# Clang Language Extensions

- Introduction
- Feature Checking Macros
- Include File Checking Macros
- Builtin Macros
- Vectors and Extended Vectors
- Messages on `deprecated` and `unavailable` attributes
- Attributes on enumerators
- Checks for Standard Language Features
  - C++ exceptions
  - C++ RTTI
- Checks for Upcoming Standard Language Features
  - C++11
    - C++11 SFINAE includes access control
    - C++11 alias templates
    - C++11 alignment specifiers
    - C++11 attributes
    - C++11 generalized constant expressions
    - C++11 `decltype()`
    - C++11 default template arguments in function templates
    - C++11 delegating constructors
    - C++11 deleted functions
    - C++11 explicit conversion functions
    - C++11 generalized initializers
    - C++11 implicit move constructors/assignment operators
    - C++11 inheriting constructors
    - C++11 inline namespaces
    - C++11 lambdas
    - C++11 noexcept specification
    - C++11 in-class non-static data member initialization
    - C++11 nullptr
    - C++11 override control
    - C++11 range-based for loop
    - C++11 raw string literals
    - C++11 rvalue references
    - C++11 reference-qualified functions
    - C++11 `static_assert()`
    - C++11 type inference
    - C++11 strongly-typed enumerations
    - C++11 trailing return type
    - C++11 Unicode string literals
    - C++11 unrestricted unions
    - C++11 user-defined literals
    - C++11 variadic templates
  - C1X
    - C1X alignment specifiers
    - C1X generic selections
    - C1X `_Static_assert()`
- Checks for Type Traits
- Blocks
- Objective-C Features
  - Related result types
  - Automatic reference counting
  - Enumerations with a fixed underlying type
- Function Overloading in C
- Initializer lists for complex numbers in C
- Builtin Functions
  - __builtin_shufflevector
  - __builtin_unreachable
  - __sync_swap
- Target-Specific Extensions
  - X86/X86-64 Language Extensions
- Static Analysis-Specific Extensions
- Thread Safety Annotation Checking

- `no_thread_safety_analysis`
- `lockable`
- `scoped_lockable`
- `guarded_var`
- `pt_guarded_var`
- `guarded_by(l)`
- `pt_guarded_by(l)`
- `acquired_before(...)`
- `acquired_after(...)`
- `exclusive_lock_function(...)`
- `shared_lock_function(...)`
- `exclusive_trylock_function(...)`
- `shared_trylock_function(...)`
- `unlock_function(...)`
- `lock_returned(l)`
- `locks_excluded(...)`
- `exclusive_locks_required(...)`
- `shared_locks_required(...)`

## 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 three 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 and __has_extension

These function-like macros take a single identifier argument that is the name of a feature. `__has_feature` evaluates to 1 if the feature is both supported by Clang and standardized in the current language standard or 0 if not (but see below), while `__has_extension` evaluates to 1 if the feature is supported by Clang in the current language (either as a language extension or a standard language feature) or 0 if not. They can be used like this:

>     #ifndef __has_feature         // Optional of course.
>       #define __has_feature(x) 0  // Compatibility with non-clang compilers.
>     #endif
>     #ifndef __has_extension
>       #define __has_extension __has_feature // Compatibility with pre-3.0 compilers.
>     #endif

>     ...
>     #if __has_feature(cxx_rvalue_references)
>     // This code will only be compiled with the -std=c++11 and -std=gnu++11
>     // options, because rvalue references are only standardized in C++11.
>     #endif

>     #if __has_extension(cxx_rvalue_references)
>     // This code will be compiled with the -std=c++11, -std=gnu++11, -std=c++98
>     // and -std=gnu++98 options, because rvalue references are supported as a
>     // language extension in C++98.
>     #endif

For backwards compatibility reasons, `__has_feature` can also be used to test for support for non-standardized features, i.e. features not prefixed `c_`, `cxx_` or `objc_`.

If the `-pedantic-errors` option is given, `__has_extension` is equivalent to `__has_feature`.

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.

### __has_warning

This function-like macro takes a string literal that represents a command line option for a warning and returns true if that is a valid warning option.

>     #if __has_warning("-Wformat")
>     ...
>     #endif

## 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_extension(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_extension(attribute_deprecated_with_message)` and `__has_extension(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_extension(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` or `__has_extension` macros can be used to query if certain upcoming standard language features are enabled. Those features are listed here. Features that are not yet implemented will be noted.

