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Showing content from https://releases.llvm.org/14.0.0/tools/clang/docs/AttributeReference.html below:

Attributes in Clang — Clang 14.0.0 documentation

This page lists the attributes currently supported by Clang.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

amdgpu_flat_work_group_size

clang::amdgpu_flat_work_group_size

Yes

The flat work-group size is the number of work-items in the work-group size specified when the kernel is dispatched. It is the product of the sizes of the x, y, and z dimension of the work-group.

Clang supports the __attribute__((amdgpu_flat_work_group_size(<min>, <max>))) attribute for the AMDGPU target. This attribute may be attached to a kernel function definition and is an optimization hint.

<min> parameter specifies the minimum flat work-group size, and <max> parameter specifies the maximum flat work-group size (must be greater than <min>) to which all dispatches of the kernel will conform. Passing 0, 0 as <min>, <max> implies the default behavior (128, 256).

If specified, the AMDGPU target backend might be able to produce better machine code for barriers and perform scratch promotion by estimating available group segment size.

An error will be given if:

• Specified values violate subtarget specifications;

• Specified values are not compatible with values provided through other attributes.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

amdgpu_num_sgpr

clang::amdgpu_num_sgpr

Yes

Clang supports the __attribute__((amdgpu_num_sgpr(<num_sgpr>))) and __attribute__((amdgpu_num_vgpr(<num_vgpr>))) attributes for the AMDGPU target. These attributes may be attached to a kernel function definition and are an optimization hint.

If these attributes are specified, then the AMDGPU target backend will attempt to limit the number of SGPRs and/or VGPRs used to the specified value(s). The number of used SGPRs and/or VGPRs may further be rounded up to satisfy the allocation requirements or constraints of the subtarget. Passing 0 as num_sgpr and/or num_vgpr implies the default behavior (no limits).

These attributes can be used to test the AMDGPU target backend. It is recommended that the amdgpu_waves_per_eu attribute be used to control resources such as SGPRs and VGPRs since it is aware of the limits for different subtargets.

An error will be given if:

• Specified values violate subtarget specifications;

• Specified values are not compatible with values provided through other attributes;

• The AMDGPU target backend is unable to create machine code that can meet the request.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

amdgpu_num_vgpr

clang::amdgpu_num_vgpr

Yes

Clang supports the __attribute__((amdgpu_num_sgpr(<num_sgpr>))) and __attribute__((amdgpu_num_vgpr(<num_vgpr>))) attributes for the AMDGPU target. These attributes may be attached to a kernel function definition and are an optimization hint.

If these attributes are specified, then the AMDGPU target backend will attempt to limit the number of SGPRs and/or VGPRs used to the specified value(s). The number of used SGPRs and/or VGPRs may further be rounded up to satisfy the allocation requirements or constraints of the subtarget. Passing 0 as num_sgpr and/or num_vgpr implies the default behavior (no limits).

These attributes can be used to test the AMDGPU target backend. It is recommended that the amdgpu_waves_per_eu attribute be used to control resources such as SGPRs and VGPRs since it is aware of the limits for different subtargets.

An error will be given if:

• Specified values violate subtarget specifications;

• Specified values are not compatible with values provided through other attributes;

• The AMDGPU target backend is unable to create machine code that can meet the request.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

amdgpu_waves_per_eu

clang::amdgpu_waves_per_eu

Yes

A compute unit (CU) is responsible for executing the wavefronts of a work-group. It is composed of one or more execution units (EU), which are responsible for executing the wavefronts. An EU can have enough resources to maintain the state of more than one executing wavefront. This allows an EU to hide latency by switching between wavefronts in a similar way to symmetric multithreading on a CPU. In order to allow the state for multiple wavefronts to fit on an EU, the resources used by a single wavefront have to be limited. For example, the number of SGPRs and VGPRs. Limiting such resources can allow greater latency hiding, but can result in having to spill some register state to memory.

Clang supports the __attribute__((amdgpu_waves_per_eu(<min>[, <max>]))) attribute for the AMDGPU target. This attribute may be attached to a kernel function definition and is an optimization hint.

<min> parameter specifies the requested minimum number of waves per EU, and optional <max> parameter specifies the requested maximum number of waves per EU (must be greater than <min> if specified). If <max> is omitted, then there is no restriction on the maximum number of waves per EU other than the one dictated by the hardware for which the kernel is compiled. Passing 0, 0 as <min>, <max> implies the default behavior (no limits).

If specified, this attribute allows an advanced developer to tune the number of wavefronts that are capable of fitting within the resources of an EU. The AMDGPU target backend can use this information to limit resources, such as number of SGPRs, number of VGPRs, size of available group and private memory segments, in such a way that guarantees that at least <min> wavefronts and at most <max> wavefronts are able to fit within the resources of an EU. Requesting more wavefronts can hide memory latency but limits available registers which can result in spilling. Requesting fewer wavefronts can help reduce cache thrashing, but can reduce memory latency hiding.

This attribute controls the machine code generated by the AMDGPU target backend to ensure it is capable of meeting the requested values. However, when the kernel is executed, there may be other reasons that prevent meeting the request, for example, there may be wavefronts from other kernels executing on the EU.

An error will be given if:

• Specified values violate subtarget specifications;

• Specified values are not compatible with values provided through other attributes;

• The AMDGPU target backend is unable to create machine code that can meet the request.

Clang supports several different calling conventions, depending on the target platform and architecture. The calling convention used for a function determines how parameters are passed, how results are returned to the caller, and other low-level details of calling a function.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

aarch64_vector_pcs

clang::aarch64_vector_pcs

clang::aarch64_vector_pcs

On AArch64 targets, this attribute changes the calling convention of a function to preserve additional floating-point and Advanced SIMD registers relative to the default calling convention used for AArch64.

This means it is more efficient to call such functions from code that performs extensive floating-point and vector calculations, because fewer live SIMD and FP registers need to be saved. This property makes it well-suited for e.g. floating-point or vector math library functions, which are typically leaf functions that require a small number of registers.

However, using this attribute also means that it is more expensive to call a function that adheres to the default calling convention from within such a function. Therefore, it is recommended that this attribute is only used for leaf functions.

For more information, see the documentation for aarch64_vector_pcs on the Arm Developer website.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

fastcall

gnu::fastcall

gnu::fastcall

__fastcall
_fastcall

On 32-bit x86 targets, this attribute changes the calling convention of a function to use ECX and EDX as register parameters and clear parameters off of the stack on return. This convention does not support variadic calls or unprototyped functions in C, and has no effect on x86_64 targets. This calling convention is supported primarily for compatibility with existing code. Users seeking register parameters should use the regparm attribute, which does not require callee-cleanup. See the documentation for __fastcall on MSDN.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

ms_abi

gnu::ms_abi

gnu::ms_abi

On non-Windows x86_64 targets, this attribute changes the calling convention of a function to match the default convention used on Windows x86_64. This attribute has no effect on Windows targets or non-x86_64 targets.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

pcs

gnu::pcs

gnu::pcs

On ARM targets, this attribute can be used to select calling conventions similar to stdcall on x86. Valid parameter values are “aapcs” and “aapcs-vfp”.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

preserve_all

clang::preserve_all

clang::preserve_all

On X86-64 and AArch64 targets, this attribute changes the calling convention of a function. The preserve_all calling convention attempts to make the code in the caller even less intrusive than the preserve_most calling convention. This calling convention also behaves identical to the C calling convention on how arguments and return values are passed, but it uses a different set of caller/callee-saved registers. This removes the burden of saving and recovering a large register set before and after the call in the caller. If the arguments are passed in callee-saved registers, then they will be preserved by the callee across the call. This doesn’t apply for values returned in callee-saved registers.

• On X86-64 the callee preserves all general purpose registers, except for R11. R11 can be used as a scratch register. Furthermore it also preserves all floating-point registers (XMMs/YMMs).

The idea behind this convention is to support calls to runtime functions that don’t need to call out to any other functions.

This calling convention, like the preserve_most calling convention, will be used by a future version of the Objective-C runtime and should be considered experimental at this time.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

preserve_most

clang::preserve_most

clang::preserve_most

On X86-64 and AArch64 targets, this attribute changes the calling convention of a function. The preserve_most calling convention attempts to make the code in the caller as unintrusive as possible. This convention behaves identically to the C calling convention on how arguments and return values are passed, but it uses a different set of caller/callee-saved registers. This alleviates the burden of saving and recovering a large register set before and after the call in the caller. If the arguments are passed in callee-saved registers, then they will be preserved by the callee across the call. This doesn’t apply for values returned in callee-saved registers.

• On X86-64 the callee preserves all general purpose registers, except for R11. R11 can be used as a scratch register. Floating-point registers (XMMs/YMMs) are not preserved and need to be saved by the caller.

The idea behind this convention is to support calls to runtime functions that have a hot path and a cold path. The hot path is usually a small piece of code that doesn’t use many registers. The cold path might need to call out to another function and therefore only needs to preserve the caller-saved registers, which haven’t already been saved by the caller. The preserve_most calling convention is very similar to the cold calling convention in terms of caller/callee-saved registers, but they are used for different types of function calls. coldcc is for function calls that are rarely executed, whereas preserve_most function calls are intended to be on the hot path and definitely executed a lot. Furthermore preserve_most doesn’t prevent the inliner from inlining the function call.

This calling convention will be used by a future version of the Objective-C runtime and should therefore still be considered experimental at this time. Although this convention was created to optimize certain runtime calls to the Objective-C runtime, it is not limited to this runtime and might be used by other runtimes in the future too. The current implementation only supports X86-64 and AArch64, but the intention is to support more architectures in the future.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

regcall

gnu::regcall

gnu::regcall

__regcall

On x86 targets, this attribute changes the calling convention to __regcall convention. This convention aims to pass as many arguments as possible in registers. It also tries to utilize registers for the return value whenever it is possible.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

regparm

gnu::regparm

gnu::regparm

On 32-bit x86 targets, the regparm attribute causes the compiler to pass the first three integer parameters in EAX, EDX, and ECX instead of on the stack. This attribute has no effect on variadic functions, and all parameters are passed via the stack as normal.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

stdcall

gnu::stdcall

gnu::stdcall

__stdcall
_stdcall

On 32-bit x86 targets, this attribute changes the calling convention of a function to clear parameters off of the stack on return. This convention does not support variadic calls or unprototyped functions in C, and has no effect on x86_64 targets. This calling convention is used widely by the Windows API and COM applications. See the documentation for __stdcall on MSDN.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

thiscall

gnu::thiscall

gnu::thiscall

__thiscall
_thiscall

On 32-bit x86 targets, this attribute changes the calling convention of a function to use ECX for the first parameter (typically the implicit this parameter of C++ methods) and clear parameters off of the stack on return. This convention does not support variadic calls or unprototyped functions in C, and has no effect on x86_64 targets. See the documentation for __thiscall on MSDN.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

vectorcall

clang::vectorcall

clang::vectorcall

__vectorcall
_vectorcall

On 32-bit x86 and x86_64 targets, this attribute changes the calling convention of a function to pass vector parameters in SSE registers.

On 32-bit x86 targets, this calling convention is similar to __fastcall. The first two integer parameters are passed in ECX and EDX. Subsequent integer parameters are passed in memory, and callee clears the stack. On x86_64 targets, the callee does not clear the stack, and integer parameters are passed in RCX, RDX, R8, and R9 as is done for the default Windows x64 calling convention.

On both 32-bit x86 and x86_64 targets, vector and floating point arguments are passed in XMM0-XMM5. Homogeneous vector aggregates of up to four elements are passed in sequential SSE registers if enough are available. If AVX is enabled, 256 bit vectors are passed in YMM0-YMM5. Any vector or aggregate type that cannot be passed in registers for any reason is passed by reference, which allows the caller to align the parameter memory.

See the documentation for __vectorcall on MSDN for more details.

Clang supports additional attributes for checking basic resource management properties, specifically for unique objects that have a single owning reference. The following attributes are currently supported, although the implementation for these annotations is currently in development and are subject to change.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

callable_when

clang::callable_when

Yes

Use __attribute__((callable_when(...))) to indicate what states a method may be called in. Valid states are unconsumed, consumed, or unknown. Each argument to this attribute must be a quoted string. E.g.:

__attribute__((callable_when("unconsumed", "unknown")))

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

consumable

clang::consumable

Yes

Each class that uses any of the typestate annotations must first be marked using the consumable attribute. Failure to do so will result in a warning.

