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The file exists for local testing + purposes only. ------------------------------------------------------------------- =================== Attributes in Clang -=================== -.. contents:: - :local: - -Introduction -============ - -This page lists the attributes currently supported by Clang. - -Function Attributes -=================== - - -interrupt ---------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -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. - - -acquire_capability (acquire_shared_capability, clang::acquire_capability, clang::acquire_shared_capability) ------------------------------------------------------------------------------------------------------------ -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -Marks a function as acquiring a capability. - - -assert_capability (assert_shared_capability, clang::assert_capability, clang::assert_shared_capability) -------------------------------------------------------------------------------------------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -Marks a function that dynamically tests whether a capability is held, and halts -the program if it is not held. - - -availability ------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -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``: - -.. code-block:: c++ - - void f(void) __attribute__((availability(macosx,introduced=10.4,deprecated=10.6,obsoleted=10.7))); - -The availability attribute states that ``f`` was introduced in Mac OS X 10.4, -deprecated in Mac OS X 10.6, and obsoleted in Mac OS X 10.7. This information -is used by Clang to determine when it is safe to use ``f``: for example, if -Clang is instructed to compile code for Mac OS X 10.5, a call to ``f()`` -succeeds. If Clang is instructed to compile code for Mac OS X 10.6, the call -succeeds but Clang emits a warning specifying that the function is deprecated. -Finally, if Clang is instructed to compile code for Mac OS X 10.7, the call -fails because ``f()`` is no longer available. - -The 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. - -Multiple availability attributes can be placed on a declaration, which may -correspond to different platforms. Only the availability attribute with the -platform corresponding to the target platform will be used; any others will be -ignored. If no availability attribute specifies availability for the current -target platform, the availability attributes are ignored. Supported platforms -are: - -``ios`` - Apple's iOS operating system. The minimum deployment target is specified by - the ``-mios-version-min=*version*`` or ``-miphoneos-version-min=*version*`` - command-line arguments. - -``macosx`` - Apple's Mac OS X operating system. The minimum deployment target is - specified by the ``-mmacosx-version-min=*version*`` command-line argument. - -A declaration can be used even when deploying back to a platform version prior -to when the declaration was introduced. When this happens, the declaration is -`weakly linked -<https://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html>`_, -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. - -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: - -.. code-block:: c - - void g(void) __attribute__((availability(macosx,introduced=10.4))); - void g(void) __attribute__((availability(macosx,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 macosx and ios availability from above. - void g(void) __attribute__((availability(macosx,introduced=10.5))); // error: mismatch - -When one method overrides another, the overriding method can be more widely available than the overridden method, e.g.,: - -.. code-block:: objc - - @interface A - - (id)method __attribute__((availability(macosx,introduced=10.4))); - - (id)method2 __attribute__((availability(macosx,introduced=10.4))); - @end - - @interface B : A - - (id)method __attribute__((availability(macosx,introduced=10.3))); // okay: method moved into base class later - - (id)method __attribute__((availability(macosx,introduced=10.5))); // error: this method was available via the base class in 10.4 - @end - - -_Noreturn ---------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "","","","X" - -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. - - -noreturn --------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "","X","","" - -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. - - -carries_dependency ------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -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. - - -enable_if ---------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -.. 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. - -.. code-block:: 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 callsite. 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: - -.. code-block:: c++ - - __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. - -.. code-block:: c++ - - 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)``. - - -flatten (gnu::flatten) ----------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -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. - - -format (gnu::format) --------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -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 -<http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_ to find details -about attribute syntax. - -Clang implements two kinds of checks with this attribute. - -#. 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. - -#. 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 - fuctions. 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: - - .. code-block:: c - - __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: - - .. code-block:: c - - __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. - - -noduplicate (clang::noduplicate) --------------------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -The ``noduplicate`` attribute can be placed on function declarations to control -whether function calls to this function can be duplicated or not as a result of -optimizations. This is required for the implementation of functions with -certain special requirements, like the OpenCL "barrier" function, that might -need to be run concurrently by all the threads that are executing in lockstep -on the hardware. For example this attribute applied on the function -"nodupfunc" in the code below avoids that: - -.. code-block:: c - - void nodupfunc() __attribute__((noduplicate)); - // Setting it as a C++11 attribute is also valid - // void nodupfunc() [[clang::noduplicate]]; - void foo(); - void bar(); - - nodupfunc(); - if (a > n) { - foo(); - } else { - bar(); - } - -gets possibly modified