### C++11

The features listed below are slated for inclusion in the upcoming C++11 standard. As a result, all these features are enabled with the `-std=c++11` option when compiling C++ code.

#### C++11 SFINAE includes access control

Use `__has_feature(cxx_access_control_sfinae)` or `__has_extension(cxx_access_control_sfinae)` to determine whether access-control errors (e.g., calling a private constructor) are considered to be template argument deduction errors (aka SFINAE errors), per C++ DR1170.

#### C++11 alias templates

Use `__has_feature(cxx_alias_templates)` or `__has_extension(cxx_alias_templates)` to determine if support for C++11's alias declarations and alias templates is enabled.

#### C++11 alignment specifiers

Use `__has_feature(cxx_alignas)` or `__has_extension(cxx_alignas)` to determine if support for alignment specifiers using `alignas` is enabled.

#### C++11 attributes

Use `__has_feature(cxx_attributes)` or `__has_extension(cxx_attributes)` to determine if support for attribute parsing with C++11's square bracket notation is enabled.

#### C++11 generalized constant expressions

Use `__has_feature(cxx_constexpr)` to determine if support for generalized constant expressions (e.g., `constexpr`) is enabled. Clang does not currently implement this feature.

#### C++11 `decltype()`

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

#### C++11 default template arguments in function templates

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

#### C++11 delegating constructors

Use `__has_feature(cxx_delegating_constructors)` to determine if support for delegating constructors is enabled.

#### C++11 `delete`d functions

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

#### C++11 explicit conversion functions

Use `__has_feature(cxx_explicit_conversions)` to determine if support for `explicit` conversion functions is enabled.

#### C++11 generalized initializers

Use `__has_feature(cxx_generalized_initializers)` to determine if support for generalized initializers (using braced lists and `std::initializer_list`) is enabled. Clang does not currently implement this feature.

#### C++11 implicit move constructors/assignment operators

Use `__has_feature(cxx_implicit_moves)` to determine if Clang will implicitly generate move constructors and move assignment operators where needed.

#### C++11 inheriting constructors

Use `__has_feature(cxx_inheriting_constructors)` to determine if support for inheriting constructors is enabled. Clang does not currently implement this feature.

#### C++11 inline namespaces

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

#### C++11 lambdas

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

#### C++11 noexcept

Use `__has_feature(cxx_noexcept)` or `__has_extension(cxx_noexcept)` to determine if support for noexcept exception specifications is enabled.

#### C++11 in-class non-static data member initialization

Use `__has_feature(cxx_nonstatic_member_init)` to determine whether in-class initialization of non-static data members is enabled.

#### C++11 `nullptr`

Use `__has_feature(cxx_nullptr)` or `__has_extension(cxx_nullptr)` to determine if support for `nullptr` is enabled.

#### C++11 `override control`

Use `__has_feature(cxx_override_control)` or `__has_extension(cxx_override_control)` to determine if support for the override control keywords is enabled.

#### C++11 reference-qualified functions

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

#### C++11 range-based `for` loop

Use `__has_feature(cxx_range_for)` or `__has_extension(cxx_range_for)` to determine if support for the range-based for loop is enabled.

#### C++11 raw string literals

Use `__has_feature(cxx_raw_string_literals)` to determine if support for raw string literals (e.g., `R"foo\bar"`) is enabled.

#### C++11 rvalue references

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

#### C++11 `static_assert()`

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

#### C++11 type inference

Use `__has_feature(cxx_auto_type)` or `__has_extension(cxx_auto_type)` to determine C++11 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++11 strongly typed enumerations

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

#### C++11 trailing return type

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

#### C++11 Unicode string literals

Use `__has_feature(cxx_unicode_literals)` to determine if support for Unicode string literals is enabled.

#### C++11 unrestricted unions

Use `__has_feature(cxx_unrestricted_unions)` to determine if support for unrestricted unions is enabled. Clang does not currently support this feature.

#### C++11 user-defined literals

Use `__has_feature(cxx_user_literals)` to determine if support for user-defined literals is enabled. Clang does not currently support this feature.

#### C++11 variadic templates

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

### C1X

The features listed below are slated for inclusion in the upcoming C1X standard. As a result, all these features are enabled with the `-std=c1x` option when compiling C code.