This attribute accepts a single parameter that must be one of the following: unknown, consumed, or unconsumed.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

param_typestate

clang::param_typestate

Yes

This attribute specifies expectations about function parameters. Calls to an function with annotated parameters will issue a warning if the corresponding argument isn’t in the expected state. The attribute is also used to set the initial state of the parameter when analyzing the function’s body.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

return_typestate

clang::return_typestate

Yes

The return_typestate attribute can be applied to functions or parameters. When applied to a function the attribute specifies the state of the returned value. The function’s body is checked to ensure that it always returns a value in the specified state. On the caller side, values returned by the annotated function are initialized to the given state.

When applied to a function parameter it modifies the state of an argument after a call to the function returns. The function’s body is checked to ensure that the parameter is in the expected state before returning.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

set_typestate

clang::set_typestate

Yes

Annotate methods that transition an object into a new state with __attribute__((set_typestate(new_state))). The new state must be unconsumed, consumed, or unknown.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

test_typestate

clang::test_typestate

Yes

Use __attribute__((test_typestate(tested_state))) to indicate that a method returns true if the object is in the specified state..

Clang supports additional attributes for customizing how APIs are imported into Swift.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

swift_async

clang::swift_async

clang::swift_async

Yes

The swift_async attribute specifies if and how a particular function or Objective-C method is imported into a swift async method. For instance:

@interface MyClass : NSObject
-(void)notActuallyAsync:(int)p1 withCompletionHandler:(void (^)())handler
    __attribute__((swift_async(none)));

-(void)actuallyAsync:(int)p1 callThisAsync:(void (^)())fun
    __attribute__((swift_async(swift_private, 1)));
@end

Here, notActuallyAsync:withCompletionHandler would have been imported as async (because it’s last parameter’s selector piece is withCompletionHandler) if not for the swift_async(none) attribute. Conversely, actuallyAsync:callThisAsync wouldn’t have been imported as async if not for the swift_async attribute because it doesn’t match the naming convention.

When using swift_async to enable importing, the first argument to the attribute is either swift_private or not_swift_private to indicate whether the function/method is private to the current framework, and the second argument is the index of the completion handler parameter.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

swift_async_error

clang::swift_async_error

clang::swift_async_error

Yes

The swift_async_error attribute specifies how an error state will be represented in a swift async method. It’s a bit analogous to the swift_error attribute for the generated async method. The swift_async_error attribute can indicate a variety of different ways of representing an error.

• __attribute__((swift_async_error(zero_argument, N))), specifies that the async method is considered to have failed if the Nth argument to the completion handler is zero.

• __attribute__((swift_async_error(nonzero_argument, N))), specifies that the async method is considered to have failed if the Nth argument to the completion handler is non-zero.

• __attribute__((swift_async_error(nonnull_error))), specifies that the async method is considered to have failed if the NSError * argument to the completion handler is non-null.

• __attribute__((swift_async_error(none))), specifies that the async method cannot fail.

For instance:

@interface MyClass : NSObject
-(void)asyncMethod:(void (^)(char, int, float))handler
    __attribute__((swift_async(swift_private, 1)))
    __attribute__((swift_async_error(zero_argument, 2)));
@end

Here, the swift_async attribute specifies that handler is the completion handler for this method, and the swift_async_error attribute specifies that the int parameter is the one that represents the error.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

swift_async_name

Yes

The swift_async_name attribute provides the name of the async overload for the given declaration in Swift. If this attribute is absent, the name is transformed according to the algorithm built into the Swift compiler.

The argument is a string literal that contains the Swift name of the function or method. The name may be a compound Swift name. The function or method with such an attribute must have more than zero parameters, as its last parameter is assumed to be a callback that’s eliminated in the Swift async name.

@interface URL
+ (void) loadContentsFrom:(URL *)url callback:(void (^)(NSData *))data __attribute__((__swift_async_name__("URL.loadContentsFrom(_:)")))
@end

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

swift_attr

Yes

The swift_attr provides a Swift-specific annotation for the declaration to which the attribute appertains to. It can be used on any declaration in Clang. This kind of annotation is ignored by Clang as it doesn’t have any semantic meaning in languages supported by Clang. The Swift compiler can interpret these annotations according to its own rules when importing C or Objective-C declarations.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

swift_bridge

The swift_bridge attribute indicates that the declaration to which the attribute appertains is bridged to the named Swift type.

__attribute__((__objc_root__))
@interface Base
- (instancetype)init;
@end

__attribute__((__swift_bridge__("BridgedI")))
@interface I : Base
@end

In this example, the Objective-C interface I will be made available to Swift with the name BridgedI. It would be possible for the compiler to refer to I still in order to bridge the type back to Objective-C.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

swift_bridged_typedef

Yes

The swift_bridged_typedef attribute indicates that when the typedef to which the attribute appertains is imported into Swift, it should refer to the bridged Swift type (e.g. Swift’s String) rather than the Objective-C type as written (e.g. NSString).

@interface NSString;
typedef NSString *AliasedString __attribute__((__swift_bridged_typedef__));

extern void acceptsAliasedString(AliasedString _Nonnull parameter);

In this case, the function acceptsAliasedString will be imported into Swift as a function which accepts a String type parameter.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

swift_error

Yes

The swift_error attribute controls whether a particular function (or Objective-C method) is imported into Swift as a throwing function, and if so, which dynamic convention it uses.

All of these conventions except none require the function to have an error parameter. Currently, the error parameter is always the last parameter of type NSError** or CFErrorRef*. Swift will remove the error parameter from the imported API. When calling the API, Swift will always pass a valid address initialized to a null pointer.

• swift_error(none) means that the function should not be imported as throwing. The error parameter and result type will be imported normally.

• swift_error(null_result) means that calls to the function should be considered to have thrown if they return a null value. The return type must be a pointer type, and it will be imported into Swift with a non-optional type. This is the default error convention for Objective-C methods that return pointers.

• swift_error(zero_result) means that calls to the function should be considered to have thrown if they return a zero result. The return type must be an integral type. If the return type would have been imported as Bool, it is instead imported as Void. This is the default error convention for Objective-C methods that return a type that would be imported as Bool.

• swift_error(nonzero_result) means that calls to the function should be considered to have thrown if they return a non-zero result. The return type must be an integral type. If the return type would have been imported as Bool, it is instead imported as Void.

• swift_error(nonnull_error) means that calls to the function should be considered to have thrown if they leave a non-null error in the error parameter. The return type is left unmodified.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

swift_name

The swift_name attribute provides the name of the declaration in Swift. If this attribute is absent, the name is transformed according to the algorithm built into the Swift compiler.

The argument is a string literal that contains the Swift name of the function, variable, or type. When renaming a function, the name may be a compound Swift name. For a type, enum constant, property, or variable declaration, the name must be a simple or qualified identifier.

@interface URL
- (void) initWithString:(NSString *)s __attribute__((__swift_name__("URL.init(_:)")))
@end

void __attribute__((__swift_name__("squareRoot()"))) sqrt(double v) {
}

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

swift_newtype
swift_wrapper

Yes

The swift_newtype attribute indicates that the typedef to which the attribute appertains is imported as a new Swift type of the typedef’s name. Previously, the attribute was spelt swift_wrapper. While the behaviour of the attribute is identical with either spelling, swift_wrapper is deprecated, only exists for compatibility purposes, and should not be used in new code.

• swift_newtype(struct) means that a Swift struct will be created for this typedef.

• swift_newtype(enum) means that a Swift enum will be created for this typedef.

  // Import UIFontTextStyle as an enum type, with enumerated values being
  // constants.
  typedef NSString * UIFontTextStyle __attribute__((__swift_newtype__(enum)));
  
  // Import UIFontDescriptorFeatureKey as a structure type, with enumerated
  // values being members of the type structure.
  typedef NSString * UIFontDescriptorFeatureKey __attribute__((__swift_newtype__(struct)));
  

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

swift_objc_members

Yes

This attribute indicates that Swift subclasses and members of Swift extensions of this class will be implicitly marked with the @objcMembers Swift attribute, exposing them back to Objective-C.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

swift_private

Declarations marked with the swift_private attribute are hidden from the framework client but are still made available for use within the framework or Swift SDK overlay.

The purpose of this attribute is to permit a more idomatic implementation of declarations in Swift while hiding the non-idiomatic one.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

gsl::Owner

Yes

Note

This attribute is experimental and its effect on analysis is subject to change in a future version of clang.

The attribute [[gsl::Owner(T)]] applies to structs and classes that own an object of type T:

class [[gsl::Owner(int)]] IntOwner {
private:
  int value;
public:
  int *getInt() { return &value; }
};

The argument T is optional and is ignored. This attribute may be used by analysis tools and has no effect on code generation. A void argument means that the class can own any type.

See Pointer for an example.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

gsl::Pointer

Yes

Note

This attribute is experimental and its effect on analysis is subject to change in a future version of clang.

The attribute [[gsl::Pointer(T)]] applies to structs and classes that behave like pointers to an object of type T:

class [[gsl::Pointer(int)]] IntPointer {
private:
  int *valuePointer;
public:
  int *getInt() { return &valuePointer; }
};

The argument T is optional and is ignored. This attribute may be used by analysis tools and has no effect on code generation. A void argument means that the pointer can point to any type.

Example: When constructing an instance of a class annotated like this (a Pointer) from an instance of a class annotated with [[gsl::Owner]] (an Owner), then the analysis will consider the Pointer to point inside the Owner. When the Owner’s lifetime ends, it will consider the Pointer to be dangling.

int f() {
  IntPointer P;
  if (true) {
    IntOwner O(7);
    P = IntPointer(O); // P "points into" O
  } // P is dangling
  return P.get(); // error: Using a dangling Pointer.
}

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

__single_inheritance
__multiple_inheritance
__virtual_inheritance
__unspecified_inheritance

This collection of keywords is enabled under -fms-extensions and controls the pointer-to-member representation used on *-*-win32 targets.

The *-*-win32 targets utilize a pointer-to-member representation which varies in size and alignment depending on the definition of the underlying class.

However, this is problematic when a forward declaration is only available and no definition has been made yet. In such cases, Clang is forced to utilize the most general representation that is available to it.

These keywords make it possible to use a pointer-to-member representation other than the most general one regardless of whether or not the definition will ever be present in the current translation unit.

This family of keywords belong between the class-key and class-name:

struct __single_inheritance S;
int S::*i;
struct S {};

This keyword can be applied to class templates but only has an effect when used on full specializations:

template <typename T, typename U> struct __single_inheritance A; // warning: inheritance model ignored on primary template
template <typename T> struct __multiple_inheritance A<T, T>; // warning: inheritance model ignored on partial specialization
template <> struct __single_inheritance A<int, float>;

Note that choosing an inheritance model less general than strictly necessary is an error:

struct __multiple_inheritance S; // error: inheritance model does not match definition
int S::*i;
struct S {};

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

asm
__asm__

This attribute can be used on a function or variable to specify its symbol name.

On some targets, all C symbols are prefixed by default with a single character, typically _. This was done historically to distinguish them from symbols used by other languages. (This prefix is also added to the standard Itanium C++ ABI prefix on “mangled” symbol names, so that e.g. on such targets the true symbol name for a C++ variable declared as int cppvar; would be __Z6cppvar; note the two underscores.) This prefix is not added to the symbol names specified by the asm attribute; programmers wishing to match a C symbol name must compensate for this.

For example, consider the following C code:

int var1 asm("altvar") = 1;  // "altvar" in symbol table.
int var2 = 1; // "_var2" in symbol table.

void func1(void) asm("altfunc");
void func1(void) {} // "altfunc" in symbol table.
void func2(void) {} // "_func2" in symbol table.

Clang’s implementation of this attribute is compatible with GCC’s, documented here.

While it is possible to use this attribute to name a special symbol used internally by the compiler, such as an LLVM intrinsic, this is neither recommended nor supported and may cause the compiler to crash or miscompile. Users who wish to gain access to intrinsic behavior are strongly encouraged to request new builtin functions.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

deprecated

gnu::deprecated
deprecated

gnu::deprecated
deprecated

deprecated

The deprecated attribute can be applied to a function, a variable, or a type. This is useful when identifying functions, variables, or types that are expected to be removed in a future version of a program.

