhead. The sample data collected by the profiler can be used during compilation to determine what the most executed areas of the code are.

Using the data from a sample profiler requires some changes in the way a program is built. Before the compiler can use profiling information, the code needs to execute under the profiler. The following is the usual build cycle when using sample profilers for optimization:

  1. Build the code with source line table information. You can use all the usual build flags that you always build your application with. The only requirement is that you add -gline-tables-only or -g to the command line. This is important for the profiler to be able to map instructions back to source line locations.

    $ clang++ -O2 -gline-tables-only -o code
  2. Run the executable under a sampling profiler. The specific profiler you use does not really matter, as long as its output can be converted into the format that the LLVM optimizer understands. Currently, there exists a conversion tool for the Linux Perf profiler (, so these examples assume that you are using Linux Perf to profile your code.

    $ perf record -b ./code

    Note the use of the -b flag. This tells Perf to use the Last Branch Record (LBR) to record call chains. While this is not strictly required, it provides better call information, which improves the accuracy of the profile data.

  3. Convert the collected profile data to LLVM’s sample profile format. This is currently supported via the AutoFDO converter create_llvm_prof. It is available at Once built and installed, you can convert the file to LLVM using the command:

    $ create_llvm_prof --binary=./code

    This will read and the binary file ./code and emit the profile data in Note that if you ran perf without the -b flag, you need to use --use_lbr=false when calling create_llvm_prof.

  4. Build the code again using the collected profile. This step feeds the profile back to the optimizers. This should result in a binary that executes faster than the original one. Note that you are not required to build the code with the exact same arguments that you used in the first step. The only requirement is that you build the code with -gline-tables-only and -fprofile-sample-use.

    $ clang++ -O2 -gline-tables-only -o code
Sample Profile Format

If you are not using Linux Perf to collect profiles, you will need to write a conversion tool from your profiler to LLVM’s format. This section explains the file format expected by the backend.

Sample profiles are written as ASCII text. The file is divided into sections, which correspond to each of the functions executed at runtime. Each section has the following format (taken from

offset1[.discriminator]: number_of_samples [fn1:num fn2:num ... ]
offset2[.discriminator]: number_of_samples [fn3:num fn4:num ... ]
offsetN[.discriminator]: number_of_samples [fn5:num fn6:num ... ]

The file may contain blank lines between sections and within a section. However, the spacing within a single line is fixed. Additional spaces will result in an error while reading the file.

Function names must be mangled in order for the profile loader to match them in the current translation unit. The two numbers in the function header specify how many total samples were accumulated in the function (first number), and the total number of samples accumulated in the prologue of the function (second number). This head sample count provides an indicator of how frequently the function is invoked.

Each sampled line may contain several items. Some are optional (marked below):

  1. Source line offset. This number represents the line number in the function where the sample was collected. The line number is always relative to the line where symbol of the function is defined. So, if the function has its header at line 280, the offset 13 is at line 293 in the file.

    Note that this offset should never be a negative number. This could happen in cases like macros. The debug machinery will register the line number at the point of macro expansion. So, if the macro was expanded in a line before the start of the function, the profile converter should emit a 0 as the offset (this means that the optimizers will not be able to associate a meaningful weight to the instructions in the macro).

  2. [OPTIONAL] Discriminator. This is used if the sampled program was compiled with DWARF discriminator support ( DWARF discriminators are unsigned integer values that allow the compiler to distinguish between multiple execution paths on the same source line location.

    For example, consider the line of code if (cond) foo(); else bar();. If the predicate cond is true 80% of the time, then the edge into function foo should be considered to be taken most of the time. But both calls to foo and bar are at the same source line, so a sample count at that line is not sufficient. The compiler needs to know which part of that line is taken more frequently.

    This is what discriminators provide. In this case, the calls to foo and bar will be at the same line, but will have different discriminator values. This allows the compiler to correctly set edge weights into foo and bar.

