Clang Compiler User's Manual

Introduction

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 the Clang 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".

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.

Terminology

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 C99 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

-Werror: Turn warnings into errors.

-Werror=foo: Turn warning "foo" into an error.

-Wno-error=foo: Turn warning "foo" into an warning even if -Werror is specified.

-Wfoo: Enable warning foo

-Wno-foo: Disable warning foo

-w: Disable all warnings.

-pedantic: Warn on language extensions.

-pedantic-errors: Error on language extensions.

-Wsystem-headers: Enable warnings from system headers.

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

-ftemplate-backtrace-limit=123: 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.

-f[no-]show-column: 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.

-f[no-]show-source-location: 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.

-f[no-]caret-diagnostics: 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
         ^
         //
-f[no-]color-diagnostics:
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
         ^
         //
-fdiagnostics-format=clang/msvc/vi: 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'
msvc
t.c(3,11) : warning: conversion specifies type 'char *' but the argument has type 'int'
vi
t.c +3:11: warning: conversion specifies type 'char *' but the argument has type 'int'
-f[no-]diagnostics-show-name: Enable the display of the diagnostic name.
This option, which defaults to off, controls whether or not Clang prints the associated name.

-f[no-]diagnostics-show-option: 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.

-fdiagnostics-show-category=none/id/name: 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.

-f[no-]diagnostics-fixit-info: 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.

-f[no-]diagnostics-print-source-range-info: Print machine parsable information about source ranges.
This option, which defaults to off, controls whether or not Clang prints 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.

-fdiagnostics-parseable-fixits: 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:

 fix-it:"t.cpp":{7:25-7:29}:"Gamma"

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").

Individual Warning Groups

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

-Wextra-tokens: 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.

This option is also enabled by -Wfoo, -Wbar, and -Wbaz.

-Wambiguous-member-template: 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;
  v.set<double>(3.2);
}

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.

-Wbind-to-temporary-copy: Warn about an unusable copy constructor when binding a reference to a temporary.
This option, which defaults to on, enables warnings about binding a reference to a temporary when the temporary doesn't have a usable copy constructor. For example: 
  struct NonCopyable {
    NonCopyable();
  private:
    NonCopyable(const NonCopyable&);
  };
  void foo(const NonCopyable&);
  void bar() {
    foo(NonCopyable());  // Disallowed in C++98; allowed in C++11.
  }

  struct NonCopyable2 {
    NonCopyable2();
    NonCopyable2(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.

-fno-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.

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:

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

-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.

Enabling All Warnings

In addition to the traditional -W flags, one can enable all warnings 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. This can be done in two ways:

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.

NOTE: 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. Relocatable precompiled headers also have some performance impact, because the difference in location between the header locations at PCH build time vs. at the time of PCH use requires one of the PCH optimizations, stat() caching, to be disabled. However, this change is only likely to affect PCH files that reference a large number of headers.

Controlling Code Generation

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

-fcatch-undefined-behavior: Turn on runtime code generation to check for undefined behavior.
This option, which defaults to off, controls whether or not Clang adds runtime checks for undefined runtime behavior. If a check fails, __builtin_trap() is used to indicate failure. The checks are:
  • Subscripting where the static type of one operand is a variable which is decayed from an array type and the other operand is greater than the size of the array or less than zero.
  • Shift operators where the amount shifted is greater or equal to the promoted bit-width of the left-hand-side or less than zero.
  • If control flow reaches __builtin_unreachable.
  • When llvm implements more __builtin_object_size support, reads and writes for objects that __builtin_object_size indicates we aren't accessing valid memory. Bit-fields and vectors are not yet checked.
-f[no-]address-sanitizer: Turn on AddressSanitizer, a memory error detector.
-f[no-]thread-sanitizer: Turn on ThreadSanitizer, an experimental data race detector. Not ready for widespread use.
-fno-assume-sane-operator-new: 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.
-ftrap-function=[name]: 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.

C Language Features

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

Extensions supported by clang

See clang language extensions.

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 and various aliases for those modes. If no -std option is specified, clang defaults to gnu99 mode.

Differences between all c* and gnu* modes:

Differences between *89 and *99 modes:

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:

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

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; enabling Microsoft extensions will silently drop certain constructs (including __declspec and Microsoft-style asm statements).

C++ Language Features

clang fully implements all of standard C++98 except for exported templates (which were removed in C++11), and many C++11 features are also implemented.

Controlling implementation limits

-fconstexpr-depth=N: Sets the limit for recursive constexpr function invocations to N. The default is 512.

-ftemplate-depth=N: Sets the limit for recursively nested template instantiations to N. The default is 1024.

Target-Specific Features and Limitations

CPU Architectures Features and Limitations

X86

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 to Microsoft x64 calling conversion. You might need to tweak WinX86_64ABIInfo::classify() in lib/CodeGen/TargetInfo.cpp.

ARM

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.

Other platforms

clang currently contains some support for PPC and 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)

No __thread support, 64-bit ObjC support requires SL tools.

Windows

Experimental supports are on Cygming.

Cygwin

Clang works on Cygwin-1.7.

MinGW32

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

On MSYS, a few tests might fail.

MinGW-w64

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

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.