Consider the function declaration for a hypothetical function f:

void f(void) __attribute__((deprecated("message", "replacement")));

When spelled as __attribute__((deprecated)), the deprecated attribute can have two optional string arguments. The first one is the message to display when emitting the warning; the second one enables the compiler to provide a Fix-It to replace the deprecated name with a new name. Otherwise, when spelled as [[gnu::deprecated]] or [[deprecated]], the attribute can have one optional string argument which is the message to display when emitting the warning.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

empty_bases

The empty_bases attribute permits the compiler to utilize the empty-base-optimization more frequently. This attribute only applies to struct, class, and union types. It is only supported when using the Microsoft C++ ABI.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

enum_extensibility

clang::enum_extensibility

clang::enum_extensibility

Yes

Attribute enum_extensibility is used to distinguish between enum definitions that are extensible and those that are not. The attribute can take either closed or open as an argument. closed indicates a variable of the enum type takes a value that corresponds to one of the enumerators listed in the enum definition or, when the enum is annotated with flag_enum, a value that can be constructed using values corresponding to the enumerators. open indicates a variable of the enum type can take any values allowed by the standard and instructs clang to be more lenient when issuing warnings.

enum __attribute__((enum_extensibility(closed))) ClosedEnum {
  A0, A1
};

enum __attribute__((enum_extensibility(open))) OpenEnum {
  B0, B1
};

enum __attribute__((enum_extensibility(closed),flag_enum)) ClosedFlagEnum {
  C0 = 1 << 0, C1 = 1 << 1
};

enum __attribute__((enum_extensibility(open),flag_enum)) OpenFlagEnum {
  D0 = 1 << 0, D1 = 1 << 1
};

void foo1() {
  enum ClosedEnum ce;
  enum OpenEnum oe;
  enum ClosedFlagEnum cfe;
  enum OpenFlagEnum ofe;

  ce = A1;           // no warnings
  ce = 100;          // warning issued
  oe = B1;           // no warnings
  oe = 100;          // no warnings
  cfe = C0 | C1;     // no warnings
  cfe = C0 | C1 | 4; // warning issued
  ofe = D0 | D1;     // no warnings
  ofe = D0 | D1 | 4; // no warnings
}

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

external_source_symbol

clang::external_source_symbol

clang::external_source_symbol

Yes

The external_source_symbol attribute specifies that a declaration originates from an external source and describes the nature of that source.

The fact that Clang is capable of recognizing declarations that were defined externally can be used to provide better tooling support for mixed-language projects or projects that rely on auto-generated code. For instance, an IDE that uses Clang and that supports mixed-language projects can use this attribute to provide a correct ‘jump-to-definition’ feature. For a concrete example, consider a protocol that’s defined in a Swift file:

@objc public protocol SwiftProtocol {
  func method()
}

This protocol can be used from Objective-C code by including a header file that was generated by the Swift compiler. The declarations in that header can use the external_source_symbol attribute to make Clang aware of the fact that SwiftProtocol actually originates from a Swift module:

__attribute__((external_source_symbol(language="Swift",defined_in="module")))
@protocol SwiftProtocol
@required
- (void) method;
@end

Consequently, when ‘jump-to-definition’ is performed at a location that references SwiftProtocol, the IDE can jump to the original definition in the Swift source file rather than jumping to the Objective-C declaration in the auto-generated header file.

The external_source_symbol attribute is a comma-separated list that includes clauses that describe the origin and the nature of the particular declaration. Those clauses can be:

language=string-literal

The name of the source language in which this declaration was defined.

defined_in=string-literal

The name of the source container in which the declaration was defined. The exact definition of source container is language-specific, e.g. Swift’s source containers are modules, so defined_in should specify the Swift module name.

generated_declaration

This declaration was automatically generated by some tool.

The clauses can be specified in any order. The clauses that are listed above are all optional, but the attribute has to have at least one clause.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

flag_enum

clang::flag_enum

clang::flag_enum

Yes

This attribute can be added to an enumerator to signal to the compiler that it is intended to be used as a flag type. This will cause the compiler to assume that the range of the type includes all of the values that you can get by manipulating bits of the enumerator when issuing warnings.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

layout_version

The layout_version attribute requests that the compiler utilize the class layout rules of a particular compiler version. This attribute only applies to struct, class, and union types. It is only supported when using the Microsoft C++ ABI.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

lto_visibility_public

clang::lto_visibility_public

clang::lto_visibility_public

Yes

See LTO Visibility.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

managed

__managed__

Yes

The __managed__ attribute can be applied to a global variable declaration in HIP. A managed variable is emitted as an undefined global symbol in the device binary and is registered by __hipRegisterManagedVariable in init functions. The HIP runtime allocates managed memory and uses it to define the symbol when loading the device binary. A managed variable can be accessed in both device and host code.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

novtable

This attribute can be added to a class declaration or definition to signal to the compiler that constructors and destructors will not reference the virtual function table. It is only supported when using the Microsoft C++ ABI.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

ns_error_domain

Yes

In Cocoa frameworks in Objective-C, one can group related error codes in enums and categorize these enums with error domains.

The ns_error_domain attribute indicates a global NSString or CFString constant representing the error domain that an error code belongs to. For pointer uniqueness and code size this is a constant symbol, not a literal.

The domain and error code need to be used together. The ns_error_domain attribute links error codes to their domain at the source level.

This metadata is useful for documentation purposes, for static analysis, and for improving interoperability between Objective-C and Swift. It is not used for code generation in Objective-C.

For example:

#define NS_ERROR_ENUM(_type, _name, _domain)  \
  enum _name : _type _name; enum __attribute__((ns_error_domain(_domain))) _name : _type

extern NSString *const MyErrorDomain;
typedef NS_ERROR_ENUM(unsigned char, MyErrorEnum, MyErrorDomain) {
  MyErrFirst,
  MyErrSecond,
};

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

objc_boxable

clang::objc_boxable

clang::objc_boxable

Yes

Structs and unions marked with the objc_boxable attribute can be used with the Objective-C boxed expression syntax, @(...).

Usage: __attribute__((objc_boxable)). This attribute can only be placed on a declaration of a trivially-copyable struct or union:

struct __attribute__((objc_boxable)) some_struct {
  int i;
};
union __attribute__((objc_boxable)) some_union {
  int i;
  float f;
};
typedef struct __attribute__((objc_boxable)) _some_struct some_struct;

// ...

some_struct ss;
NSValue *boxed = @(ss);

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

objc_direct

clang::objc_direct

clang::objc_direct

Yes

The objc_direct attribute can be used to mark an Objective-C method as being direct. A direct method is treated statically like an ordinary method, but dynamically it behaves more like a C function. This lowers some of the costs associated with the method but also sacrifices some of the ordinary capabilities of Objective-C methods.

A message send of a direct method calls the implementation directly, as if it were a C function, rather than using ordinary Objective-C method dispatch. This is substantially faster and potentially allows the implementation to be inlined, but it also means the method cannot be overridden in subclasses or replaced dynamically, as ordinary Objective-C methods can.

Furthermore, a direct method is not listed in the class’s method lists. This substantially reduces the code-size overhead of the method but also means it cannot be called dynamically using ordinary Objective-C method dispatch at all; in particular, this means that it cannot override a superclass method or satisfy a protocol requirement.

Because a direct method cannot be overridden, it is an error to perform a super message send of one.

Although a message send of a direct method causes the method to be called directly as if it were a C function, it still obeys Objective-C semantics in other ways:

• If the receiver is nil, the message send does nothing and returns the zero value for the return type.

• A message send of a direct class method will cause the class to be initialized, including calling the +initialize method if present.

• The implicit _cmd parameter containing the method’s selector is still defined. In order to minimize code-size costs, the implementation will not emit a reference to the selector if the parameter is unused within the method.

Symbols for direct method implementations are implicitly given hidden visibility, meaning that they can only be called within the same linkage unit.

It is an error to do any of the following:

• declare a direct method in a protocol,

• declare an override of a direct method with a method in a subclass,

• declare an override of a non-direct method with a direct method in a subclass,

• declare a method with different directness in different class interfaces, or

• implement a non-direct method (as declared in any class interface) with a direct method.

If any of these rules would be violated if every method defined in an @implementation within a single linkage unit were declared in an appropriate class interface, the program is ill-formed with no diagnostic required. If a violation of this rule is not diagnosed, behavior remains well-defined; this paragraph is simply reserving the right to diagnose such conflicts in the future, not to treat them as undefined behavior.

Additionally, Clang will warn about any @selector expression that names a selector that is only known to be used for direct methods.

For the purpose of these rules, a “class interface” includes a class’s primary @interface block, its class extensions, its categories, its declared protocols, and all the class interfaces of its superclasses.

An Objective-C property can be declared with the direct property attribute. If a direct property declaration causes an implicit declaration of a getter or setter method (that is, if the given method is not explicitly declared elsewhere), the method is declared to be direct.

Some programmers may wish to make many methods direct at once. In order to simplify this, the objc_direct_members attribute is provided; see its documentation for more information.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

objc_direct_members

clang::objc_direct_members

clang::objc_direct_members

Yes

The objc_direct_members attribute can be placed on an Objective-C @interface or @implementation to mark that methods declared therein should be considered direct by default. See the documentation for objc_direct for more information about direct methods.

When objc_direct_members is placed on an @interface block, every method in the block is considered to be declared as direct. This includes any implicit method declarations introduced by property declarations. If the method redeclares a non-direct method, the declaration is ill-formed, exactly as if the method was annotated with the objc_direct attribute.

When objc_direct_members is placed on an @implementation block, methods defined in the block are considered to be declared as direct unless they have been previously declared as non-direct in any interface of the class. This includes the implicit method definitions introduced by synthesized properties, including auto-synthesized properties.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

objc_non_runtime_protocol

clang::objc_non_runtime_protocol

clang::objc_non_runtime_protocol

Yes

The objc_non_runtime_protocol attribute can be used to mark that an Objective-C protocol is only used during static type-checking and doesn’t need to be represented dynamically. This avoids several small code-size and run-time overheads associated with handling the protocol’s metadata. A non-runtime protocol cannot be used as the operand of a @protocol expression, and dynamic attempts to find it with objc_getProtocol will fail.

If a non-runtime protocol inherits from any ordinary protocols, classes and derived protocols that declare conformance to the non-runtime protocol will dynamically list their conformance to those bare protocols.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

objc_nonlazy_class

clang::objc_nonlazy_class

clang::objc_nonlazy_class

Yes

This attribute can be added to an Objective-C @interface or @implementation declaration to add the class to the list of non-lazily initialized classes. A non-lazy class will be initialized eagerly when the Objective-C runtime is loaded. This is required for certain system classes which have instances allocated in non-standard ways, such as the classes for blocks and constant strings. Adding this attribute is essentially equivalent to providing a trivial +load method but avoids the (fairly small) load-time overheads associated with defining and calling such a method.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

objc_runtime_name

clang::objc_runtime_name

clang::objc_runtime_name

Yes

By default, the Objective-C interface or protocol identifier is used in the metadata name for that object. The objc_runtime_name attribute allows annotated interfaces or protocols to use the specified string argument in the object’s metadata name instead of the default name.

Usage: __attribute__((objc_runtime_name("MyLocalName"))). This attribute can only be placed before an @protocol or @interface declaration:

__attribute__((objc_runtime_name("MyLocalName")))
@interface Message
@end

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

objc_runtime_visible

clang::objc_runtime_visible

clang::objc_runtime_visible

Yes

This attribute specifies that the Objective-C class to which it applies is visible to the Objective-C runtime but not to the linker. Classes annotated with this attribute cannot be subclassed and cannot have categories defined for them.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

objc_subclassing_restricted

clang::objc_subclassing_restricted

clang::objc_subclassing_restricted

Yes

This attribute can be added to an Objective-C @interface declaration to ensure that this class cannot be subclassed.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

preferred_name

clang::preferred_name

The preferred_name attribute can be applied to a class template, and specifies a preferred way of naming a specialization of the template. The preferred name will be used whenever the corresponding template specialization would otherwise be printed in a diagnostic or similar context.

The preferred name must be a typedef or type alias declaration that refers to a specialization of the class template (not including any type qualifiers). In general this requires the template to be declared at least twice. For example:

template<typename T> struct basic_string;
using string = basic_string<char>;
using wstring = basic_string<wchar_t>;
template<typename T> struct [[clang::preferred_name(string),
                              clang::preferred_name(wstring)]] basic_string {
  // ...
};

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

selectany

gnu::selectany

gnu::selectany

selectany

This attribute appertains to a global symbol, causing it to have a weak definition ( linkonce ), allowing the linker to select any definition.

For more information see gcc documentation or msvc documentation.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

transparent_union

gnu::transparent_union

gnu::transparent_union

This attribute can be applied to a union to change the behavior of calls to functions that have an argument with a transparent union type. The compiler behavior is changed in the following manner:

• A value whose type is any member of the transparent union can be passed as an argument without the need to cast that value.

• The argument is passed to the function using the calling convention of the first member of the transparent union. Consequently, all the members of the transparent union should have the same calling convention as its first member.

