Clang Language Extensions

Introduction

This document describes the language extensions provided by Clang. In addition to the language extensions listed here, Clang aims to support a broad range of GCC extensions. Please see the GCC manual for more information on these extensions.

Feature Checking Macros

Language extensions can be very useful, but only if you know you can depend on them. In order to allow fine-grain features checks, we support two builtin function-like macros. This allows you to directly test for a feature in your code without having to resort to something like autoconf or fragile "compiler version checks".

__has_builtin

This function-like macro takes a single identifier argument that is the name of a builtin function. It evaluates to 1 if the builtin is supported or 0 if not. It can be used like this:

#ifndef __has_builtin         // Optional of course.
  #define __has_builtin(x) 0  // Compatibility with non-clang compilers.
#endif

...
#if __has_builtin(__builtin_trap)
  __builtin_trap();
#else
  abort();
#endif
...

__has_feature

This function-like macro takes a single identifier argument that is the name of a feature. It evaluates to 1 if the feature is supported or 0 if not. It can be used like this:

#ifndef __has_feature         // Optional of course.
  #define __has_feature(x) 0  // Compatibility with non-clang compilers.
#endif

...
#if __has_feature(attribute_overloadable) || \
    __has_feature(blocks)
...
#endif
...

The feature tag is described along with the language feature below.

__has_attribute

This function-like macro takes a single identifier argument that is the name of an attribute. It evaluates to 1 if the attribute is supported or 0 if not. It can be used like this:

#ifndef __has_attribute         // Optional of course.
  #define __has_attribute(x) 0  // Compatibility with non-clang compilers.
#endif

...
#if __has_attribute(always_inline)
#define ALWAYS_INLINE __attribute__((always_inline))
#else
#define ALWAYS_INLINE
#endif
...

Include File Checking Macros

Not all developments systems have the same include files. The __has_include and __has_include_next macros allow you to check for the existence of an include file before doing a possibly failing #include directive.

__has_include

This function-like macro takes a single file name string argument that is the name of an include file. It evaluates to 1 if the file can be found using the include paths, or 0 otherwise:

// Note the two possible file name string formats.
#if __has_include("myinclude.h") && __has_include(<stdint.h>)
# include "myinclude.h"
#endif

// To avoid problem with non-clang compilers not having this macro.
#if defined(__has_include) && __has_include("myinclude.h")
# include "myinclude.h"
#endif

To test for this feature, use #if defined(__has_include).

__has_include_next

This function-like macro takes a single file name string argument that is the name of an include file. It is like __has_include except that it looks for the second instance of the given file found in the include paths. It evaluates to 1 if the second instance of the file can be found using the include paths, or 0 otherwise:

// Note the two possible file name string formats.
#if __has_include_next("myinclude.h") && __has_include_next(<stdint.h>)
# include_next "myinclude.h"
#endif

// To avoid problem with non-clang compilers not having this macro.
#if defined(__has_include_next) && __has_include_next("myinclude.h")
# include_next "myinclude.h"
#endif

Note that __has_include_next, like the GNU extension #include_next directive, is intended for use in headers only, and will issue a warning if used in the top-level compilation file. A warning will also be issued if an absolute path is used in the file argument.

Builtin Macros

__BASE_FILE__
Defined to a string that contains the name of the main input file passed to Clang.
__COUNTER__
Defined to an integer value that starts at zero and is incremented each time the __COUNTER__ macro is expanded.
__INCLUDE_LEVEL__
Defined to an integral value that is the include depth of the file currently being translated. For the main file, this value is zero.
__TIMESTAMP__
Defined to the date and time of the last modification of the current source file.
__clang__
Defined when compiling with Clang
__clang_major__
Defined to the major version number of Clang (e.g., the 2 in 2.0.1).
__clang_minor__
Defined to the minor version number of Clang (e.g., the 0 in 2.0.1).
__clang_patchlevel__
Defined to the patch level of Clang (e.g., the 1 in 2.0.1).
__clang_version__
Defined to a string that captures the Clang version, including the Subversion tag or revision number, e.g., "1.5 (trunk 102332)".

Vectors and Extended Vectors

Supports the GCC vector extensions, plus some stuff like V[1].