  3. Number of samples. This is an integer quantity representing the number of samples collected by the profiler at this source location.

  4. [OPTIONAL] Potential call targets and samples. If present, this line contains a call instruction. This models both direct and number of samples. For example,

    130: 7  foo:3  bar:2  baz:7

    The above means that at relative line offset 130 there is a call instruction that calls one of foo(), bar() and baz(), with baz() being the relatively more frequently called target.

Profiling with Instrumentation

Clang also supports profiling via instrumentation. This requires building a special instrumented version of the code and has some runtime overhead during the profiling, but it provides more detailed results than a sampling profiler. It also provides reproducible results, at least to the extent that the code behaves consistently across runs.

Here are the steps for using profile guided optimization with instrumentation:

  1. Build an instrumented version of the code by compiling and linking with the -fprofile-instr-generate option.

    $ clang++ -O2 -fprofile-instr-generate -o code
  2. Run the instrumented executable with inputs that reflect the typical usage. By default, the profile data will be written to a default.profraw file in the current directory. You can override that default by setting the LLVM_PROFILE_FILE environment variable to specify an alternate file. Any instance of %p in that file name will be replaced by the process ID, so that you can easily distinguish the profile output from multiple runs.

    $ LLVM_PROFILE_FILE="code-%p.profraw" ./code
  3. Combine profiles from multiple runs and convert the “raw” profile format to the input expected by clang. Use the merge command of the llvm-profdata tool to do this.

    $ llvm-profdata merge -output=code.profdata code-*.profraw

    Note that this step is necessary even when there is only one “raw” profile, since the merge operation also changes the file format.

  4. Build the code again using the -fprofile-instr-use option to specify the collected profile data.

    $ clang++ -O2 -fprofile-instr-use=code.profdata -o code

    You can repeat step 4 as often as you like without regenerating the profile. As you make changes to your code, clang may no longer be able to use the profile data. It will warn you when this happens.

Controlling Size of Debug Information

Debug info kind generated by Clang can be set by one of the flags listed below. If multiple flags are present, the last one is used.


Don’t generate any debug info (default).


Generate line number tables only.

This kind of debug info allows to obtain stack traces with function names, file names and line numbers (by such tools as gdb or addr2line). It doesn’t contain any other data (e.g. description of local variables or function parameters).


Clang supports a number of optimizations to reduce the size of debug information in the binary. They work based on the assumption that the debug type information can be spread out over multiple compilation units. For instance, Clang will not emit type definitions for types that are not needed by a module and could be replaced with a forward declaration. Further, Clang will only emit type info for a dynamic C++ class in the module that contains the vtable for the class.

The -fstandalone-debug option turns off these optimizations. This is useful when working with 3rd-party libraries that don’t come with debug information. Note that Clang will never emit type information for types that are not referenced at all by the program.


On Darwin -fstandalone-debug is enabled by default. The -fno-standalone-debug option can be used to get to turn on the vtable-based optimization described above.


Generate complete debug info.

Comment Parsing Options

Clang parses Doxygen and non-Doxygen style documentation comments and attaches them to the appropriate declaration nodes. By default, it only parses Doxygen-style comments and ignores ordinary comments starting with // and /*.


Emit warnings about use of documentation comments. This warning group is off by default.

This includes checking that \param commands name parameters that actually present in the function signature, checking that \returns is used only on functions that actually return a value etc.


Don’t warn when encountering an unknown Doxygen command.


Parse all comments as documentation comments (including ordinary comments starting with // and /*).


Define custom documentation commands as block commands. This allows Clang to construct the correct AST for these custom commands, and silences warnings about unknown commands. Several commands must be separated by a comma without trailing space; e.g. -fcomment-block-commands=foo,bar defines custom commands \foo and \bar.

It is also possible to use -fcomment-block-commands several times; e.g. -fcomment-block-commands=foo -fcomment-block-commands=bar does the same as above.

C Language Features

The support for standard C in clang is feature-complete except for the C99 floating-point pragmas.