Transparent unions are not supported in C++.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

trivial_abi

clang::trivial_abi

Yes

The trivial_abi attribute can be applied to a C++ class, struct, or union. It instructs the compiler to pass and return the type using the C ABI for the underlying type when the type would otherwise be considered non-trivial for the purpose of calls. A class annotated with trivial_abi can have non-trivial destructors or copy/move constructors without automatically becoming non-trivial for the purposes of calls. For example:

// A is trivial for the purposes of calls because ``trivial_abi`` makes the
// user-provided special functions trivial.
struct __attribute__((trivial_abi)) A {
  ~A();
  A(const A &);
  A(A &&);
  int x;
};

// B's destructor and copy/move constructor are considered trivial for the
// purpose of calls because A is trivial.
struct B {
  A a;
};

If a type is trivial for the purposes of calls, has a non-trivial destructor, and is passed as an argument by value, the convention is that the callee will destroy the object before returning.

Attribute trivial_abi has no effect in the following cases:

• The class directly declares a virtual base or virtual methods.

• Copy constructors and move constructors of the class are all deleted.

• The class has a base class that is non-trivial for the purposes of calls.

• The class has a non-static data member whose type is non-trivial for the purposes of calls, which includes:

  • classes that are non-trivial for the purposes of calls

  • __weak-qualified types in Objective-C++

  • arrays of any of the above

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

using_if_exists

clang::using_if_exists

The using_if_exists attribute applies to a using-declaration. It allows programmers to import a declaration that potentially does not exist, instead deferring any errors to the point of use. For instance:

namespace empty_namespace {};
__attribute__((using_if_exists))
using empty_namespace::does_not_exist; // no error!

does_not_exist x; // error: use of unresolved 'using_if_exists'

The C++ spelling of the attribte ([[clang::using_if_exists]]) is also supported as a clang extension, since ISO C++ doesn’t support attributes in this position. If the entity referred to by the using-declaration is found by name lookup, the attribute has no effect. This attribute is useful for libraries (primarily, libc++) that wish to redeclare a set of declarations in another namespace, when the availability of those declarations is difficult or impossible to detect at compile time with the preprocessor.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

no_unique_address

The no_unique_address attribute allows tail padding in a non-static data member to overlap other members of the enclosing class (and in the special case when the type is empty, permits it to fully overlap other members). The field is laid out as if a base class were encountered at the corresponding point within the class (except that it does not share a vptr with the enclosing object).

Example usage:

template<typename T, typename Alloc> struct my_vector {
  T *p;
  [[no_unique_address]] Alloc alloc;
  // ...
};
static_assert(sizeof(my_vector<int, std::allocator<int>>) == sizeof(int*));

[[no_unique_address]] is a standard C++20 attribute. Clang supports its use in C++11 onwards.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

omp declare simd

The declare simd construct can be applied to a function to enable the creation of one or more versions that can process multiple arguments using SIMD instructions from a single invocation in a SIMD loop. The declare simd directive is a declarative directive. There may be multiple declare simd directives for a function. The use of a declare simd construct on a function enables the creation of SIMD versions of the associated function that can be used to process multiple arguments from a single invocation from a SIMD loop concurrently. The syntax of the declare simd construct is as follows:

#pragma omp declare simd [clause[[,] clause] ...] new-line
[#pragma omp declare simd [clause[[,] clause] ...] new-line]
[...]
function definition or declaration

where clause is one of the following:

simdlen(length)
linear(argument-list[:constant-linear-step])
aligned(argument-list[:alignment])
uniform(argument-list)
inbranch
notinbranch

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

omp declare target

The declare target directive specifies that variables and functions are mapped to a device for OpenMP offload mechanism.

The syntax of the declare target directive is as follows:

#pragma omp declare target new-line
declarations-definition-seq
#pragma omp end declare target new-line

or

#pragma omp declare target (extended-list) new-line

or

#pragma omp declare target clause[ [,] clause ... ] new-line

where clause is one of the following:

to(extended-list)
link(list)
device_type(host | nohost | any)

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

omp declare variant

The declare variant directive declares a specialized variant of a base function and specifies the context in which that specialized variant is used. The declare variant directive is a declarative directive. The syntax of the declare variant construct is as follows:

#pragma omp declare variant(variant-func-id) clause new-line
[#pragma omp declare variant(variant-func-id) clause new-line]
[...]
function definition or declaration

where clause is one of the following:

match(context-selector-specification)

and where variant-func-id is the name of a function variant that is either a base language identifier or, for C++, a template-id.

Clang provides the following context selector extensions, used via implementation={extension(EXTENSION)}:

match_all
match_any
match_none
disable_implicit_base
allow_templates

The match extensions change when the entire context selector is considered a match for an OpenMP context. The default is all, with none no trait in the selector is allowed to be in the OpenMP context, with any a single trait in both the selector and OpenMP context is sufficient. Only a single match extension trait is allowed per context selector. The disable extensions remove default effects of the begin declare variant applied to a definition. If disable_implicit_base is given, we will not introduce an implicit base function for a variant if no base function was found. The variant is still generated but will never be called, due to the absence of a base function and consequently calls to a base function. The allow extensions change when the begin declare variant effect is applied to a definition. If allow_templates is given, template function definitions are considered as specializations of existing or assumed template declarations with the same name. The template parameters for the base functions are used to instantiate the specialization.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

_Noreturn

A function declared as _Noreturn shall not return to its caller. The compiler will generate a diagnostic for a function declared as _Noreturn that appears to be capable of returning to its caller. Despite being a type specifier, the _Noreturn attribute cannot be specified on a function pointer type.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

abi_tag

gnu::abi_tag

Yes

The abi_tag attribute can be applied to a function, variable, class or inline namespace declaration to modify the mangled name of the entity. It gives the ability to distinguish between different versions of the same entity but with different ABI versions supported. For example, a newer version of a class could have a different set of data members and thus have a different size. Using the abi_tag attribute, it is possible to have different mangled names for a global variable of the class type. Therefore, the old code could keep using the old mangled name and the new code will use the new mangled name with tags.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

alloc_align

gnu::alloc_align

gnu::alloc_align

Use __attribute__((alloc_align(<alignment>)) on a function declaration to specify that the return value of the function (which must be a pointer type) is at least as aligned as the value of the indicated parameter. The parameter is given by its index in the list of formal parameters; the first parameter has index 1 unless the function is a C++ non-static member function, in which case the first parameter has index 2 to account for the implicit this parameter.

// The returned pointer has the alignment specified by the first parameter.
void *a(size_t align) __attribute__((alloc_align(1)));

// The returned pointer has the alignment specified by the second parameter.
void *b(void *v, size_t align) __attribute__((alloc_align(2)));

// The returned pointer has the alignment specified by the second visible
// parameter, however it must be adjusted for the implicit 'this' parameter.
void *Foo::b(void *v, size_t align) __attribute__((alloc_align(3)));

Note that this attribute merely informs the compiler that a function always returns a sufficiently aligned pointer. It does not cause the compiler to emit code to enforce that alignment. The behavior is undefined if the returned pointer is not sufficiently aligned.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

alloc_size

gnu::alloc_size

gnu::alloc_size

The alloc_size attribute can be placed on functions that return pointers in order to hint to the compiler how many bytes of memory will be available at the returned pointer. alloc_size takes one or two arguments.

• alloc_size(N) implies that argument number N equals the number of available bytes at the returned pointer.

• alloc_size(N, M) implies that the product of argument number N and argument number M equals the number of available bytes at the returned pointer.

Argument numbers are 1-based.

An example of how to use alloc_size

void *my_malloc(int a) __attribute__((alloc_size(1)));
void *my_calloc(int a, int b) __attribute__((alloc_size(1, 2)));

int main() {
  void *const p = my_malloc(100);
  assert(__builtin_object_size(p, 0) == 100);
  void *const a = my_calloc(20, 5);
  assert(__builtin_object_size(a, 0) == 100);
}

Note

This attribute works differently in clang than it does in GCC. Specifically, clang will only trace const pointers (as above); we give up on pointers that are not marked as const. In the vast majority of cases, this is unimportant, because LLVM has support for the alloc_size attribute. However, this may cause mildly unintuitive behavior when used with other attributes, such as enable_if.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

allocator

The __declspec(allocator) attribute is applied to functions that allocate memory, such as operator new in C++. When CodeView debug information is emitted (enabled by clang -gcodeview or clang-cl /Z7), Clang will attempt to record the code offset of heap allocation call sites in the debug info. It will also record the type being allocated using some local heuristics. The Visual Studio debugger uses this information to profile memory usage.

This attribute does not affect optimizations in any way, unlike GCC’s __attribute__((malloc)).

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

artificial

gnu::artificial

gnu::artificial

The artificial attribute can be applied to an inline function. If such a function is inlined, the attribute indicates that debuggers should associate the resulting instructions with the call site, rather than with the corresponding line within the inlined callee.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

assume

clang::assume

clang::assume

Yes

Clang supports the __attribute__((assume("assumption"))) attribute to provide additional information to the optimizer. The string-literal, here “assumption”, will be attached to the function declaration such that later analysis and optimization passes can assume the “assumption” to hold. This is similar to __builtin_assume but instead of an expression that can be assumed to be non-zero, the assumption is expressed as a string and it holds for the entire function.

A function can have multiple assume attributes and they propagate from prior declarations to later definitions. Multiple assumptions are aggregated into a single comma separated string. Thus, one can provide multiple assumptions via a comma separated string, i.a., __attribute__((assume("assumption1,assumption2"))).

While LLVM plugins might provide more assumption strings, the default LLVM optimization passes are aware of the following assumptions:

"omp_no_openmp"
"omp_no_openmp_routines"
"omp_no_parallelism"

The OpenMP standard defines the meaning of OpenMP assumptions (“omp_XYZ” is spelled “XYZ” in the OpenMP 5.1 Standard).

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

assume_aligned

gnu::assume_aligned

gnu::assume_aligned

Yes

Use __attribute__((assume_aligned(<alignment>[,<offset>])) on a function declaration to specify that the return value of the function (which must be a pointer type) has the specified offset, in bytes, from an address with the specified alignment. The offset is taken to be zero if omitted.

// The returned pointer value has 32-byte alignment.
void *a() __attribute__((assume_aligned (32)));

// The returned pointer value is 4 bytes greater than an address having
// 32-byte alignment.
void *b() __attribute__((assume_aligned (32, 4)));

Note that this attribute provides information to the compiler regarding a condition that the code already ensures is true. It does not cause the compiler to enforce the provided alignment assumption.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

availability

clang::availability

clang::availability

Yes

The availability attribute 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(macos,introduced=10.4,deprecated=10.6,obsoleted=10.7)));

The availability attribute states that f was introduced in macOS 10.4, deprecated in macOS 10.6, and obsoleted in macOS 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 macOS 10.5, a call to f() succeeds. If Clang is instructed to compile code for macOS 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 macOS 10.7, the call fails because f() is no longer available.

The availability 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.

replacement=string-literal

Additional message text that Clang will use to provide Fix-It when emitting a warning about use of a deprecated declaration. The Fix-It will replace the deprecated declaration with the new declaration specified.

Multiple availability attributes can be placed on a declaration, which may correspond to different platforms. For most platforms, the availability attribute with the platform corresponding to the target platform will be used; any others will be ignored. However, the availability for watchOS and tvOS can be implicitly inferred from an iOS availability attribute. Any explicit availability attributes for those platforms are still preferred over the implicitly inferred availability attributes. 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.

macos

Apple’s macOS operating system. The minimum deployment target is specified by the -mmacosx-version-min=*version* command-line argument. macosx is supported for backward-compatibility reasons, but it is deprecated.

tvos

Apple’s tvOS operating system. The minimum deployment target is specified by the -mtvos-version-min=*version* command-line argument.

watchos

Apple’s watchOS operating system. The minimum deployment target is specified by the -mwatchos-version-min=*version* command-line argument.

A declaration can typically 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.

The flag strict disallows using API when deploying back to a platform version prior to when the declaration was introduced. An attempt to use such API before its introduction causes a hard error. Weakly-linking is almost always a better API choice, since it allows users to query availability at runtime.