Also supports ext_vector, which additionally support for V.xyzw syntax and other tidbits as seen in OpenCL. An example is:

typedef float float4 __attribute__((ext_vector_type(4)));
typedef float float2 __attribute__((ext_vector_type(2)));

float4 foo(float2 a, float2 b) {
  float4 c;
  c.xz = a;
  c.yw = b;
  return c;
}

Query for this feature with __has_feature(attribute_ext_vector_type).

See also __builtin_shufflevector.

Messages on deprecated and unavailable Attributes

An optional string message can be added to the deprecated and unavailable attributes. For example:

void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));

If the deprecated or unavailable declaration is used, the message will be incorporated into the appropriate diagnostic:

harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!! [-Wdeprecated-declarations]
  explode();
  ^

Query for this feature with __has_feature(attribute_deprecated_with_message) and __has_feature(attribute_unavailable_with_message).

Attributes on Enumerators

Clang allows attributes to be written on individual enumerators. This allows enumerators to be deprecated, made unavailable, etc. The attribute must appear after the enumerator name and before any initializer, like so:

enum OperationMode {
  OM_Invalid,
  OM_Normal,
  OM_Terrified __attribute__((deprecated)),
  OM_AbortOnError __attribute__((deprecated)) = 4
};

Attributes on the enum declaration do not apply to individual enumerators.

Query for this feature with __has_feature(enumerator_attributes).

Checks for Standard Language Features

The __has_feature macro can be used to query if certain standard language features are enabled. Those features are listed here.

C++ exceptions

Use __has_feature(cxx_exceptions) to determine if C++ exceptions have been enabled. For example, compiling code with -fexceptions enables C++ exceptions.

C++ RTTI

Use __has_feature(cxx_rtti) to determine if C++ RTTI has been enabled. For example, compiling code with -fno-rtti disables the use of RTTI.

Checks for Upcoming Standard Language Features

The __has_feature macro can be used to query if certain upcoming standard language features are enabled. Those features are listed here.

Currently, all features listed here are slated for inclusion in the upcoming C++0x standard. As a result, all the features that clang supports are enabled with the -std=c++0x option when compiling C++ code. Features that are not yet implemented will be noted.

C++0x decltype()

Use __has_feature(cxx_decltype) to determine if support for the decltype() specifier is enabled.

C++0x attributes

Use __has_feature(cxx_attributes) to determine if support for attribute parsing with C++0x's square bracket notation is enabled.

C++0x default template arguments in function templates

Use __has_feature(cxx_default_function_template_args) to determine if support for default template arguments in function templates is enabled.

C++0x deleted functions

Use __has_feature(cxx_deleted_functions) to determine if support for deleted function definitions (with = delete) is enabled.

C++0x lambdas

Use __has_feature(cxx_lambdas) to determine if support for lambdas is enabled. clang does not currently implement this feature.

C++0x nullptr

Use __has_feature(cxx_nullptr) to determine if support for nullptr is enabled. clang does not yet fully implement this feature.

C++0x reference-qualified functions

Use __has_feature(cxx_reference_qualified_functions) to determine if support for reference-qualified functions (e.g., member functions with & or && applied to *this) is enabled.

C++0x rvalue references

Use __has_feature(cxx_rvalue_references) to determine if support for rvalue references is enabled.

C++0x static_assert()

Use __has_feature(cxx_static_assert) to determine if support for compile-time assertions using static_assert is enabled.

C++0x type inference

Use __has_feature(cxx_auto_type) to determine C++0x type inference is supported using the auto specifier. If this is disabled, auto will instead be a storage class specifier, as in C or C++98.

C++0x variadic templates

Use __has_feature(cxx_variadic_templates) to determine if support for variadic templates is enabled.

C++0x inline namespaces

Use __has_feature(cxx_inline_namespaces) to determine if support for inline namespaces is enabled.

C++0x trailing return type

Use __has_feature(cxx_trailing_return) to determine if support for the alternate function declaration syntax with trailing return type is enabled.

C++0x strongly typed enumerations

Use __has_feature(cxx_strong_enums) to determine if support for strongly typed, scoped enumerations is enabled.

Checks for Type Traits

Clang supports the GNU C++ type traits and a subset of the Microsoft Visual C++ Type traits. For each supported type trait __X, __has_feature(X) indicates the presence of the type trait. For example: 

#if __has_feature(is_convertible_to)
template<typename From, typename To>
struct is_convertible_to {
  static const bool value = __is_convertible_to(From, To);
};
#else
// Emulate type trait
#endif

The following type traits are supported by Clang:

Blocks

The syntax and high level language feature description is in BlockLanguageSpec.txt. Implementation and ABI details for the clang implementation are in Block-ABI-Apple.txt.