Differences between various standard modes

clang supports the -std option, which changes what language mode clang uses. The supported modes for C are c89, gnu89, c94, c99, gnu99, c11, gnu11, and various aliases for those modes. If no -std option is specified, clang defaults to gnu11 mode. Many C99 and C11 features are supported in earlier modes as a conforming extension, with a warning. Use -pedantic-errors to request an error if a feature from a later standard revision is used in an earlier mode.

Differences between all c* and gnu* modes:

  • c* modes define “__STRICT_ANSI__”.
  • Target-specific defines not prefixed by underscores, like “linux”, are defined in gnu* modes.
  • Trigraphs default to being off in gnu* modes; they can be enabled by the -trigraphs option.
  • The parser recognizes “asm” and “typeof” as keywords in gnu* modes; the variants “__asm__” and “__typeof__” are recognized in all modes.
  • The Apple “blocks” extension is recognized by default in gnu* modes on some platforms; it can be enabled in any mode with the “-fblocks” option.
  • Arrays that are VLA’s according to the standard, but which can be constant folded by the frontend are treated as fixed size arrays. This occurs for things like “int X[(1, 2)];”, which is technically a VLA. c* modes are strictly compliant and treat these as VLAs.

Differences between *89 and *99 modes:

  • The *99 modes default to implementing “inline” as specified in C99, while the *89 modes implement the GNU version. This can be overridden for individual functions with the __gnu_inline__ attribute.
  • Digraphs are not recognized in c89 mode.
  • The scope of names defined inside a “for”, “if”, “switch”, “while”, or “do” statement is different. (example: “if ((struct x {int x;}*)0) {}”.)
  • __STDC_VERSION__ is not defined in *89 modes.
  • “inline” is not recognized as a keyword in c89 mode.
  • “restrict” is not recognized as a keyword in *89 modes.
  • Commas are allowed in integer constant expressions in *99 modes.
  • Arrays which are not lvalues are not implicitly promoted to pointers in *89 modes.
  • Some warnings are different.

Differences between *99 and *11 modes:

  • Warnings for use of C11 features are disabled.
  • __STDC_VERSION__ is defined to 201112L rather than 199901L.

c94 mode is identical to c89 mode except that digraphs are enabled in c94 mode (FIXME: And __STDC_VERSION__ should be defined!).

GCC extensions not implemented yet

clang tries to be compatible with gcc as much as possible, but some gcc extensions are not implemented yet:

  • clang does not support #pragma weak (bug 3679). Due to the uses described in the bug, this is likely to be implemented at some point, at least partially.

  • clang does not support decimal floating point types (_Decimal32 and friends) or fixed-point types (_Fract and friends); nobody has expressed interest in these features yet, so it’s hard to say when they will be implemented.

  • clang does not support nested functions; this is a complex feature which is infrequently used, so it is unlikely to be implemented anytime soon. In C++11 it can be emulated by assigning lambda functions to local variables, e.g:

    auto const local_function = [&](int parameter) {
      // Do something
  • clang does not support global register variables; this is unlikely to be implemented soon because it requires additional LLVM backend support.

  • clang does not support static initialization of flexible array members. This appears to be a rarely used extension, but could be implemented pending user demand.

  • clang does not support __builtin_va_arg_pack/__builtin_va_arg_pack_len. This is used rarely, but in some potentially interesting places, like the glibc headers, so it may be implemented pending user demand. Note that because clang pretends to be like GCC 4.2, and this extension was introduced in 4.3, the glibc headers will not try to use this extension with clang at the moment.

  • clang does not support the gcc extension for forward-declaring function parameters; this has not shown up in any real-world code yet, though, so it might never be implemented.

This is not a complete list; if you find an unsupported extension missing from this list, please send an e-mail to cfe-dev. This list currently excludes C++; see C++ Language Features. Also, this list does not include bugs in mostly-implemented features; please see the bug tracker for known existing bugs (FIXME: Is there a section for bug-reporting guidelines somewhere?).