If there are multiple declarations of the same entity, the availability attributes must either match on a per-platform basis or later declarations must not have availability attributes for that platform. For example:

void g(void) __attribute__((availability(macos,introduced=10.4)));
void g(void) __attribute__((availability(macos,introduced=10.4))); // okay, matches
void g(void) __attribute__((availability(ios,introduced=4.0))); // okay, adds a new platform
void g(void); // okay, inherits both macos and ios availability from above.
void g(void) __attribute__((availability(macos,introduced=10.5))); // error: mismatch

When one method overrides another, the overriding method can be more widely available than the overridden method, e.g.,:

@interface A
- (id)method __attribute__((availability(macos,introduced=10.4)));
- (id)method2 __attribute__((availability(macos,introduced=10.4)));
@end

@interface B : A
- (id)method __attribute__((availability(macos,introduced=10.3))); // okay: method moved into base class later
- (id)method __attribute__((availability(macos,introduced=10.5))); // error: this method was available via the base class in 10.4
@end

Starting with the macOS 10.12 SDK, the API_AVAILABLE macro from <os/availability.h> can simplify the spelling:

@interface A
- (id)method API_AVAILABLE(macos(10.11)));
- (id)otherMethod API_AVAILABLE(macos(10.11), ios(11.0));
@end

Availability attributes can also be applied using a #pragma clang attribute. Any explicit availability attribute whose platform corresponds to the target platform is applied to a declaration regardless of the availability attributes specified in the pragma. For example, in the code below, hasExplicitAvailabilityAttribute will use the macOS availability attribute that is specified with the declaration, whereas getsThePragmaAvailabilityAttribute will use the macOS availability attribute that is applied by the pragma.

#pragma clang attribute push (__attribute__((availability(macOS, introduced=10.12))), apply_to=function)
void getsThePragmaAvailabilityAttribute(void);
void hasExplicitAvailabilityAttribute(void) __attribute__((availability(macos,introduced=10.4)));
#pragma clang attribute pop

For platforms like watchOS and tvOS, whose availability attributes can be implicitly inferred from an iOS availability attribute, the logic is slightly more complex. The explicit and the pragma-applied availability attributes whose platform corresponds to the target platform are applied as described in the previous paragraph. However, the implicitly inferred attributes are applied to a declaration only when there is no explicit or pragma-applied availability attribute whose platform corresponds to the target platform. For example, the function below will receive the tvOS availability from the pragma rather than using the inferred iOS availability from the declaration:

#pragma clang attribute push (__attribute__((availability(tvOS, introduced=12.0))), apply_to=function)
void getsThePragmaTVOSAvailabilityAttribute(void) __attribute__((availability(iOS,introduced=11.0)));
#pragma clang attribute pop

The compiler is also able to apply implicitly inferred attributes from a pragma as well. For example, when targeting tvOS, the function below will receive a tvOS availability attribute that is implicitly inferred from the iOS availability attribute applied by the pragma:

#pragma clang attribute push (__attribute__((availability(iOS, introduced=12.0))), apply_to=function)
void infersTVOSAvailabilityFromPragma(void);
#pragma clang attribute pop

The implicit attributes that are inferred from explicitly specified attributes whose platform corresponds to the target platform are applied to the declaration even if there is an availability attribute that can be inferred from a pragma. For example, the function below will receive the tvOS, introduced=11.0 availability that is inferred from the attribute on the declaration rather than inferring availability from the pragma:

#pragma clang attribute push (__attribute__((availability(iOS, unavailable))), apply_to=function)
void infersTVOSAvailabilityFromAttributeNextToDeclaration(void)
  __attribute__((availability(iOS,introduced=11.0)));
#pragma clang attribute pop

Also see the documentation for @available

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

btf_decl_tag

clang::btf_decl_tag

clang::btf_decl_tag

Yes

Clang supports the __attribute__((btf_decl_tag("ARGUMENT"))) attribute for all targets. This attribute may be attached to a struct/union, struct/union field, function, function parameter, variable or typedef declaration. If -g is specified, the ARGUMENT info will be preserved in IR and be emitted to dwarf. For BPF targets, the ARGUMENT info will be emitted to .BTF ELF section too.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

callback

clang::callback

clang::callback

Yes

The callback attribute specifies that the annotated function may invoke the specified callback zero or more times. The callback, as well as the passed arguments, are identified by their parameter name or position (starting with 1!) in the annotated function. The first position in the attribute identifies the callback callee, the following positions declare describe its arguments. The callback callee is required to be callable with the number, and order, of the specified arguments. The index 0, or the identifier this, is used to represent an implicit “this” pointer in class methods. If there is no implicit “this” pointer it shall not be referenced. The index ‘-1’, or the name “__”, represents an unknown callback callee argument. This can be a value which is not present in the declared parameter list, or one that is, but is potentially inspected, captured, or modified. Parameter names and indices can be mixed in the callback attribute.

The callback attribute, which is directly translated to callback metadata <http://llvm.org/docs/LangRef.html#callback-metadata>, make the connection between the call to the annotated function and the callback callee. This can enable interprocedural optimizations which were otherwise impossible. If a function parameter is mentioned in the callback attribute, through its position, it is undefined if that parameter is used for anything other than the actual callback. Inspected, captured, or modified parameters shall not be listed in the callback metadata.

Example encodings for the callback performed by pthread_create are shown below. The explicit attribute annotation indicates that the third parameter (start_routine) is called zero or more times by the pthread_create function, and that the fourth parameter (arg) is passed along. Note that the callback behavior of pthread_create is automatically recognized by Clang. In addition, the declarations of __kmpc_fork_teams and __kmpc_fork_call, generated for #pragma omp target teams and #pragma omp parallel, respectively, are also automatically recognized as broker functions. Further functions might be added in the future.

__attribute__((callback (start_routine, arg)))
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                   void *(*start_routine) (void *), void *arg);

__attribute__((callback (3, 4)))
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                   void *(*start_routine) (void *), void *arg);

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

carries_dependency

carries_dependency

Yes

The carries_dependency attribute specifies dependency propagation into and out of functions.

When specified on a function or Objective-C method, the carries_dependency attribute means that the return value carries a dependency out of the function, so that the implementation need not constrain ordering upon return from that function. Implementations of the function and its caller may choose to preserve dependencies instead of emitting memory ordering instructions such as fences.

Note, this attribute does not change the meaning of the program, but may result in generation of more efficient code.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

cf_consumed

clang::cf_consumed

clang::cf_consumed

Yes

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

cf_returns_not_retained

clang::cf_returns_not_retained

clang::cf_returns_not_retained

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

cf_returns_retained

clang::cf_returns_retained

clang::cf_returns_retained

The behavior of a function with respect to reference counting for Foundation (Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming convention (e.g. functions starting with “get” are assumed to return at +0).

It can be overridden using a family of the following attributes. In Objective-C, the annotation __attribute__((ns_returns_retained)) applied to a function communicates that the object is returned at +1, and the caller is responsible for freeing it. Similarly, the annotation __attribute__((ns_returns_not_retained)) specifies that the object is returned at +0 and the ownership remains with the callee. The annotation __attribute__((ns_consumes_self)) specifies that the Objective-C method call consumes the reference to self, e.g. by attaching it to a supplied parameter. Additionally, parameters can have an annotation __attribute__((ns_consumed)), which specifies that passing an owned object as that parameter effectively transfers the ownership, and the caller is no longer responsible for it. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes: __attribute__((cf_returns_not_retained)), __attribute__((cf_returns_retained)) and __attribute__((cf_consumed)) have the same respective semantics when applied to CoreFoundation objects. These attributes affect code generation when interacting with ARC code, and they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject), the same attribute family is present: __attribute__((os_returns_not_retained)), __attribute__((os_returns_retained)) and __attribute__((os_consumed)), with the same respective semantics. Similar to __attribute__((ns_consumes_self)), __attribute__((os_consumes_this)) specifies that the method call consumes the reference to “this” (e.g., when attaching it to a different object supplied as a parameter). Out parameters (parameters the function is meant to write into, either via pointers-to-pointers or references-to-pointers) may be annotated with __attribute__((os_returns_retained)) or __attribute__((os_returns_not_retained)) which specifies that the object written into the out parameter should (or respectively should not) be released after use. Since often out parameters may or may not be written depending on the exit code of the function, annotations __attribute__((os_returns_retained_on_zero)) and __attribute__((os_returns_retained_on_non_zero)) specify that an out parameter at +1 is written if and only if the function returns a zero (respectively non-zero) error code. Observe that return-code-dependent out parameter annotations are only available for retained out parameters, as non-retained object do not have to be released by the callee. These attributes are only used by the Clang Static Analyzer.

The family of attributes X_returns_X_retained can be added to functions, C++ methods, and Objective-C methods and properties. Attributes X_consumed can be added to parameters of methods, functions, and Objective-C methods.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

cfi_canonical_jump_table

clang::cfi_canonical_jump_table

clang::cfi_canonical_jump_table

Yes

Use __attribute__((cfi_canonical_jump_table)) on a function declaration to make the function’s CFI jump table canonical. See the CFI documentation for more details.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

clang_builtin_alias

clang::builtin_alias

clang::builtin_alias

Yes

This attribute is used in the implementation of the C intrinsics. It allows the C intrinsic functions to be declared using the names defined in target builtins, and still be recognized as clang builtins equivalent to the underlying name. For example, riscv_vector.h declares the function vadd with __attribute__((clang_builtin_alias(__builtin_rvv_vadd_vv_i8m1))). This ensures that both functions are recognized as that clang builtin, and in the latter case, the choice of which builtin to identify the function as can be deferred until after overload resolution.

This attribute can only be used to set up the aliases for certain ARM/RISC-V C intrinsic functions; it is intended for use only inside arm_*.h and riscv_*.h and is not a general mechanism for declaring arbitrary aliases for clang builtin functions.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

__clang_arm_builtin_alias

clang::__clang_arm_builtin_alias

clang::__clang_arm_builtin_alias

Yes

This attribute is used in the implementation of the ACLE intrinsics. It allows the intrinsic functions to be declared using the names defined in ACLE, and still be recognized as clang builtins equivalent to the underlying name. For example, arm_mve.h declares the function vaddq_u32 with __attribute__((__clang_arm_mve_alias(__builtin_arm_mve_vaddq_u32))), and similarly, one of the type-overloaded declarations of vaddq will have the same attribute. This ensures that both functions are recognized as that clang builtin, and in the latter case, the choice of which builtin to identify the function as can be deferred until after overload resolution.

This attribute can only be used to set up the aliases for certain Arm intrinsic functions; it is intended for use only inside arm_*.h and is not a general mechanism for declaring arbitrary aliases for clang builtin functions.

In order to avoid duplicating the attribute definitions for similar purpose for other architecture, there is a general form for the attribute clang_builtin_alias.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

cmse_nonsecure_entry

Yes

This attribute declares a function that can be called from non-secure state, or from secure state. Entering from and returning to non-secure state would switch to and from secure state, respectively, and prevent flow of information to non-secure state, except via return values. See ARMv8-M Security Extensions: Requirements on Development Tools - Engineering Specification Documentation for more information.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

code_seg

The __declspec(code_seg) attribute enables the placement of code into separate named segments that can be paged or locked in memory individually. This attribute is used to control the placement of instantiated templates and compiler-generated code. See the documentation for __declspec(code_seg) on MSDN.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

convergent

clang::convergent

clang::convergent

Yes

The convergent attribute can be placed on a function declaration. It is translated into the LLVM convergent attribute, which indicates that the call instructions of a function with this attribute cannot be made control-dependent on any additional values.

In languages designed for SPMD/SIMT programming model, e.g. OpenCL or CUDA, the call instructions of a function with this attribute must be executed by all work items or threads in a work group or sub group.

This attribute is different from noduplicate because it allows duplicating function calls if it can be proved that the duplicated function calls are not made control-dependent on any additional values, e.g., unrolling a loop executed by all work items.

Sample usage:

void convfunc(void) __attribute__((convergent));
// Setting it as a C++11 attribute is also valid in a C++ program.
// void convfunc(void) [[clang::convergent]];

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

cpu_dispatch

clang::cpu_dispatch

clang::cpu_dispatch

cpu_dispatch

Yes

The cpu_specific and cpu_dispatch attributes are used to define and resolve multiversioned functions. This form of multiversioning provides a mechanism for declaring versions across translation units and manually specifying the resolved function list. A specified CPU defines a set of minimum features that are required for the function to be called. The result of this is that future processors execute the most restrictive version of the function the new processor can execute.

Function versions are defined with cpu_specific, which takes one or more CPU names as a parameter. For example:

// Declares and defines the ivybridge version of single_cpu.
__attribute__((cpu_specific(ivybridge)))
void single_cpu(void){}

// Declares and defines the atom version of single_cpu.
__attribute__((cpu_specific(atom)))
void single_cpu(void){}

// Declares and defines both the ivybridge and atom version of multi_cpu.
__attribute__((cpu_specific(ivybridge, atom)))
void multi_cpu(void){}

A dispatching (or resolving) function can be declared anywhere in a project’s source code with cpu_dispatch. This attribute takes one or more CPU names as a parameter (like cpu_specific). Functions marked with cpu_dispatch are not expected to be defined, only declared. If such a marked function has a definition, any side effects of the function are ignored; trivial function bodies are permissible for ICC compatibility.