Query for this feature with __has_feature(blocks).

Function Overloading in C

Clang provides support for C++ function overloading in C. Function overloading in C is introduced using the overloadable attribute. For example, one might provide several overloaded versions of a tgsin function that invokes the appropriate standard function computing the sine of a value with float, double, or long double precision:

#include <math.h>
float __attribute__((overloadable)) tgsin(float x) { return sinf(x); }
double __attribute__((overloadable)) tgsin(double x) { return sin(x); }
long double __attribute__((overloadable)) tgsin(long double x) { return sinl(x); }

Given these declarations, one can call tgsin with a float value to receive a float result, with a double to receive a double result, etc. Function overloading in C follows the rules of C++ function overloading to pick the best overload given the call arguments, with a few C-specific semantics:

The declaration of overloadable functions is restricted to function declarations and definitions. Most importantly, if any function with a given name is given the overloadable attribute, then all function declarations and definitions with that name (and in that scope) must have the overloadable attribute. This rule even applies to redeclarations of functions whose original declaration had the overloadable attribute, e.g.,

int f(int) __attribute__((overloadable));
float f(float); // error: declaration of "f" must have the "overloadable" attribute

int g(int) __attribute__((overloadable));
int g(int) { } // error: redeclaration of "g" must also have the "overloadable" attribute

Functions marked overloadable must have prototypes. Therefore, the following code is ill-formed:

int h() __attribute__((overloadable)); // error: h does not have a prototype

However, overloadable functions are allowed to use a ellipsis even if there are no named parameters (as is permitted in C++). This feature is particularly useful when combined with the unavailable attribute:

void honeypot(...) __attribute__((overloadable, unavailable)); // calling me is an error

Functions declared with the overloadable attribute have their names mangled according to the same rules as C++ function names. For example, the three tgsin functions in our motivating example get the mangled names _Z5tgsinf, _Z5tgsind, and _Z5tgsine, respectively. There are two caveats to this use of name mangling:

Query for this feature with __has_feature(attribute_overloadable).

Builtin Functions

Clang supports a number of builtin library functions with the same syntax as GCC, including things like __builtin_nan, __builtin_constant_p, __builtin_choose_expr, __builtin_types_compatible_p, __sync_fetch_and_add, etc. In addition to the GCC builtins, Clang supports a number of builtins that GCC does not, which are listed here.

Please note that Clang does not and will not support all of the GCC builtins for vector operations. Instead of using builtins, you should use the functions defined in target-specific header files like <xmmintrin.h>, which define portable wrappers for these. Many of the Clang versions of these functions are implemented directly in terms of extended vector support instead of builtins, in order to reduce the number of builtins that we need to implement.

__builtin_shufflevector

__builtin_shufflevector is used to express generic vector permutation/shuffle/swizzle operations. This builtin is also very important for the implementation of various target-specific header files like <xmmintrin.h>.

Syntax:

__builtin_shufflevector(vec1, vec2, index1, index2, ...)

Examples:

  // Identity operation - return 4-element vector V1.
  __builtin_shufflevector(V1, V1, 0, 1, 2, 3)

  // "Splat" element 0 of V1 into a 4-element result.
  __builtin_shufflevector(V1, V1, 0, 0, 0, 0)

  // Reverse 4-element vector V1.
  __builtin_shufflevector(V1, V1, 3, 2, 1, 0)

  // Concatenate every other element of 4-element vectors V1 and V2.
  __builtin_shufflevector(V1, V2, 0, 2, 4, 6)

  // Concatenate every other element of 8-element vectors V1 and V2.
  __builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)

Description:

The first two arguments to __builtin_shufflevector are vectors that have the same element type. The remaining arguments are a list of integers that specify the elements indices of the first two vectors that should be extracted and returned in a new vector. These element indices are numbered sequentially starting with the first vector, continuing into the second vector. Thus, if vec1 is a 4-element vector, index 5 would refer to the second element of vec2.

The result of __builtin_shufflevector is a vector with the same element type as vec1/vec2 but that has an element count equal to the number of indices specified.

Query for this feature with __has_builtin(__builtin_shufflevector).