Intentionally unsupported GCC extensions

  • clang does not support the gcc extension that allows variable-length arrays in structures. This is for a few reasons: one, it is tricky to implement, two, the extension is completely undocumented, and three, the extension appears to be rarely used. Note that clang does support flexible array members (arrays with a zero or unspecified size at the end of a structure).
  • clang does not have an equivalent to gcc’s “fold”; this means that clang doesn’t accept some constructs gcc might accept in contexts where a constant expression is required, like “x-x” where x is a variable.
  • clang does not support __builtin_apply and friends; this extension is extremely obscure and difficult to implement reliably.

Microsoft extensions

clang has some experimental support for extensions from Microsoft Visual C++; to enable it, use the -fms-extensions command-line option. This is the default for Windows targets. Note that the support is incomplete. Some constructs such as dllexport on classes are ignored with a warning, and others such as Microsoft IDL annotations are silently ignored.

clang has a -fms-compatibility flag that makes clang accept enough invalid C++ to be able to parse most Microsoft headers. For example, it allows unqualified lookup of dependent base class members, which is a common compatibility issue with clang. This flag is enabled by default for Windows targets.

-fdelayed-template-parsing lets clang delay parsing of function template definitions until the end of a translation unit. This flag is enabled by default for Windows targets.

  • clang allows setting _MSC_VER with -fmsc-version=. It defaults to 1700 which is the same as Visual C/C++ 2012. Any number is supported and can greatly affect what Windows SDK and c++stdlib headers clang can compile.
  • clang does not support the Microsoft extension where anonymous record members can be declared using user defined typedefs.
  • clang supports the Microsoft #pragma pack feature for controlling record layout. GCC also contains support for this feature, however where MSVC and GCC are incompatible clang follows the MSVC definition.
  • clang supports the Microsoft #pragma comment(lib, "foo.lib") feature for automatically linking against the specified library. Currently this feature only works with the Visual C++ linker.
  • clang supports the Microsoft #pragma comment(linker, "/flag:foo") feature for adding linker flags to COFF object files. The user is responsible for ensuring that the linker understands the flags.
  • clang defaults to C++11 for Windows targets.

C++ Language Features

clang fully implements all of standard C++98 except for exported templates (which were removed in C++11), and all of standard C++11 and the current draft standard for C++1y.

Controlling implementation limits


Sets the limit for nested parentheses, brackets, and braces to N. The default is 256.


Sets the limit for recursive constexpr function invocations to N. The default is 512.


Sets the limit for recursively nested template instantiations to N. The default is 256.


Sets the limit for iterative calls to ‘operator->’ functions to N. The default is 256.

Target-Specific Features and Limitations

CPU Architectures Features and Limitations


The support for X86 (both 32-bit and 64-bit) is considered stable on Darwin (Mac OS X), Linux, FreeBSD, and Dragonfly BSD: it has been tested to correctly compile many large C, C++, Objective-C, and Objective-C++ codebases.

On x86_64-mingw32, passing i128(by value) is incompatible with the Microsoft x64 calling convention. You might need to tweak WinX86_64ABIInfo::classify() in lib/CodeGen/TargetInfo.cpp.

For the X86 target, clang supports the -m16 command line argument which enables 16-bit code output. This is broadly similar to using asm(".code16gcc") with the GNU toolchain. The generated code and the ABI remains 32-bit but the assembler emits instructions appropriate for a CPU running in 16-bit mode, with address-size and operand-size prefixes to enable 32-bit addressing and operations.


The support for ARM (specifically ARMv6 and ARMv7) is considered stable on Darwin (iOS): it has been tested to correctly compile many large C, C++, Objective-C, and Objective-C++ codebases. Clang only supports a limited number of ARM architectures. It does not yet fully support ARMv5, for example.