#### C1X alignment specifiers

Use `__has_feature(c_alignas)` or `__has_extension(c_alignas)` to determine if support for alignment specifiers using `_Alignas` is enabled.

#### C1X generic selections

Use `__has_feature(c_generic_selections)` or `__has_extension(c_generic_selections)` to determine if support for generic selections is enabled.

As an extension, the C1X generic selection expression is available in all languages supported by Clang. The syntax is the same as that given in the C1X draft standard.

In C, type compatibility is decided according to the rules given in the appropriate standard, but in C++, which lacks the type compatibility rules used in C, types are considered compatible only if they are equivalent.

#### C1X `_Static_assert()`

Use `__has_feature(c_static_assert)` or `__has_extension(c_static_assert)` to determine if support for compile-time assertions using `_Static_assert` 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_extension(X)` indicates the presence of the type trait. For example:

>     #if __has_extension(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:

- `__has_nothrow_assign` (GNU, Microsoft)
- `__has_nothrow_copy` (GNU, Microsoft)
- `__has_nothrow_constructor` (GNU, Microsoft)
- `__has_trivial_assign` (GNU, Microsoft)
- `__has_trivial_copy` (GNU, Microsoft)
- `__has_trivial_constructor` (GNU, Microsoft)
- `__has_trivial_destructor` (GNU, Microsoft)
- `__has_virtual_destructor` (GNU, Microsoft)
- `__is_abstract` (GNU, Microsoft)
- `__is_base_of` (GNU, Microsoft)
- `__is_class` (GNU, Microsoft)
- `__is_convertible_to` (Microsoft)
- `__is_empty` (GNU, Microsoft)
- `__is_enum` (GNU, Microsoft)
- `__is_pod` (GNU, Microsoft)
- `__is_polymorphic` (GNU, Microsoft)
- `__is_union` (GNU, Microsoft)
- `__is_literal(type)`: Determines whether the given type is a literal type
- `__underlying_type(type)`: Retrieves the underlying type for a given `enum` type. This trait is required to implement the C++11 standard library.

## 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_extension(blocks).

## Objective-C Features

### Related result types

According to Cocoa conventions, Objective-C methods with certain names ("init", "alloc", etc.) always return objects that are an instance of the receiving class's type. Such methods are said to have a "related result type", meaning that a message send to one of these methods will have the same static type as an instance of the receiver class. For example, given the following classes:

>     @interface NSObject
>     + (id)alloc;
>     - (id)init;
>     @end

>     @interface NSArray : NSObject
>     @end

and this common initialization pattern

>     NSArray *array = [[NSArray alloc] init];

the type of the expression `[NSArray alloc]` is `NSArray*` because `alloc` implicitly has a related result type. Similarly, the type of the expression `[[NSArray alloc] init]` is `NSArray*`, since `init` has a related result type and its receiver is known to have the type `NSArray *`. If neither `alloc` nor `init` had a related result type, the expressions would have had type `id`, as declared in the method signature.

A method with a related result type can be declared by using the type `instancetype` as its result type. `instancetype` is a contextual keyword that is only permitted in the result type of an Objective-C method, e.g.

    @interface A
    + (instancetype)constructAnA;
    @end

The related result type can also be inferred for some methods. To determine whether a method has an inferred related result type, the first word in the camel-case selector (e.g., "init" in "initWithObjects") is considered, and the method will a related result type if its return type is compatible with the type of its class and if

- the first word is "alloc" or "new", and the method is a class method, or
- the first word is "autorelease", "init", "retain", or "self", and the method is an instance method.

If a method with a related result type is overridden by a subclass method, the subclass method must also return a type that is compatible with the subclass type. For example:

>     @interface NSString : NSObject
>     - (NSUnrelated *)init; // incorrect usage: NSUnrelated is not NSString or a superclass of NSString
>     @end

Related result types only affect the type of a message send or property access via the given method. In all other respects, a method with a related result type is treated the same way as method that returns `id`.

Use `__has_feature(objc_instancetype)` to determine whether the `instancetype` contextual keyword is available.

## Automatic reference counting

Clang provides support for automated reference counting in Objective-C, which eliminates the need for manual retain/release/autorelease message sends. There are two feature macros associated with automatic reference counting: `__has_feature(objc_arc)` indicates the availability of automated reference counting in general, while `__has_feature(objc_arc_weak)` indicates that automated reference counting also includes support for `__weak` pointers to Objective-C objects.