// Creates a resolver for single_cpu above.
__attribute__((cpu_dispatch(ivybridge, atom)))
void single_cpu(void){}

// Creates a resolver for multi_cpu, but adds a 3rd version defined in another
// translation unit.
__attribute__((cpu_dispatch(ivybridge, atom, sandybridge)))
void multi_cpu(void){}

Note that it is possible to have a resolving function that dispatches based on more or fewer options than are present in the program. Specifying fewer will result in the omitted options not being considered during resolution. Specifying a version for resolution that isn’t defined in the program will result in a linking failure.

It is also possible to specify a CPU name of generic which will be resolved if the executing processor doesn’t satisfy the features required in the CPU name. The behavior of a program executing on a processor that doesn’t satisfy any option of a multiversioned function is undefined.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

cpu_specific

clang::cpu_specific

clang::cpu_specific

cpu_specific

Yes

The cpu_specific and cpu_dispatch attributes are used to define and resolve multiversioned functions. This form of multiversioning provides a mechanism for declaring versions across translation units and manually specifying the resolved function list. A specified CPU defines a set of minimum features that are required for the function to be called. The result of this is that future processors execute the most restrictive version of the function the new processor can execute.

Function versions are defined with cpu_specific, which takes one or more CPU names as a parameter. For example:

// Declares and defines the ivybridge version of single_cpu.
__attribute__((cpu_specific(ivybridge)))
void single_cpu(void){}

// Declares and defines the atom version of single_cpu.
__attribute__((cpu_specific(atom)))
void single_cpu(void){}

// Declares and defines both the ivybridge and atom version of multi_cpu.
__attribute__((cpu_specific(ivybridge, atom)))
void multi_cpu(void){}

A dispatching (or resolving) function can be declared anywhere in a project’s source code with cpu_dispatch. This attribute takes one or more CPU names as a parameter (like cpu_specific). Functions marked with cpu_dispatch are not expected to be defined, only declared. If such a marked function has a definition, any side effects of the function are ignored; trivial function bodies are permissible for ICC compatibility.

// Creates a resolver for single_cpu above.
__attribute__((cpu_dispatch(ivybridge, atom)))
void single_cpu(void){}

// Creates a resolver for multi_cpu, but adds a 3rd version defined in another
// translation unit.
__attribute__((cpu_dispatch(ivybridge, atom, sandybridge)))
void multi_cpu(void){}

Note that it is possible to have a resolving function that dispatches based on more or fewer options than are present in the program. Specifying fewer will result in the omitted options not being considered during resolution. Specifying a version for resolution that isn’t defined in the program will result in a linking failure.

It is also possible to specify a CPU name of generic which will be resolved if the executing processor doesn’t satisfy the features required in the CPU name. The behavior of a program executing on a processor that doesn’t satisfy any option of a multiversioned function is undefined.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

diagnose_as_builtin

clang::diagnose_as_builtin

clang::diagnose_as_builtin

Yes

The diagnose_as_builtin attribute indicates that Fortify diagnostics are to be applied to the declared function as if it were the function specified by the attribute. The builtin function whose diagnostics are to be mimicked should be given. In addition, the order in which arguments should be applied must also be given.

For example, the attribute can be used as follows.

__attribute__((diagnose_as_builtin(__builtin_memset, 3, 2, 1)))
void *mymemset(int n, int c, void *s) {
  // ...
}

This indicates that calls to mymemset should be diagnosed as if they were calls to __builtin_memset. The arguments 3, 2, 1 indicate by index the order in which arguments of mymemset should be applied to __builtin_memset. The third argument should be applied first, then the second, and then the first. Thus (when Fortify warnings are enabled) the call mymemset(n, c, s) will diagnose overflows as if it were the call __builtin_memset(s, c, n).

For variadic functions, the variadic arguments must come in the same order as they would to the builtin function, after all normal arguments. For instance, to diagnose a new function as if it were sscanf, we can use the attribute as follows.

__attribute__((diagnose_as_builtin(sscanf, 1, 2)))
int mysscanf(const char *str, const char *format, ...)  {
  // ...
}

Then the call mysscanf(“abc def”, “%4s %4s”, buf1, buf2) will be diagnosed as if it were the call sscanf(“abc def”, “%4s %4s”, buf1, buf2).

This attribute cannot be applied to non-static member functions.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

diagnose_if

The diagnose_if attribute can be placed on function declarations to emit warnings or errors at compile-time if calls to the attributed function meet certain user-defined criteria. For example:

int abs(int a)
  __attribute__((diagnose_if(a >= 0, "Redundant abs call", "warning")));
int must_abs(int a)
  __attribute__((diagnose_if(a >= 0, "Redundant abs call", "error")));

int val = abs(1); // warning: Redundant abs call
int val2 = must_abs(1); // error: Redundant abs call
int val3 = abs(val);
int val4 = must_abs(val); // Because run-time checks are not emitted for
                          // diagnose_if attributes, this executes without
                          // issue.

diagnose_if is closely related to enable_if, with a few key differences:

• Overload resolution is not aware of diagnose_if attributes: they’re considered only after we select the best candidate from a given candidate set.

• Function declarations that differ only in their diagnose_if attributes are considered to be redeclarations of the same function (not overloads).

• If the condition provided to diagnose_if cannot be evaluated, no diagnostic will be emitted.

Otherwise, diagnose_if is essentially the logical negation of enable_if.

As a result of bullet number two, diagnose_if attributes will stack on the same function. For example:

int foo() __attribute__((diagnose_if(1, "diag1", "warning")));
int foo() __attribute__((diagnose_if(1, "diag2", "warning")));

int bar = foo(); // warning: diag1
                 // warning: diag2
int (*fooptr)(void) = foo; // warning: diag1
                           // warning: diag2

constexpr int supportsAPILevel(int N) { return N < 5; }
int baz(int a)
  __attribute__((diagnose_if(!supportsAPILevel(10),
                             "Upgrade to API level 10 to use baz", "error")));
int baz(int a)
  __attribute__((diagnose_if(!a, "0 is not recommended.", "warning")));

int (*bazptr)(int) = baz; // error: Upgrade to API level 10 to use baz
int v = baz(0); // error: Upgrade to API level 10 to use baz

Query for this feature with __has_attribute(diagnose_if).

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

disable_sanitizer_instrumentation

clang::disable_sanitizer_instrumentation

clang::disable_sanitizer_instrumentation

Yes

Use the disable_sanitizer_instrumentation attribute on a function, Objective-C method, or global variable, to specify that no sanitizer instrumentation should be applied.

This is not the same as __attribute__((no_sanitize(...))), which depending on the tool may still insert instrumentation to prevent false positive reports.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

disable_tail_calls

clang::disable_tail_calls

clang::disable_tail_calls

Yes

The disable_tail_calls attribute instructs the backend to not perform tail call optimization inside the marked function.

For example:

int callee(int);

int foo(int a) __attribute__((disable_tail_calls)) {
  return callee(a); // This call is not tail-call optimized.
}

Marking virtual functions as disable_tail_calls is legal.

int callee(int);

class Base {
public:
  [[clang::disable_tail_calls]] virtual int foo1() {
    return callee(); // This call is not tail-call optimized.
  }
};

class Derived1 : public Base {
public:
  int foo1() override {
    return callee(); // This call is tail-call optimized.
  }
};

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

enable_if

Yes

Note

Some features of this attribute are experimental. The meaning of multiple enable_if attributes on a single declaration is subject to change in a future version of clang. Also, the ABI is not standardized and the name mangling may change in future versions. To avoid that, use asm labels.

The enable_if attribute can be placed on function declarations to control which overload is selected based on the values of the function’s arguments. When combined with the overloadable attribute, this feature is also available in C.

int isdigit(int c);
int isdigit(int c) __attribute__((enable_if(c <= -1 || c > 255, "chosen when 'c' is out of range"))) __attribute__((unavailable("'c' must have the value of an unsigned char or EOF")));

void foo(char c) {
  isdigit(c);
  isdigit(10);
  isdigit(-10);  // results in a compile-time error.
}

The enable_if attribute takes two arguments, the first is an expression written in terms of the function parameters, the second is a string explaining why this overload candidate could not be selected to be displayed in diagnostics. The expression is part of the function signature for the purposes of determining whether it is a redeclaration (following the rules used when determining whether a C++ template specialization is ODR-equivalent), but is not part of the type.

The enable_if expression is evaluated as if it were the body of a bool-returning constexpr function declared with the arguments of the function it is being applied to, then called with the parameters at the call site. If the result is false or could not be determined through constant expression evaluation, then this overload will not be chosen and the provided string may be used in a diagnostic if the compile fails as a result.

Because the enable_if expression is an unevaluated context, there are no global state changes, nor the ability to pass information from the enable_if expression to the function body. For example, suppose we want calls to strnlen(strbuf, maxlen) to resolve to strnlen_chk(strbuf, maxlen, size of strbuf) only if the size of strbuf can be determined:

__attribute__((always_inline))
static inline size_t strnlen(const char *s, size_t maxlen)
  __attribute__((overloadable))
  __attribute__((enable_if(__builtin_object_size(s, 0) != -1))),
                           "chosen when the buffer size is known but 'maxlen' is not")))
{
  return strnlen_chk(s, maxlen, __builtin_object_size(s, 0));
}

Multiple enable_if attributes may be applied to a single declaration. In this case, the enable_if expressions are evaluated from left to right in the following manner. First, the candidates whose enable_if expressions evaluate to false or cannot be evaluated are discarded. If the remaining candidates do not share ODR-equivalent enable_if expressions, the overload resolution is ambiguous. Otherwise, enable_if overload resolution continues with the next enable_if attribute on the candidates that have not been discarded and have remaining enable_if attributes. In this way, we pick the most specific overload out of a number of viable overloads using enable_if.

void f() __attribute__((enable_if(true, "")));  // #1
void f() __attribute__((enable_if(true, ""))) __attribute__((enable_if(true, "")));  // #2

void g(int i, int j) __attribute__((enable_if(i, "")));  // #1
void g(int i, int j) __attribute__((enable_if(j, ""))) __attribute__((enable_if(true)));  // #2

In this example, a call to f() is always resolved to #2, as the first enable_if expression is ODR-equivalent for both declarations, but #1 does not have another enable_if expression to continue evaluating, so the next round of evaluation has only a single candidate. In a call to g(1, 1), the call is ambiguous even though #2 has more enable_if attributes, because the first enable_if expressions are not ODR-equivalent.

Query for this feature with __has_attribute(enable_if).

Note that functions with one or more enable_if attributes may not have their address taken, unless all of the conditions specified by said enable_if are constants that evaluate to true. For example:

const int TrueConstant = 1;
const int FalseConstant = 0;
int f(int a) __attribute__((enable_if(a > 0, "")));
int g(int a) __attribute__((enable_if(a == 0 || a != 0, "")));
int h(int a) __attribute__((enable_if(1, "")));
int i(int a) __attribute__((enable_if(TrueConstant, "")));
int j(int a) __attribute__((enable_if(FalseConstant, "")));

void fn() {
  int (*ptr)(int);
  ptr = &f; // error: 'a > 0' is not always true
  ptr = &g; // error: 'a == 0 || a != 0' is not a truthy constant
  ptr = &h; // OK: 1 is a truthy constant
  ptr = &i; // OK: 'TrueConstant' is a truthy constant
  ptr = &j; // error: 'FalseConstant' is a constant, but not truthy
}

Because enable_if evaluation happens during overload resolution, enable_if may give unintuitive results when used with templates, depending on when overloads are resolved. In the example below, clang will emit a diagnostic about no viable overloads for foo in bar, but not in baz:

double foo(int i) __attribute__((enable_if(i > 0, "")));
void *foo(int i) __attribute__((enable_if(i <= 0, "")));
template <int I>
auto bar() { return foo(I); }

template <typename T>
auto baz() { return foo(T::number); }

struct WithNumber { constexpr static int number = 1; };
void callThem() {
  bar<sizeof(WithNumber)>();
  baz<WithNumber>();
}

This is because, in bar, foo is resolved prior to template instantiation, so the value for I isn’t known (thus, both enable_if conditions for foo fail). However, in baz, foo is resolved during template instantiation, so the value for T::number is known.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

enforce_tcb

clang::enforce_tcb

clang::enforce_tcb

Yes

The enforce_tcb attribute can be placed on functions to enforce that a

trusted compute base (TCB) does not call out of the TCB. This generates a warning every time a function not marked with an enforce_tcb attribute is called from a function with the enforce_tcb attribute. A function may be a part of multiple TCBs. Invocations through function pointers are currently not checked. Builtins are considered to a part of every TCB.

