# 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 “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”.

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.

-Weverything

Enable all warnings.

-pedantic

Warn on language extensions.

-pedantic-errors

Error on language extensions.

-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]
^
//

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.

-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]
^
//

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]
^
//
-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]
^
//


When this is disabled, Clang will just print:

test.c:2:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
^
//
-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]
^
//

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]
^
//

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.

-fdiagnostics-print-source-range-info

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.

-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”).

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

-fno-elide-type

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:

t.cc:4:5: note: candidate function not viable: no known conversion from 'vector<map<[...], map<float, [...]>>>' to 'vector<map<[...], map<double, [...]>>>' for 1st argument;

-fno-elide-type:

t.cc:4:5: 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;
-fdiagnostics-show-template-tree

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:

t.cc:4:5: note: candidate function not viable: no known conversion from 'vector<map<[...], map<float, [...]>>>' to 'vector<map<[...], map<double, [...]>>>' for 1st argument;
t.cc:4:5: note: candidate function not viable: no known conversion for 1st argument;
vector<
map<
[...],
map<
[float != float],
[...]>>>

#### 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]
^

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

-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].

#### Diagnostic Mappings¶

All diagnostics are mapped into one of these 5 classes:

• Ignored
• Note
• 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

char b = 'ab'; // no warning

The -isystem-prefix and -ino-system-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 -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.

### Controlling Code Generation¶

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

-fsanitize=check1,check2,...

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=init-order: Make AddressSanitizer check for dynamic initialization order problems. Implied by -fsanitize=address.

• -fsanitize=integer: Enables checks for undefined or suspicious integer behavior.

• -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 are generally used in conjunction with the -fsanitize-undefined-trap-on-error flag, which causes traps to be emitted, rather than calls to runtime libraries. This includes all of the checks listed below other than unsigned-integer-overflow and vptr.

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=integer-divide-by-zero: Integer division by zero.
• -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=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.

• -fsanitize=use-after-return: Check for use-after-return errors (accessing local variable after the function exit).
• -fsanitize=use-after-scope: Check for use-after-scope errors (accesing local variable after it went out of scope).

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

• -fsanitize-memory-track-origins: 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.

The -fsanitize= argument must also be provided when linking, in order to link to the appropriate runtime library. It is not possible to combine the -fsanitize=address and -fsanitize=thread checkers in the same program.

-fcatch-undefined-behavior

Deprecated synonym for -fsanitize=undefined.

-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.

-ftls-model=[model]

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.

### 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.

-g0

Don’t generate any debug info (default).

-gline-tables-only

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).

-g

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 /*.

## 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 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.
• 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.

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
};
...
local_function(1);

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

clang has a -fms-compatibility flag that makes clang accept enough invalid C++ to be able to parse most Microsoft headers. This flag is enabled by default for Windows targets.

-fdelayed-template-parsing lets clang delay all template instantiation 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 1300 which is the same as Visual C/C++ 2003. Any number is supported and can greatly affect what Windows SDK and c++stdlib headers clang can compile. This option will be removed when clang supports the full set of MS extensions required for these headers.
• 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 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 many C++11 features are also implemented.

### Controlling implementation limits¶

-fbracket-depth=N

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

-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¶

None

#### 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;

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

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;

• 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.