# Clang Compiler User's Manual

- Introduction
  - Terminology
  - Basic Usage
- Command Line Options
  - Options to Control Error and Warning Messages
- Language and Target-Independent Features
  - Controlling Errors and Warnings
  - Controlling How Clang Displays Diagnostics
  - Diagnostic Mappings
  - Controlling Diagnostics via Command Line Flags
  - Controlling Diagnostics via Pragmas
  - Precompiled Headers
  - Controlling Code Generation
- C Language Features
  - Extensions supported by clang
  - Differences between various standard modes
  - GCC extensions not implemented yet
  - Intentionally unsupported GCC extensions
  - Microsoft extensions
- Objective-C Language Features
  - Intentional Incompatibilities with GCC
- C++ Language Features
- Objective C++ Language Features
- Target-Specific Features and Limitations
  - CPU Architectures Features and Limitations
    - X86
    - ARM
    - Other platforms
  - Operating System Features and Limitations
    - Darwin (Mac OS/X)
    - Linux, etc.

## 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:

- C Language: K&R C, ANSI C89, ISO C90, ISO C94 (C89+AMD1), ISO C99 (+TC1, TC2, TC3).
- Objective-C Language: ObjC 1, ObjC 2, ObjC 2.1, plus variants depending on base language.
- C++ Language Features
- Objective C++ Language

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:

- Ignored
- Note
- Warning
- Error
- Fatal

#### 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.

### 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 the check fails, `__builtin_trap()` is used to indicate failure. The checks are:

  - Subscripting where the static type of one operand is 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.
-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 do not alias any other pointer when the function returns.

## 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:

- 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.

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.

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 __label__ (bug 3429). This is a relatively small feature, so it is likely to be implemented relatively soon.
- clang does not support attributes on function pointers (bug 2461). This is a relatively important feature, so it is likely to be implemented relatively soon.
- 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 #pragma align (bug 3811). This is a relatively small feature, so it is likely to be implemented relatively soon.
- clang does not support code generation for local variables pinned to registers (bug 3933). This is a relatively small feature, so it is likely to be implemented relatively soon.
- 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.
- 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 showed 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 of reasons: one, it is tricky to implement, two, the extension is completely undocumented, and three, the extension appears to be rarely used.
- clang does not support duplicate definitions of a function where one is inline. This complicates clients of the AST which normally can expect there is at most one definition for each function. Source code using this feature should be changed to define the inline and out-of-line definitions in separate translation units.
- 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, or calls to C library functions like strlen.
- clang does not support multiple alternative constraints in inline asm; this is an extremely obscure feature which would be complicated to implement correctly.
- 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; 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 production-quality and is not recommended for use beyond experimentation. 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 released Clang compilers will refuse to even try to use clang to compile C++ code unless you pass the `-ccc-clang-cxx` option to the driver. To turn on Clang's C++ support, please pass that flag. Clang compilers built from the Subversion trunk enable C++ support by default, and do not require the `-ccc-clang-cxx` 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.