• enforce_tcb(Name) indicates that this function is a part of the TCB named Name

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

enforce_tcb_leaf

clang::enforce_tcb_leaf

clang::enforce_tcb_leaf

Yes

The enforce_tcb_leaf attribute satisfies the requirement enforced by

enforce_tcb for the marked function to be in the named TCB but does not continue to check the functions called from within the leaf function.

• enforce_tcb_leaf(Name) indicates that this function is a part of the TCB named Name

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

error
warning

gnu::error
gnu::warning

gnu::error
gnu::warning

Yes

The error and warning function attributes can be used to specify a custom diagnostic to be emitted when a call to such a function is not eliminated via optimizations. This can be used to create compile time assertions that depend on optimizations, while providing diagnostics pointing to precise locations of the call site in the source.

__attribute__((warning("oh no"))) void dontcall();
void foo() {
  if (someCompileTimeAssertionThatsTrue)
    dontcall(); // Warning

  dontcall(); // Warning

  if (someCompileTimeAssertionThatsFalse)
    dontcall(); // No Warning
  sizeof(dontcall()); // No Warning
}

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

exclude_from_explicit_instantiation

clang::exclude_from_explicit_instantiation

clang::exclude_from_explicit_instantiation

Yes

The exclude_from_explicit_instantiation attribute opts-out a member of a class template from being part of explicit template instantiations of that class template. This means that an explicit instantiation will not instantiate members of the class template marked with the attribute, but also that code where an extern template declaration of the enclosing class template is visible will not take for granted that an external instantiation of the class template would provide those members (which would otherwise be a link error, since the explicit instantiation won’t provide those members). For example, let’s say we don’t want the data() method to be part of libc++’s ABI. To make sure it is not exported from the dylib, we give it hidden visibility:

// in <string>
template <class CharT>
class basic_string {
public:
  __attribute__((__visibility__("hidden")))
  const value_type* data() const noexcept { ... }
};

template class basic_string<char>;

Since an explicit template instantiation declaration for basic_string<char> is provided, the compiler is free to assume that basic_string<char>::data() will be provided by another translation unit, and it is free to produce an external call to this function. However, since data() has hidden visibility and the explicit template instantiation is provided in a shared library (as opposed to simply another translation unit), basic_string<char>::data() won’t be found and a link error will ensue. This happens because the compiler assumes that basic_string<char>::data() is part of the explicit template instantiation declaration, when it really isn’t. To tell the compiler that data() is not part of the explicit template instantiation declaration, the exclude_from_explicit_instantiation attribute can be used:

// in <string>
template <class CharT>
class basic_string {
public:
  __attribute__((__visibility__("hidden")))
  __attribute__((exclude_from_explicit_instantiation))
  const value_type* data() const noexcept { ... }
};

template class basic_string<char>;

Now, the compiler won’t assume that basic_string<char>::data() is provided externally despite there being an explicit template instantiation declaration: the compiler will implicitly instantiate basic_string<char>::data() in the TUs where it is used.

This attribute can be used on static and non-static member functions of class templates, static data members of class templates and member classes of class templates.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

export_name

clang::export_name

clang::export_name

Yes

Clang supports the __attribute__((export_name(<name>))) attribute for the WebAssembly target. This attribute may be attached to a function declaration, where it modifies how the symbol is to be exported from the linked WebAssembly.

WebAssembly functions are exported via string name. By default when a symbol is exported, the export name for C/C++ symbols are the same as their C/C++ symbol names. This attribute can be used to override the default behavior, and request a specific string name be used instead.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

flatten

gnu::flatten

gnu::flatten

Yes

The flatten attribute causes calls within the attributed function to be inlined unless it is impossible to do so, for example if the body of the callee is unavailable or if the callee has the noinline attribute.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

force_align_arg_pointer

gnu::force_align_arg_pointer

gnu::force_align_arg_pointer

Use this attribute to force stack alignment.

Legacy x86 code uses 4-byte stack alignment. Newer aligned SSE instructions (like ‘movaps’) that work with the stack require operands to be 16-byte aligned. This attribute realigns the stack in the function prologue to make sure the stack can be used with SSE instructions.

Note that the x86_64 ABI forces 16-byte stack alignment at the call site. Because of this, ‘force_align_arg_pointer’ is not needed on x86_64, except in rare cases where the caller does not align the stack properly (e.g. flow jumps from i386 arch code).

__attribute__ ((force_align_arg_pointer))
void f () {
  ...
}

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

format

gnu::format

gnu::format

Clang supports the format attribute, which indicates that the function accepts a printf or scanf-like format string and corresponding arguments or a va_list that contains these arguments.

Please see GCC documentation about format attribute to find details about attribute syntax.

Clang implements two kinds of checks with this attribute.

1. Clang checks that the function with the format attribute is called with a format string that uses format specifiers that are allowed, and that arguments match the format string. This is the -Wformat warning, it is on by default.

2. Clang checks that the format string argument is a literal string. This is the -Wformat-nonliteral warning, it is off by default.

   Clang implements this mostly the same way as GCC, but there is a difference for functions that accept a va_list argument (for example, vprintf). GCC does not emit -Wformat-nonliteral warning for calls to such functions. Clang does not warn if the format string comes from a function parameter, where the function is annotated with a compatible attribute, otherwise it warns. For example:

   __attribute__((__format__ (__scanf__, 1, 3)))
   void foo(const char* s, char *buf, ...) {
     va_list ap;
     va_start(ap, buf);
   
     vprintf(s, ap); // warning: format string is not a string literal
   }
   

   In this case we warn because s contains a format string for a scanf-like function, but it is passed to a printf-like function.

   If the attribute is removed, clang still warns, because the format string is not a string literal.

   Another example:

   __attribute__((__format__ (__printf__, 1, 3)))
   void foo(const char* s, char *buf, ...) {
     va_list ap;
     va_start(ap, buf);
   
     vprintf(s, ap); // warning
   }
   

   In this case Clang does not warn because the format string s and the corresponding arguments are annotated. If the arguments are incorrect, the caller of foo will receive a warning.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

gnu_inline

gnu::gnu_inline

gnu::gnu_inline

Yes

The gnu_inline changes the meaning of extern inline to use GNU inline semantics, meaning:

• If any declaration that is declared inline is not declared extern, then the inline keyword is just a hint. In particular, an out-of-line definition is still emitted for a function with external linkage, even if all call sites are inlined, unlike in C99 and C++ inline semantics.

• If all declarations that are declared inline are also declared extern, then the function body is present only for inlining and no out-of-line version is emitted.

Some important consequences: static inline emits an out-of-line version if needed, a plain inline definition emits an out-of-line version always, and an extern inline definition (in a header) followed by a (non-extern) inline declaration in a source file emits an out-of-line version of the function in that source file but provides the function body for inlining to all includers of the header.

Either __GNUC_GNU_INLINE__ (GNU inline semantics) or __GNUC_STDC_INLINE__ (C99 semantics) will be defined (they are mutually exclusive). If __GNUC_STDC_INLINE__ is defined, then the gnu_inline function attribute can be used to get GNU inline semantics on a per function basis. If __GNUC_GNU_INLINE__ is defined, then the translation unit is already being compiled with GNU inline semantics as the implied default. It is unspecified which macro is defined in a C++ compilation.

GNU inline semantics are the default behavior with -std=gnu89, -std=c89, -std=c94, or -fgnu89-inline.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

guard

Code can indicate CFG checks are not wanted with the __declspec(guard(nocf)) attribute. This directs the compiler to not insert any CFG checks for the entire function. This approach is typically used only sparingly in specific situations where the programmer has manually inserted “CFG-equivalent” protection. The programmer knows that they are calling through some read-only function table whose address is obtained through read-only memory references and for which the index is masked to the function table limit. This approach may also be applied to small wrapper functions that are not inlined and that do nothing more than make a call through a function pointer. Since incorrect usage of this directive can compromise the security of CFG, the programmer must be very careful using the directive. Typically, this usage is limited to very small functions that only call one function.

Control Flow Guard documentation <https://docs.microsoft.com/en-us/windows/win32/secbp/pe-metadata>

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

ifunc

gnu::ifunc

gnu::ifunc

Yes

__attribute__((ifunc("resolver"))) is used to mark that the address of a declaration should be resolved at runtime by calling a resolver function.

The symbol name of the resolver function is given in quotes. A function with this name (after mangling) must be defined in the current translation unit; it may be static. The resolver function should return a pointer.

The ifunc attribute may only be used on a function declaration. A function declaration with an ifunc attribute is considered to be a definition of the declared entity. The entity must not have weak linkage; for example, in C++, it cannot be applied to a declaration if a definition at that location would be considered inline.

Not all targets support this attribute. ELF target support depends on both the linker and runtime linker, and is available in at least lld 4.0 and later, binutils 2.20.1 and later, glibc v2.11.1 and later, and FreeBSD 9.1 and later. Non-ELF targets currently do not support this attribute.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

import_module

clang::import_module

clang::import_module

Yes

Clang supports the __attribute__((import_module(<module_name>))) attribute for the WebAssembly target. This attribute may be attached to a function declaration, where it modifies how the symbol is to be imported within the WebAssembly linking environment.

WebAssembly imports use a two-level namespace scheme, consisting of a module name, which typically identifies a module from which to import, and a field name, which typically identifies a field from that module to import. By default, module names for C/C++ symbols are assigned automatically by the linker. This attribute can be used to override the default behavior, and request a specific module name be used instead.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

import_name

clang::import_name

clang::import_name

Yes

Clang supports the __attribute__((import_name(<name>))) attribute for the WebAssembly target. This attribute may be attached to a function declaration, where it modifies how the symbol is to be imported within the WebAssembly linking environment.

WebAssembly imports use a two-level namespace scheme, consisting of a module name, which typically identifies a module from which to import, and a field name, which typically identifies a field from that module to import. By default, field names for C/C++ symbols are the same as their C/C++ symbol names. This attribute can be used to override the default behavior, and request a specific field name be used instead.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

internal_linkage

clang::internal_linkage

clang::internal_linkage

Yes

The internal_linkage attribute changes the linkage type of the declaration to internal. This is similar to C-style static, but can be used on classes and class methods. When applied to a class definition, this attribute affects all methods and static data members of that class. This can be used to contain the ABI of a C++ library by excluding unwanted class methods from the export tables.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

interrupt

gnu::interrupt

gnu::interrupt

Clang supports the GNU style __attribute__((interrupt("TYPE"))) attribute on ARM targets. This attribute may be attached to a function definition and instructs the backend to generate appropriate function entry/exit code so that it can be used directly as an interrupt service routine.

The parameter passed to the interrupt attribute is optional, but if provided it must be a string literal with one of the following values: “IRQ”, “FIQ”, “SWI”, “ABORT”, “UNDEF”.

The semantics are as follows:

• If the function is AAPCS, Clang instructs the backend to realign the stack to 8 bytes on entry. This is a general requirement of the AAPCS at public interfaces, but may not hold when an exception is taken. Doing this allows other AAPCS functions to be called.

• If the CPU is M-class this is all that needs to be done since the architecture itself is designed in such a way that functions obeying the normal AAPCS ABI constraints are valid exception handlers.

• If the CPU is not M-class, the prologue and epilogue are modified to save all non-banked registers that are used, so that upon return the user-mode state will not be corrupted. Note that to avoid unnecessary overhead, only general-purpose (integer) registers are saved in this way. If VFP operations are needed, that state must be saved manually.

  Specifically, interrupt kinds other than “FIQ” will save all core registers except “lr” and “sp”. “FIQ” interrupts will save r0-r7.

• If the CPU is not M-class, the return instruction is changed to one of the canonical sequences permitted by the architecture for exception return. Where possible the function itself will make the necessary “lr” adjustments so that the “preferred return address” is selected.

  Unfortunately the compiler is unable to make this guarantee for an “UNDEF” handler, where the offset from “lr” to the preferred return address depends on the execution state of the code which generated the exception. In this case a sequence equivalent to “movs pc, lr” will be used.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

interrupt

gnu::interrupt

gnu::interrupt

Yes

Clang supports the GNU style __attribute__((interrupt)) attribute on AVR targets. This attribute may be attached to a function definition and instructs the backend to generate appropriate function entry/exit code so that it can be used directly as an interrupt service routine.

