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 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_.

Another use of __has_feature is to check for compiler features not related to the language standard, such as e.g. AddressSanitizer.

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.

The feature name or extension name can also be specified with a preceding and following __ (double underscore) to avoid interference from a macro with the same name. For instance, __cxx_rvalue_references__ can be used instead of cxx_rvalue_references.

__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
...

The attribute name can also be specified with a preceding and following __ (double underscore) to avoid interference from a macro with the same name. For instance, __always_inline__ can be used instead of always_inline.

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 marketing version number of Clang (e.g., the 2 in 2.0.1). Note that marketing version numbers should not be used to check for language features, as different vendors use different numbering schemes. Instead, use the feature checking macros.
__clang_minor__
Defined to the minor version number of Clang (e.g., the 0 in 2.0.1). Note that marketing version numbers should not be used to check for language features, as different vendors use different numbering schemes. Instead, use the feature checking macros.
__clang_patchlevel__
Defined to the marketing patch level of Clang (e.g., the 1 in 2.0.1).
__clang_version__
Defined to a string that captures the Clang marketing version, including the Subversion tag or revision number, e.g., "1.5 (trunk 102332)".

Vectors and Extended Vectors

Supports the GCC, OpenCL, AltiVec and NEON vector extensions.

OpenCL vector types are created using ext_vector_type attribute. It 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).

Giving -faltivec option to clang enables support for AltiVec vector syntax and functions. For example:

vector float foo(vector int a) { 
  vector int b;
  b = vec_add(a, a) + a; 
  return (vector float)b;
}

NEON vector types are created using neon_vector_type and neon_polyvector_type attributes. For example:

typedef __attribute__((neon_vector_type(8))) int8_t int8x8_t;
typedef __attribute__((neon_polyvector_type(16))) poly8_t poly8x16_t;

int8x8_t foo(int8x8_t a) {
  int8x8_t v;
  v = a;
  return v;
}

Vector Literals

Vector literals can be used to create vectors from a set of scalars, or vectors. Either parentheses or braces form can be used. In the parentheses form the number of literal values specified must be one, i.e. referring to a scalar value, or must match the size of the vector type being created. If a single scalar literal value is specified, the scalar literal value will be replicated to all the components of the vector type. In the brackets form any number of literals can be specified. For example:

typedef int v4si __attribute__((__vector_size__(16)));
typedef float float4 __attribute__((ext_vector_type(4)));
typedef float float2 __attribute__((ext_vector_type(2)));

v4si vsi = (v4si){1, 2, 3, 4};
float4 vf = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
vector int vi1 = (vector int)(1);    // vi1 will be (1, 1, 1, 1).
vector int vi2 = (vector int){1};    // vi2 will be (1, 0, 0, 0).
vector int vi3 = (vector int)(1, 2); // error
vector int vi4 = (vector int){1, 2}; // vi4 will be (1, 2, 0, 0).
vector int vi5 = (vector int)(1, 2, 3, 4);
float4 vf = (float4)((float2)(1.0f, 2.0f), (float2)(3.0f, 4.0f));

Vector Operations

The table below shows the support for each operation by vector extension. A dash indicates that an operation is not accepted according to a corresponding specification.

Operator OpenCL AltiVec GCC NEON
[] yes yes yes -
unary operators +, - yes yes yes -
++, -- yes yes - -
+, -, *, /, % yes yes yes -
bitwise operators &, |, ^, ~ yes yes yes -
>>, << yes yes yes -
!, &&,|| no - - -
==,!=, >, <, >=, <= yes yes - -
= yes yes yes yes
:? yes - - -
sizeof yes yes yes yes

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).

'User-Specified' System Frameworks

Clang provides a mechanism by which frameworks can be built in such a way that they will always be treated as being 'system frameworks', even if they are not present in a system framework directory. This can be useful to system framework developers who want to be able to test building other applications with development builds of their framework, including the manner in which the compiler changes warning behavior for system headers.