__builtin_unreachable

__builtin_unreachable is used to indicate that a specific point in the program cannot be reached, even if the compiler might otherwise think it can. This is useful to improve optimization and eliminates certain warnings. For example, without the __builtin_unreachable in the example below, the compiler assumes that the inline asm can fall through and prints a "function declared 'noreturn' should not return" warning.

Syntax:

__builtin_unreachable()

Example of Use:

void myabort(void) __attribute__((noreturn));
void myabort(void) {
    asm("int3");
    __builtin_unreachable();
}

Description:

The __builtin_unreachable() builtin has completely undefined behavior. Since it has undefined behavior, it is a statement that it is never reached and the optimizer can take advantage of this to produce better code. This builtin takes no arguments and produces a void result.

Query for this feature with __has_builtin(__builtin_unreachable).

Target-Specific Extensions

Clang supports some language features conditionally on some targets.

X86/X86-64 Language Extensions

The X86 backend has these language extensions:

Memory references off the GS segment

Annotating a pointer with address space #256 causes it to be code generated relative to the X86 GS segment register, and address space #257 causes it to be relative to the X86 FS segment. Note that this is a very very low-level feature that should only be used if you know what you're doing (for example in an OS kernel).

Here is an example:

#define GS_RELATIVE __attribute__((address_space(256)))
int foo(int GS_RELATIVE *P) {
  return *P;
}

Which compiles to (on X86-32):

_foo:
	movl	4(%esp), %eax
	movl	%gs:(%eax), %eax
	ret

Static Analysis-Specific Extensions

Clang supports additional attributes that are useful for documenting program invariants and rules for static analysis tools. The extensions documented here are used by the path-sensitive static analyzer engine that is part of Clang's Analysis library.

Analyzer Attributes

analyzer_noreturn

Clang's static analysis engine understands the standard noreturn attribute. This attribute, which is typically affixed to a function prototype, indicates that a call to a given function never returns. Function prototypes for common functions like exit are typically annotated with this attribute, as well as a variety of common assertion handlers. Users can educate the static analyzer about their own custom assertion handles (thus cutting down on false positives due to false paths) by marking their own "panic" functions with this attribute.

While useful, noreturn is not applicable in all cases. Sometimes there are special functions that for all intents and purposes should be considered panic functions (i.e., they are only called when an internal program error occurs) but may actually return so that the program can fail gracefully. The analyzer_noreturn attribute allows one to annotate such functions as being interpreted as "no return" functions by the analyzer (thus pruning bogus paths) but will not affect compilation (as in the case of noreturn).

Usage: The analyzer_noreturn attribute can be placed in the same places where the noreturn attribute can be placed. It is commonly placed at the end of function prototypes:

  void foo() __attribute__((analyzer_noreturn));

Query for this feature with __has_feature(attribute_analyzer_noreturn).

Objective-C retaining behavior attributes

In Objective-C, functions and methods are generally assumed to take and return objects with +0 retain counts, with some exceptions for special methods like +alloc and init. However, there are exceptions, and so Clang provides attributes to allow these exceptions to be documented, which helps the analyzer find leaks (and ignore non-leaks).

Usage: The ns_returns_retained, ns_returns_not_retained, ns_returns_autoreleased, cf_returns_retained, and cf_returns_not_retained attributes can be placed on methods and functions that return Objective-C or CoreFoundation objects. They are commonly placed at the end of a function prototype or method declaration:

  id foo() __attribute__((ns_returns_retained));

  - (NSString*) bar: (int) x __attribute__((ns_returns_retained));

The *_returns_retained attributes specify that the returned object has a +1 retain count. The *_returns_not_retained attributes specify that the return object has a +0 retain count, even if the normal convention for its selector would be +1. ns_returns_autoreleased specifies that the returned object is +0, but is guaranteed to live at least as long as the next flush of an autorelease pool.

Usage: The ns_consumed and cf_consumed attributes can be placed on an parameter declaration; they specify that the argument is expected to have a +1 retain count, which will be balanced in some way by the function or method. The ns_consumes_self attribute can only be placed on an Objective-C method; it specifies that the method expects its self parameter to have a +1 retain count, which it will balance in some way.

  void foo(__attribute__((ns_consumed)) NSString *string);

  - (void) bar __attribute__((ns_consumes_self));
  - (void) baz: (id) __attribute__((ns_consumed)) x;