## Enumerations with a fixed underlying type

Clang provides support for C++11 enumerations with a fixed underlying type within Objective-C. For example, one can write an enumeration type as:

    typedef enum : unsigned char { Red, Green, Blue } Color;

This specifies that the underlying type, which is used to store the enumeration value, is `unsigned char`.

Use `__has_feature(objc_fixed_enum)` to determine whether support for fixed underlying types is available in Objective-C.

## 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:

- Conversion from `float` or `double` to `long double` is ranked as a floating-point promotion (per C99) rather than as a floating-point conversion (as in C++).
- A conversion from a pointer of type `T*` to a pointer of type `U*` is considered a pointer conversion (with conversion rank) if `T` and `U` are compatible types.
- A conversion from type `T` to a value of type `U` is permitted if `T` and `U` are compatible types. This conversion is given "conversion" rank.

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:

- Future versions of Clang may change the name mangling of functions overloaded in C, so you should not depend on an specific mangling. To be completely safe, we strongly urge the use of `static inline` with `overloadable` functions.
- The `overloadable` attribute has almost no meaning when used in C++, because names will already be mangled and functions are already overloadable. However, when an `overloadable` function occurs within an `extern "C"` linkage specification, it's name will be mangled in the same way as it would in C.

Query for this feature with __has_extension(attribute_overloadable).

## Initializer lists for complex numbers in C

clang supports an extension which allows the following in C:

>     #include <math.h>
>     #include <complex.h>
>     complex float x = { 1.0f, INFINITY }; // Init to (1, Inf)

This construct is useful because there is no way to separately initialize the real and imaginary parts of a complex variable in standard C, given that clang does not support `_Imaginary`. (clang also supports the `__real__` and `__imag__` extensions from gcc, which help in some cases, but are not usable in static initializers.)

Note that this extension does not allow eliding the braces; the meaning of the following two lines is different:

>     complex float x[] = { { 1.0f, 1.0f } }; // [0] = (1, 1)
>     complex float x[] = { 1.0f, 1.0f }; // [0] = (1, 0), [1] = (1, 0)

This extension also works in C++ mode, as far as that goes, but does not apply to the C++ `std::complex`. (In C++11, list initialization allows the same syntax to be used with `std::complex` with the same meaning.)

## 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).

### __sync_swap

`__sync_swap` is used to atomically swap integers or pointers in memory.

Syntax:

    type __sync_swap(type *ptr, type value, ...)

Example of Use:

    int old_value = __sync_swap(&value, new_value);

Description:

The __sync_swap() builtin extends the existing __sync_*() family of atomic intrinsics to allow code to atomically swap the current value with the new value. More importantly, it helps developers write more efficient and correct code by avoiding expensive loops around __sync_bool_compare_and_swap() or relying on the platform specific implementation details of __sync_lock_test_and_set(). The __sync_swap() builtin is a full barrier.

## 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.

### The `analyzer_noreturn` attribute

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_attribute(analyzer_noreturn)`.

### The `objc_method_family` attribute

Many methods in Objective-C have conventional meanings determined by their selectors. For the purposes of static analysis, it is sometimes useful to be able to mark a method as having a particular conventional meaning despite not having the right selector, or as not having the conventional meaning that its selector would suggest. For these use cases, we provide an attribute to specifically describe the method family that a method belongs to.

Usage: `__attribute__((objc_method_family(X)))`, where `X` is one of `none`, `alloc`, `copy`, `init`, `mutableCopy`, or `new`. This attribute can only be placed at the end of a method declaration:

      - (NSString*) initMyStringValue __attribute__((objc_method_family(none)));

Users who do not wish to change the conventional meaning of a method, and who merely want to document its non-standard retain and release semantics, should use the retaining behavior attributes described below.

Query for this feature with `__has_attribute(objc_method_family)`.

### 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). Some exceptions may be better described using the `objc_method_family` attribute instead.

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;

Query for these features with `__has_attribute(ns_consumed)`, `__has_attribute(ns_returns_retained)`, etc.

## Thread-Safety Annotation Checking

Clang supports additional attributes for checking basic locking policies in multithreaded programs. Clang currently parses the following list of attributes, although the implementation for these annotations is currently in development. For more details, see the GCC implementation.