Framework developers can opt-in to this mechanism by creating a '.system_framework' file at the top-level of their framework. That is, the framework should have contents like:

 .../TestFramework.framework
 .../TestFramework.framework/.system_framework
 .../TestFramework.framework/Headers
 .../TestFramework.framework/Headers/TestFramework.h
 ...

Clang will treat the presence of this file as an indicator that the framework should be treated as a system framework, regardless of how it was found in the framework search path. For consistency, we recommend that such files never be included in installed versions of the framework.

Availability attribute

Clang introduces the availability attribute, which can be placed on declarations to describe the lifecycle of that declaration relative to operating system versions. Consider the function declaration for a hypothetical function f:

void f(void) __attribute__((availability(macosx,introduced=10.4,deprecated=10.6,obsoleted=10.7)));

The availability attribute states that f was introduced in Mac OS X 10.4, deprecated in Mac OS X 10.6, and obsoleted in Mac OS X 10.7. This information is used by Clang to determine when it is safe to use f: for example, if Clang is instructed to compile code for Mac OS X 10.5, a call to f() succeeds. If Clang is instructed to compile code for Mac OS X 10.6, the call succeeds but Clang emits a warning specifying that the function is deprecated. Finally, if Clang is instructed to compile code for Mac OS X 10.7, the call fails because f() is no longer available.

The availablility attribute is a comma-separated list starting with the platform name and then including clauses specifying important milestones in the declaration's lifetime (in any order) along with additional information. Those clauses can be:

introduced=version
The first version in which this declaration was introduced.
deprecated=version
The first version in which this declaration was deprecated, meaning that users should migrate away from this API.
obsoleted=version
The first version in which this declaration was obsoleted, meaning that it was removed completely and can no longer be used.
unavailable
This declaration is never available on this platform.
message=string-literal
Additional message text that Clang will provide when emitting a warning or error about use of a deprecated or obsoleted declaration. Useful to direct users to replacement APIs.

Multiple availability attributes can be placed on a declaration, which may correspond to different platforms. Only the availability attribute with the platform corresponding to the target platform will be used; any others will be ignored. If no availability attribute specifies availability for the current target platform, the availability attributes are ignored. Supported platforms are:

ios
Apple's iOS operating system. The minimum deployment target is specified by the -mios-version-min=version or -miphoneos-version-min=version command-line arguments.
macosx
Apple's Mac OS X operating system. The minimum deployment target is specified by the -mmacosx-version-min=version command-line argument.

A declaration can be used even when deploying back to a platform version prior to when the declaration was introduced. When this happens, the declaration is weakly linked, as if the weak_import attribute were added to the declaration. A weakly-linked declaration may or may not be present a run-time, and a program can determine whether the declaration is present by checking whether the address of that declaration is non-NULL.

Checks for Standard Language Features

The __has_feature macro can be used to query if certain standard language features are enabled. The __has_extension macro can be used to query if language features are available as an extension when compiling for a standard which does not provide them. The features which can be tested are listed here.

C++98

The features listed below are part of the C++98 standard. These features are enabled by default when compiling C++ code.

C++ exceptions

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

C++11

The features listed below are part of the C++11 standard. As a result, all these features are enabled with the -std=c++11 or -std=gnu++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.

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's decltype does not require type-completeness of a function call expression. Use __has_feature(cxx_decltype_incomplete_return_types) or __has_extension(cxx_decltype_incomplete_return_types) to determine if support for this feature 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 defaulted functions

Use __has_feature(cxx_defaulted_functions) or __has_extension(cxx_defaulted_functions) to determine if support for defaulted function definitions (with = default) is enabled.

C++11 delegating constructors

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

C++11 deleted 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.

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.

C++11 local and unnamed types as template arguments

Use __has_feature(cxx_local_type_template_args) or __has_extension(cxx_local_type_template_args) to determine if support for local and unnamed types as template arguments is enabled.

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"x(foo\bar)x") 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.

C++11 user-defined literals

Use __has_feature(cxx_user_literals) to determine if support for user-defined literals is enabled.