The support for PowerPC (especially PowerPC64) is considered stable on Linux and FreeBSD: it has been tested to correctly compile many large C and C++ codebases. PowerPC (32bit) is still missing certain features (e.g. PIC code on ELF platforms).

Other platforms

clang currently contains some support for other architectures (e.g. Sparc); however, significant pieces of code generation are still missing, and they haven’t undergone significant testing.

clang contains limited support for the MSP430 embedded processor, but both the clang support and the LLVM backend support are highly experimental.

Other platforms are completely unsupported at the moment. Adding the minimal support needed for parsing and semantic analysis on a new platform is quite easy; see lib/Basic/Targets.cpp in the clang source tree. This level of support is also sufficient for conversion to LLVM IR for simple programs. Proper support for conversion to LLVM IR requires adding code to lib/CodeGen/CGCall.cpp at the moment; this is likely to change soon, though. Generating assembly requires a suitable LLVM backend.

Operating System Features and Limitations

Darwin (Mac OS X)

Thread Sanitizer is not supported.


Clang has experimental support for targeting “Cygming” (Cygwin / MinGW) platforms.

See also Microsoft Extensions.


Clang works on Cygwin-1.7.


Clang works on some mingw32 distributions. Clang assumes directories as below;

  • C:/mingw/include
  • C:/mingw/lib
  • C:/mingw/lib/gcc/mingw32/4.[3-5].0/include/c++

On MSYS, a few tests might fail.


For 32-bit (i686-w64-mingw32), and 64-bit (x86_64-w64-mingw32), Clang assumes as below;

  • GCC versions 4.5.0 to 4.5.3, 4.6.0 to 4.6.2, or 4.7.0 (for the C++ header search path)
  • some_directory/bin/gcc.exe
  • some_directory/bin/clang.exe
  • some_directory/bin/clang++.exe
  • some_directory/bin/../include/c++/GCC_version
  • some_directory/bin/../include/c++/GCC_version/x86_64-w64-mingw32
  • some_directory/bin/../include/c++/GCC_version/i686-w64-mingw32
  • some_directory/bin/../include/c++/GCC_version/backward
  • some_directory/bin/../x86_64-w64-mingw32/include
  • some_directory/bin/../i686-w64-mingw32/include
  • some_directory/bin/../include

This directory layout is standard for any toolchain you will find on the official MinGW-w64 website.

Clang expects the GCC executable “gcc.exe” compiled for i686-w64-mingw32 (or x86_64-w64-mingw32) to be present on PATH.

Some tests might fail on x86_64-w64-mingw32.


clang-cl is an alternative command-line interface to Clang driver, designed for compatibility with the Visual C++ compiler, cl.exe.

To enable clang-cl to find system headers, libraries, and the linker when run from the command-line, it should be executed inside a Visual Studio Native Tools Command Prompt or a regular Command Prompt where the environment has been set up using e.g. vcvars32.bat.

clang-cl can also be used from inside Visual Studio by using an LLVM Platform Toolset.

Command-Line Options

To be compatible with cl.exe, clang-cl supports most of the same command-line options. Those options can start with either / or -. It also supports some of Clang’s core options, such as the -W options.

Options that are known to clang-cl, but not currently supported, are ignored with a warning. For example:

clang-cl.exe: warning: argument unused during compilation: '/Zi'

To suppress warnings about unused arguments, use the -Qunused-arguments option.

Options that are not known to clang-cl will cause errors. If they are spelled with a leading /, they will be mistaken for a filename:

clang-cl.exe: error: no such file or directory: '/foobar'

Please file a bug for any valid cl.exe flags that clang-cl does not understand.