#### no_thread_safety_analysis

Use `__attribute__((no_thread_safety_analysis))` on a function declaration to specify that the thread safety analysis should not be run on that function. This attribute provides an escape hatch (e.g. for situations when it is difficult to annotate the locking policy).

#### lockable

Use `__attribute__((lockable))` on a class definition to specify that it has a lockable type (e.g. a Mutex class). This annotation is primarily used to check consistency.

#### scoped_lockable

Use `__attribute__((scoped_lockable))` on a class definition to specify that it has a "scoped" lockable type. Objects of this type will acquire the lock upon construction and release it upon going out of scope. This annotation is primarily used to check consistency.

#### guarded_var

Use `__attribute__((guarded_var))` on a variable declaration to specify that the variable must be accessed while holding some lock.

#### pt_guarded_var

Use `__attribute__((pt_guarded_var))` on a pointer declaration to specify that the pointer must be dereferenced while holding some lock.

#### guarded_by(l)

Use `__attribute__((guarded_by(l)))` on a variable declaration to specify that the variable must be accessed while holding lock `l`.

#### pt_guarded_by(l)

Use `__attribute__((pt_guarded_by(l)))` on a pointer declaration to specify that the pointer must be dereferenced while holding lock `l`.

#### acquired_before(...)

Use `__attribute__((acquired_before(...)))` on a declaration of a lockable variable to specify that the lock must be acquired before all attribute arguments. Arguments must be lockable type, and there must be at least one argument.

#### acquired_after(...)

Use `__attribute__((acquired_after(...)))` on a declaration of a lockable variable to specify that the lock must be acquired after all attribute arguments. Arguments must be lockable type, and there must be at least one argument.

#### exclusive_lock_function(...)

Use `__attribute__((exclusive_lock_function(...)))` on a function declaration to specify that the function acquires all listed locks exclusively. This attribute takes zero or more arguments: either of lockable type or integers indexing into function parameters of lockable type. If no arguments are given, the acquired lock is implicitly `this` of the enclosing object.

#### shared_lock_function(...)

Use `__attribute__((shared_lock_function(...)))` on a function declaration to specify that the function acquires all listed locks, although the locks may be shared (e.g. read locks). This attribute takes zero or more arguments: either of lockable type or integers indexing into function parameters of lockable type. If no arguments are given, the acquired lock is implicitly `this` of the enclosing object.

#### exclusive_trylock_function(...)

Use `__attribute__((exclusive_lock_function(...)))` on a function declaration to specify that the function will try (without blocking) to acquire all listed locks exclusively. This attribute takes one or more arguments. The first argument is an integer or boolean value specifying the return value of a successful lock acquisition. The remaining arugments are either of lockable type or integers indexing into function parameters of lockable type. If only one argument is given, the acquired lock is implicitly `this` of the enclosing object.

#### shared_trylock_function(...)

Use `__attribute__((shared_lock_function(...)))` on a function declaration to specify that the function will try (without blocking) to acquire all listed locks, although the locks may be shared (e.g. read locks). This attribute takes one or more arguments. The first argument is an integer or boolean value specifying the return value of a successful lock acquisition. The remaining arugments are either of lockable type or integers indexing into function parameters of lockable type. If only one argument is given, the acquired lock is implicitly `this` of the enclosing object.

#### unlock_function(...)

Use `__attribute__((unlock_function(...)))` on a function declaration to specify that the function release all listed locks. This attribute takes zero or more arguments: either of lockable type or integers indexing into function parameters of lockable type. If no arguments are given, the acquired lock is implicitly `this` of the enclosing object.

#### lock_returned(l)

Use `__attribute__((lock_returned(l)))` on a function declaration to specify that the function returns lock `l` (`l` must be of lockable type). This annotation is used to aid in resolving lock expressions.

#### locks_excluded(...)

Use `__attribute__((locks_excluded(...)))` on a function declaration to specify that the function must not be called with the listed locks. Arguments must be lockable type, and there must be at least one argument.

#### exclusive_locks_required(...)

Use `__attribute__((exclusive_locks_required(...)))` on a function declaration to specify that the function must be called while holding the listed exclusive locks. Arguments must be lockable type, and there must be at least one argument.

#### shared_locks_required(...)

Use `__attribute__((shared_locks_required(...)))` on a function declaration to specify that the function must be called while holding the listed shared locks. Arguments must be lockable type, and there must be at least one argument.