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.

C11

The features listed below are part of the C11 standard. As a result, all these features are enabled with the -std=c11 or -std=gnu11 option when compiling C code. Additionally, because these features are all backward-compatible, they are available as extensions in all language modes.

C11 alignment specifiers

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

C11 atomic operations

Use __has_feature(c_atomic) or __has_extension(c_atomic) to determine if support for atomic types using _Atomic is enabled. Clang also provides a set of builtins which can be used to implement the <stdatomic.h> operations on _Atomic types.

C11 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 C11 generic selection expression is available in all languages supported by Clang. The syntax is the same as that given in the C11 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.

C11 _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:

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 have a related result type if its return type is compatible with the type of its class and if

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.

Interoperability with C++11 lambdas

Clang provides interoperability between C++11 lambdas and blocks-based APIs, by permitting a lambda to be implicitly converted to a block pointer with the corresponding signature. For example, consider an API such as NSArray's array-sorting method:

 - (NSArray *)sortedArrayUsingComparator:(NSComparator)cmptr;

NSComparator is simply a typedef for the block pointer NSComparisonResult (^)(id, id), and parameters of this type are generally provided with block literals as arguments. However, one can also use a C++11 lambda so long as it provides the same signature (in this case, accepting two parameters of type id and returning an NSComparisonResult):

  NSArray *array = @[@"string 1", @"string 21", @"string 12", @"String 11",
                     @"String 02"];
  const NSStringCompareOptions comparisonOptions
    = NSCaseInsensitiveSearch | NSNumericSearch |
      NSWidthInsensitiveSearch | NSForcedOrderingSearch;
  NSLocale *currentLocale = [NSLocale currentLocale];
  NSArray *sorted 
    = [array sortedArrayUsingComparator:[=](id s1, id s2) -> NSComparisonResult {
               NSRange string1Range = NSMakeRange(0, [s1 length]);
               return [s1 compare:s2 options:comparisonOptions 
                          range:string1Range locale:currentLocale];
       }];
  NSLog(@"sorted: %@", sorted);

This code relies on an implicit conversion from the type of the lambda expression (an unnamed, local class type called the closure type) to the corresponding block pointer type. The conversion itself is expressed by a conversion operator in that closure type that produces a block pointer with the same signature as the lambda itself, e.g.,

  operator NSComparisonResult (^)(id, id)() const;

This conversion function returns a new block that simply forwards the two parameters to the lambda object (which it captures by copy), then returns the result. The returned block is first copied (with Block_copy) and then autoreleased. As an optimization, if a lambda expression is immediately converted to a block pointer (as in the first example, above), then the block is not copied and autoreleased: rather, it is given the same lifetime as a block literal written at that point in the program, which avoids the overhead of copying a block to the heap in the common case.

The conversion from a lambda to a block pointer is only available in Objective-C++, and not in C++ with blocks, due to its use of Objective-C memory management (autorelease).

Object Literals and Subscripting

Clang provides support for Object Literals and Subscripting in Objective-C, which simplifies common Objective-C programming patterns, makes programs more concise, and improves the safety of container creation. There are several feature macros associated with object literals and subscripting: __has_feature(objc_array_literals) tests the availability of array literals; __has_feature(objc_dictionary_literals) tests the availability of dictionary literals; __has_feature(objc_subscripting) tests the availability of object subscripting.

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_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.

__c11_atomic builtins

Clang provides a set of builtins which are intended to be used to implement C11's <stdatomic.h> header. These builtins provide the semantics of the _explicit form of the corresponding C11 operation, and are named with a __c11_ prefix. The supported operations are:

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.

Dynamic Analysis-Specific Extensions

AddressSanitizer

Use __has_feature(address_sanitizer) to check if the code is being built with AddressSanitizer.

Use __attribute__((no_address_safety_analysis)) on a function declaration to specify that address safety instrumentation (e.g. AddressSanitizer) should not be applied to that function.

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.