On the AVR, the hardware globally disables interrupts when an interrupt is executed. The first instruction of an interrupt handler declared with this attribute is a SEI instruction to re-enable interrupts. See also the signal attribute that does not insert a SEI instruction.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

interrupt

gnu::interrupt

gnu::interrupt

Yes

Clang supports the GNU style __attribute__((interrupt("ARGUMENT"))) attribute on MIPS targets. This attribute may be attached to a function definition and instructs the backend to generate appropriate function entry/exit code so that it can be used directly as an interrupt service routine.

By default, the compiler will produce a function prologue and epilogue suitable for an interrupt service routine that handles an External Interrupt Controller (eic) generated interrupt. This behavior can be explicitly requested with the “eic” argument.

Otherwise, for use with vectored interrupt mode, the argument passed should be of the form “vector=LEVEL” where LEVEL is one of the following values: “sw0”, “sw1”, “hw0”, “hw1”, “hw2”, “hw3”, “hw4”, “hw5”. The compiler will then set the interrupt mask to the corresponding level which will mask all interrupts up to and including the argument.

The semantics are as follows:

• The prologue is modified so that the Exception Program Counter (EPC) and Status coprocessor registers are saved to the stack. The interrupt mask is set so that the function can only be interrupted by a higher priority interrupt. The epilogue will restore the previous values of EPC and Status.

• The prologue and epilogue are modified to save and restore all non-kernel registers as necessary.

• The FPU is disabled in the prologue, as the floating pointer registers are not spilled to the stack.

• The function return sequence is changed to use an exception return instruction.

• The parameter sets the interrupt mask for the function corresponding to the interrupt level specified. If no mask is specified the interrupt mask defaults to “eic”.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

interrupt

gnu::interrupt

gnu::interrupt

Yes

Clang supports the GNU style __attribute__((interrupt)) attribute on RISCV targets. This attribute may be attached to a function definition and instructs the backend to generate appropriate function entry/exit code so that it can be used directly as an interrupt service routine.

Permissible values for this parameter are user, supervisor, and machine. If there is no parameter, then it defaults to machine.

Repeated interrupt attribute on the same declaration will cause a warning to be emitted. In case of repeated declarations, the last one prevails.

Refer to: https://gcc.gnu.org/onlinedocs/gcc/RISC-V-Function-Attributes.html https://riscv.org/specifications/privileged-isa/ The RISC-V Instruction Set Manual Volume II: Privileged Architecture Version 1.10.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

kernel

Yes

__attribute__((kernel)) is used to mark a kernel function in RenderScript.

In RenderScript, kernel functions are used to express data-parallel computations. The RenderScript runtime efficiently parallelizes kernel functions to run on computational resources such as multi-core CPUs and GPUs. See the RenderScript documentation for more information.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

lifetimebound

clang::lifetimebound

The lifetimebound attribute on a function parameter or implicit object parameter indicates that objects that are referred to by that parameter may also be referred to by the return value of the annotated function (or, for a parameter of a constructor, by the value of the constructed object). It is only supported in C++.

By default, a reference is considered to refer to its referenced object, a pointer is considered to refer to its pointee, a std::initializer_list<T> is considered to refer to its underlying array, and aggregates (arrays and simple structs) are considered to refer to all objects that their transitive subobjects refer to.

Clang warns if it is able to detect that an object or reference refers to another object with a shorter lifetime. For example, Clang will warn if a function returns a reference to a local variable, or if a reference is bound to a temporary object whose lifetime is not extended. By using the lifetimebound attribute, this determination can be extended to look through user-declared functions. For example:

// Returns m[key] if key is present, or default_value if not.
template<typename T, typename U>
const U &get_or_default(const std::map<T, U> &m [[clang::lifetimebound]],
                        const T &key, /* note, not lifetimebound */
                        const U &default_value [[clang::lifetimebound]]);

std::map<std::string, std::string> m;
// warning: temporary "bar"s that might be bound to local reference 'val'
// will be destroyed at the end of the full-expression
const std::string &val = get_or_default(m, "foo"s, "bar"s);

// No warning in this case.
std::string def_val = "bar"s;
const std::string &val = get_or_default(m, "foo"s, def_val);

The attribute can be applied to the implicit this parameter of a member function by writing the attribute after the function type:

struct string {
  // The returned pointer should not outlive ``*this``.
  const char *data() const [[clang::lifetimebound]];
};

This attribute is inspired by the C++ committee paper P0936R0, but does not affect whether temporary objects have their lifetimes extended.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

long_call
far

gnu::long_call
gnu::far

gnu::long_call
gnu::far

Yes

Clang supports the __attribute__((long_call)), __attribute__((far)), and __attribute__((near)) attributes on MIPS targets. These attributes may only be added to function declarations and change the code generated by the compiler when directly calling the function. The near attribute allows calls to the function to be made using the jal instruction, which requires the function to be located in the same naturally aligned 256MB segment as the caller. The long_call and far attributes are synonyms and require the use of a different call sequence that works regardless of the distance between the functions.

These attributes have no effect for position-independent code.

These attributes take priority over command line switches such as -mlong-calls and -mno-long-calls.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

malloc

gnu::malloc

gnu::malloc

restrict

Yes

The malloc attribute indicates that the function acts like a system memory allocation function, returning a pointer to allocated storage disjoint from the storage for any other object accessible to the caller.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

micromips

gnu::micromips

gnu::micromips

Yes

Clang supports the GNU style __attribute__((micromips)) and __attribute__((nomicromips)) attributes on MIPS targets. These attributes may be attached to a function definition and instructs the backend to generate or not to generate microMIPS code for that function.

These attributes override the -mmicromips and -mno-micromips options on the command line.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

mig_server_routine

clang::mig_server_routine

clang::mig_server_routine

Yes

The Mach Interface Generator release-on-success convention dictates functions that follow it to only release arguments passed to them when they return “success” (a kern_return_t error code that indicates that no errors have occurred). Otherwise the release is performed by the MIG client that called the function. The annotation __attribute__((mig_server_routine)) is applied in order to specify which functions are expected to follow the convention. This allows the Static Analyzer to find bugs caused by violations of that convention. The attribute would normally appear on the forward declaration of the actual server routine in the MIG server header, but it may also be added to arbitrary functions that need to follow the same convention - for example, a user can add them to auxiliary functions called by the server routine that have their return value of type kern_return_t unconditionally returned from the routine. The attribute can be applied to C++ methods, and in this case it will be automatically applied to overrides if the method is virtual. The attribute can also be written using C++11 syntax: [[mig::server_routine]].

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

min_vector_width

clang::min_vector_width

clang::min_vector_width

Yes

Clang supports the __attribute__((min_vector_width(width))) attribute. This attribute may be attached to a function and informs the backend that this function desires vectors of at least this width to be generated. Target-specific maximum vector widths still apply. This means even if you ask for something larger than the target supports, you will only get what the target supports. This attribute is meant to be a hint to control target heuristics that may generate narrower vectors than what the target hardware supports.

This is currently used by the X86 target to allow some CPUs that support 512-bit vectors to be limited to using 256-bit vectors to avoid frequency penalties. This is currently enabled with the -prefer-vector-width=256 command line option. The min_vector_width attribute can be used to prevent the backend from trying to split vector operations to match the prefer-vector-width. All X86 vector intrinsics from x86intrin.h already set this attribute. Additionally, use of any of the X86-specific vector builtins will implicitly set this attribute on the calling function. The intent is that explicitly writing vector code using the X86 intrinsics will prevent prefer-vector-width from affecting the code.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

no_builtin

clang::no_builtin

clang::no_builtin

Yes

Note

This attribute is not yet fully implemented, it is validated but has no effect on the generated code.

The __attribute__((no_builtin)) is similar to the -fno-builtin flag except it is specific to the body of a function. The attribute may also be applied to a virtual function but has no effect on the behavior of overriding functions in a derived class.

It accepts one or more strings corresponding to the specific names of the builtins to disable (e.g. “memcpy”, “memset”). If the attribute is used without parameters it will disable all buitins at once.

// The compiler is not allowed to add any builtin to foo's body.
void foo(char* data, size_t count) __attribute__((no_builtin)) {
  // The compiler is not allowed to convert the loop into
  // `__builtin_memset(data, 0xFE, count);`.
  for (size_t i = 0; i < count; ++i)
    data[i] = 0xFE;
}

// The compiler is not allowed to add the `memcpy` builtin to bar's body.
void bar(char* data, size_t count) __attribute__((no_builtin("memcpy"))) {
  // The compiler is allowed to convert the loop into
  // `__builtin_memset(data, 0xFE, count);` but cannot generate any
  // `__builtin_memcpy`
  for (size_t i = 0; i < count; ++i)
    data[i] = 0xFE;
}

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

no_caller_saved_registers

gnu::no_caller_saved_registers

gnu::no_caller_saved_registers

Use this attribute to indicate that the specified function has no caller-saved registers. That is, all registers are callee-saved except for registers used for passing parameters to the function or returning parameters from the function. The compiler saves and restores any modified registers that were not used for passing or returning arguments to the function.

The user can call functions specified with the ‘no_caller_saved_registers’ attribute from an interrupt handler without saving and restoring all call-clobbered registers.

Note that ‘no_caller_saved_registers’ attribute is not a calling convention. In fact, it only overrides the decision of which registers should be saved by the caller, but not how the parameters are passed from the caller to the callee.

For example:

__attribute__ ((no_caller_saved_registers, fastcall))
void f (int arg1, int arg2) {
  ...
}

In this case parameters ‘arg1’ and ‘arg2’ will be passed in registers. In this case, on 32-bit x86 targets, the function ‘f’ will use ECX and EDX as register parameters. However, it will not assume any scratch registers and should save and restore any modified registers except for ECX and EDX.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

no_profile_instrument_function

gnu::no_profile_instrument_function

gnu::no_profile_instrument_function

Yes

Use the no_profile_instrument_function attribute on a function declaration to denote that the compiler should not instrument the function with profile-related instrumentation, such as via the -fprofile-generate / -fprofile-instr-generate / -fcs-profile-generate / -fprofile-arcs flags.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

no_sanitize

clang::no_sanitize

clang::no_sanitize

Yes

Use the no_sanitize attribute on a function or a global variable declaration to specify that a particular instrumentation or set of instrumentations should not be applied.

The attribute takes a list of string literals with the following accepted values: * all values accepted by -fno-sanitize=; * coverage, to disable SanitizerCoverage instrumentation.

For example, __attribute__((no_sanitize("address", "thread"))) specifies that AddressSanitizer and ThreadSanitizer should not be applied to the function or variable. Using __attribute__((no_sanitize("coverage"))) specifies that SanitizerCoverage should not be applied to the function.

See Controlling Code Generation for a full list of supported sanitizer flags.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

no_address_safety_analysis
no_sanitize_address
no_sanitize_thread
no_sanitize_memory

gnu::no_address_safety_analysis
gnu::no_sanitize_address
gnu::no_sanitize_thread
clang::no_sanitize_memory

gnu::no_address_safety_analysis
gnu::no_sanitize_address
gnu::no_sanitize_thread
clang::no_sanitize_memory

Yes

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

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

no_address_safety_analysis
no_sanitize_address
no_sanitize_thread
no_sanitize_memory

gnu::no_address_safety_analysis
gnu::no_sanitize_address
gnu::no_sanitize_thread
clang::no_sanitize_memory

gnu::no_address_safety_analysis
gnu::no_sanitize_address
gnu::no_sanitize_thread
clang::no_sanitize_memory

Yes

Use __attribute__((no_sanitize_memory)) on a function declaration to specify that checks for uninitialized memory should not be inserted (e.g. by MemorySanitizer). The function may still be instrumented by the tool to avoid false positives in other places.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

no_address_safety_analysis
no_sanitize_address
no_sanitize_thread
no_sanitize_memory

gnu::no_address_safety_analysis
gnu::no_sanitize_address
gnu::no_sanitize_thread
clang::no_sanitize_memory

gnu::no_address_safety_analysis
gnu::no_sanitize_address
gnu::no_sanitize_thread
clang::no_sanitize_memory

Yes

Use __attribute__((no_sanitize_thread)) on a function declaration to specify that checks for data races on plain (non-atomic) memory accesses should not be inserted by ThreadSanitizer. The function is still instrumented by the tool to avoid false positives and provide meaningful stack traces.

GNU

C++11

C2x

__declspec

Keyword

#pragma

#pragma clang attribute

no_speculative_load_hardening

clang::no_speculative_load_hardening