Execute clang-cl /? to see a list of supported options:

  /?                     Display available options
  /arch:<value>          Set architecture for code generation
  /C                     Don't discard comments when preprocessing
  /c                     Compile only
  /D <macro[=value]>     Define macro
  /EH<value>             Exception handling model
  /EP                    Disable linemarker output and preprocess to stdout
  /E                     Preprocess to stdout
  /fallback              Fall back to cl.exe if clang-cl fails to compile
  /FA                    Output assembly code file during compilation
  /Fa<file or directory> Output assembly code to this file during compilation
  /Fe<file or directory> Set output executable file or directory (ends in / or \)
  /FI <value>            Include file before parsing
  /Fi<file>              Set preprocess output file name
  /Fo<file or directory> Set output object file, or directory (ends in / or \)
  /GF-                   Disable string pooling
  /GR-                   Disable emission of RTTI data
  /GR                    Enable emission of RTTI data
  /Gw-                   Don't put each data item in its own section
  /Gw                    Put each data item in its own section
  /Gy-                   Don't put each function in its own section
  /Gy                    Put each function in its own section
  /help                  Display available options
  /I <dir>               Add directory to include search path
  /J                     Make char type unsigned
  /LDd                   Create debug DLL
  /LD                    Create DLL
  /link <options>        Forward options to the linker
  /MDd                   Use DLL debug run-time
  /MD                    Use DLL run-time
  /MTd                   Use static debug run-time
  /MT                    Use static run-time
  /Ob0                   Disable inlining
  /Od                    Disable optimization
  /Oi-                   Disable use of builtin functions
  /Oi                    Enable use of builtin functions
  /Os                    Optimize for size
  /Ot                    Optimize for speed
  /Ox                    Maximum optimization
  /Oy-                   Disable frame pointer omission
  /Oy                    Enable frame pointer omission
  /O<n>                  Optimization level
  /P                     Preprocess to file
  /showIncludes          Print info about included files to stderr
  /TC                    Treat all source files as C
  /Tc <filename>         Specify a C source file
  /TP                    Treat all source files as C++
  /Tp <filename>         Specify a C++ source file
  /U <macro>             Undefine macro
  /vd<value>             Control vtordisp placement
  /vmb                   Use a best-case representation method for member pointers
  /vmg                   Use a most-general representation for member pointers
  /vmm                   Set the default most-general representation to multiple inheritance
  /vms                   Set the default most-general representation to single inheritance
  /vmv                   Set the default most-general representation to virtual inheritance
  /W0                    Disable all warnings
  /W1                    Enable -Wall
  /W2                    Enable -Wall
  /W3                    Enable -Wall
  /W4                    Enable -Wall
  /Wall                  Enable -Wall
  /WX-                   Do not treat warnings as errors
  /WX                    Treat warnings as errors
  /w                     Disable all warnings
  /Zi                    Enable debug information
  /Zp                    Set the default maximum struct packing alignment to 1
  /Zp<value>             Specify the default maximum struct packing alignment
  /Zs                    Syntax-check only

  -###                  Print (but do not run) the commands to run for this compilation
                        Dot-separated value representing the Microsoft compiler version
                        number to report in _MSC_VER (0 = don't define it (default))
  -fmsc-version=<value> Microsoft compiler version number to report in _MSC_VER (0 = don't
                        define it (default))
                        Path to blacklist file for sanitizers
  -fsanitize=<check>    Enable runtime instrumentation for bug detection: address (memory
                        errors) | thread (race detection) | undefined (miscellaneous
                        undefined behavior)
  -mllvm <value>        Additional arguments to forward to LLVM's option processing
  -Qunused-arguments    Don't emit warning for unused driver arguments
  --target=<value>      Generate code for the given target
  -v                    Show commands to run and use verbose output
  -W<warning>           Enable the specified warning
  -Xclang <arg>         Pass <arg> to the clang compiler

The /fallback Option

When clang-cl is run with the /fallback option, it will first try to compile files itself. For any file that it fails to compile, it will fall back and try to compile the file by invoking cl.exe.

This option is intended to be used as a temporary means to build projects where clang-cl cannot successfully compile all the files. clang-cl may fail to compile a file either because it cannot generate code for some C++ feature, or because it cannot parse some Microsoft language extension.

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