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.

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
         ^
         //

When this is disabled, Clang will just print:

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

-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.

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.

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. The line of source code that the issue occurs on, along with a caret and ranges that indicate the important locations [-fcaret-diagnostics].
  6. "FixIt" information, which is a concise explanation of how to fix the problem (when Clang is certain it knows) [-fdiagnostics-fixit-info].
  7. 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:

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 of 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.

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 a 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.

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).

  • 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.

    • Objective-C Language Features

    Intentional Incompatibilities with GCC

    No cast of super, no lvalue casts.

    C++ Language Features

    At this point, Clang C++ is not generally useful. However, Clang C++ support is under active development and is progressing rapidly. Please see the C++ Status page for details or ask on the mailing list about how you can help.

    Note that the clang driver will refuse to even try to use clang to compile C++ code unless you pass the -ccc-clang-cxx option to the driver. If you really want to play with Clang's C++ support, please pass that flag.

    Objective C++ Language Features

    At this point, Clang C++ support is not generally useful (and therefore, neither is Objective-C++). Please see the C++ section for more information.

    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 large C and Objective-C codebases. (FIXME: Anything specific we want to say here? Possibly mention some LLVM x86 limitations?)

    ARM

    ARM support is mostly feature-complete, but still experimental; it hasn't undergone significant testing.

    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 some support for the embedded PIC16 processor (FIXME: I haven't been keeping track of this; what should this say?).

    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.