by some optimizations into code similar to this: - -.. code-block:: c - - if (a > n) { - nodupfunc(); - foo(); - } else { - nodupfunc(); - bar(); - } - -where the call to "nodupfunc" is duplicated and sunk into the two branches -of the condition. - - -no_sanitize_address (no_address_safety_analysis, gnu::no_address_safety_analysis, gnu::no_sanitize_address) ------------------------------------------------------------------------------------------------------------ -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -.. _langext-address_sanitizer: - -Use ``__attribute__((no_sanitize_address))`` on a function declaration to -specify that address safety instrumentation (e.g. AddressSanitizer) should -not be applied to that function. - - -no_sanitize_memory ------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -.. _langext-memory_sanitizer: - -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. - - -no_sanitize_thread ------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -.. _langext-thread_sanitizer: - -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. - - -no_split_stack (gnu::no_split_stack) ------------------------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -The ``no_split_stack`` attribute disables the emission of the split stack -preamble for a particular function. It has no effect if ``-fsplit-stack`` -is not specified. - - -objc_method_family ------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -Many methods in Objective-C have conventional meanings determined by their -selectors. It is sometimes useful to be able to mark a method as having a -particular conventional meaning despite not having the right selector, or as -not having the conventional meaning that its selector would suggest. For these -use cases, we provide an attribute to specifically describe the "method family" -that a method belongs to. - -**Usage**: ``__attribute__((objc_method_family(X)))``, where ``X`` is one of -``none``, ``alloc``, ``copy``, ``init``, ``mutableCopy``, or ``new``. This -attribute can only be placed at the end of a method declaration: - -.. code-block:: objc - - - (NSString *)initMyStringValue __attribute__((objc_method_family(none))); - -Users who do not wish to change the conventional meaning of a method, and who -merely want to document its non-standard retain and release semantics, should -use the retaining behavior attributes (``ns_returns_retained``, -``ns_returns_not_retained``, etc). - -Query for this feature with ``__has_attribute(objc_method_family)``. - - -objc_requires_super -------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -Some Objective-C classes allow a subclass to override a particular method in a -parent class but expect that the overriding method also calls the overridden -method in the parent class. For these cases, we provide an attribute to -designate that a method requires a "call to ``super``" in the overriding -method in the subclass. - -**Usage**: ``__attribute__((objc_requires_super))``. This attribute can only -be placed at the end of a method declaration: - -.. code-block:: objc - - - (void)foo __attribute__((objc_requires_super)); - -This attribute can only be applied the method declarations within a class, and -not a protocol. Currently this attribute does not enforce any placement of -where the call occurs in the overriding method (such as in the case of -``-dealloc`` where the call must appear at the end). It checks only that it -exists. - -Note that on both OS X and iOS that the Foundation framework provides a -convenience macro ``NS_REQUIRES_SUPER`` that provides syntactic sugar for this -attribute: - -.. code-block:: objc - - - (void)foo NS_REQUIRES_SUPER; - -This macro is conditionally defined depending on the compiler's support for -this attribute. If the compiler does not support the attribute the macro -expands to nothing. - -Operationally, when a method has this annotation the compiler will warn if the -implementation of an override in a subclass does not call super. For example: - -.. code-block:: objc - - warning: method possibly missing a [super AnnotMeth] call - - (void) AnnotMeth{}; - ^ - - -optnone (clang::optnone) ------------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -The ``optnone`` attribute suppresses essentially all optimizations -on a function or method, regardless of the optimization level applied to -the compilation unit as a whole. This is particularly useful when you -need to debug a particular function, but it is infeasible to build the -entire application without optimization. Avoiding optimization on the -specified function can improve the quality of the debugging information -for that function. - -This attribute is incompatible with the ``always_inline`` attribute. - - -overloadable ------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -Clang provides support for C++ function overloading in C. Function overloading -in C is introduced using the ``overloadable`` attribute. For example, one -might provide several overloaded versions of a ``tgsin`` function that invokes -the appropriate standard function computing the sine of a value with ``float``, -``double``, or ``long double`` precision: - -.. code-block:: c - - #include <math.h> - float __attribute__((overloadable)) tgsin(float x) { return sinf(x); } - double __attribute__((overloadable)) tgsin(double x) { return sin(x); } - long double __attribute__((overloadable)) tgsin(long double x) { return sinl(x); } - -Given these declarations, one can call ``tgsin`` with a ``float`` value to -receive a ``float`` result, with a ``double`` to receive a ``double`` result, -etc. Function overloading in C follows the rules of C++ function overloading -to pick the best overload given the call arguments, with a few C-specific -semantics: - -* Conversion from ``float`` or ``double`` to ``long double`` is ranked as a - floating-point promotion (per C99) rather than as a floating-point conversion - (as in C++). - -* A conversion from a pointer of type ``T*`` to a pointer of type ``U*`` is - considered a pointer conversion (with conversion rank) if ``T`` and ``U`` are - compatible types. - -* A conversion from type ``T`` to a value of type ``U`` is permitted if ``T`` - and ``U`` are compatible types. This conversion is given "conversion" rank. - -The declaration of ``overloadable`` functions is restricted to function -declarations and definitions. Most importantly, if any function with a given -name is given the ``overloadable`` attribute, then all function declarations -and definitions with that name (and in that scope) must have the -``overloadable`` attribute. This rule even applies to redeclarations of -functions whose original declaration had the ``overloadable`` attribute, e.g., - -.. code-block:: c - - int f(int) __attribute__((overloadable)); - float f(float); // error: declaration of "f" must have the "overloadable" attribute - - int g(int) __attribute__((overloadable)); - int g(int) { } // error: redeclaration of "g" must also have the "overloadable" attribute - -Functions marked ``overloadable`` must have prototypes. Therefore, the -following code is ill-formed: - -.. code-block:: c - - int h() __attribute__((overloadable)); // error: h does not have a prototype - -However, ``overloadable`` functions are allowed to use a ellipsis even if there -are no named parameters (as is permitted in C++). This feature is particularly -useful when combined with the ``unavailable`` attribute: - -.. code-block:: c++ - - void honeypot(...) __attribute__((overloadable, unavailable)); // calling me is an error - -Functions declared with the ``overloadable`` attribute have their names mangled -according to the same rules as C++ function names. For example, the three -``tgsin`` functions in our motivating example get the mangled names -``_Z5tgsinf``, ``_Z5tgsind``, and ``_Z5tgsine``, respectively. There are two -caveats to this use of name mangling: - -* Future versions of Clang may change the name mangling of functions overloaded - in C, so you should not depend on an specific mangling. To be completely - safe, we strongly urge the use of ``static inline`` with ``overloadable`` - functions. - -* The ``overloadable`` attribute has almost no meaning when used in C++, - because names will already be mangled and functions are already overloadable. - However, when an ``overloadable`` function occurs within an ``extern "C"`` - linkage specification, it's name *will* be mangled in the same way as it - would in C. - -Query for this feature with ``__has_extension(attribute_overloadable)``. - - -pcs (gnu::pcs) --------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -On ARM targets, this can attribute can be used to select calling conventions, -similar to ``stdcall`` on x86. Valid parameter values are "aapcs" and -"aapcs-vfp". - - -release_capability (release_shared_capability, clang::release_capability, clang::release_shared_capability) ------------------------------------------------------------------------------------------------------------ -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -Marks a function as releasing a capability. - - -try_acquire_capability (try_acquire_shared_capability, clang::try_acquire_capability, clang::try_acquire_shared_capability) ---------------------------------------------------------------------------------------------------------------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -Marks a function that attempts to acquire a capability. This function may fail to -actually acquire the capability; they accept a Boolean value determining -whether acquiring the capability means success (true), or failing to acquire -the capability means success (false). - - -Variable Attributes -=================== - - -section (gnu::section, __declspec(allocate)) --------------------------------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","X","" - -The ``section`` attribute allows you to specify a specific section a -global variable or function should be in after translation. - - -tls_model (gnu::tls_model) --------------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","X","","" - -The ``tls_model`` attribute allows you to specify which thread-local storage -model to use. It accepts the following strings: - -* global-dynamic -* local-dynamic -* initial-exec -* local-exec - -TLS models are mutually exclusive. - - -thread ------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "","","X","" - -The ``__declspec(thread)`` attribute declares a variable with thread local -storage. It is available under the ``-fms-extensions`` flag for MSVC -compatibility. Documentation for the Visual C++ attribute is available on MSDN_. - -.. _MSDN: http://msdn.microsoft.com/en-us/library/9w1sdazb.aspx - -In Clang, ``__declspec(thread)`` is generally equivalent in functionality to the -GNU ``__thread`` keyword. The variable must not have a destructor and must have -a constant initializer, if any. The attribute only applies to variables -declared with static storage duration, such as globals, class static data -members, and static locals. - - -Type Attributes -=============== - - -__single_inhertiance, __multiple_inheritance, __virtual_inheritance -------------------------------------------------------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "","","","X" - -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``: - -.. code-block:: c++ - - 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: - -.. code-block:: c++ - - 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: - -.. code-block:: c++ - - struct __multiple_inheritance S; // error: inheritance model does not match definition - int S::*i; - struct S {}; - - -Statement Attributes -==================== - - -fallthrough (clang::fallthrough) --------------------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "","X","","" - -The ``clang::fallthrough`` attribute is used along with the -``-Wimplicit-fallthrough`` argument to annotate intentional fall-through -between switch labels. It can only be applied to a null statement placed at a -point of execution between any statement and the next switch label. It is -common to mark these places with a specific comment, but this attribute is -meant to replace comments with a more strict annotation, which can be checked -by the compiler. This attribute doesn't change semantics of the code and can -be used wherever an intended fall-through occurs. It is designed to mimic -control-flow statements like ``break;``, so it can be placed in most places -where ``break;`` can, but only if there are no statements on the execution path -between it and the next switch label. - -Here is an example: - -.. code-block:: c++ - - // compile with -Wimplicit-fallthrough - switch (n) { - case 22: - case 33: // no warning: no statements between case labels - f(); - case 44: // warning: unannotated fall-through - g(); - [[clang::fallthrough]]; - case 55: // no warning - if (x) { - h(); - break; - } - else { - i(); - [[clang::fallthrough]]; - } - case 66: // no warning - p(); - [[clang::fallthrough]]; // warning: fallthrough annotation does not - // directly precede case label - q(); - case 77: // warning: unannotated fall-through - r(); - } - - -Consumed Annotation Checking -============================ -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.** - -callable_when -------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -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")))`` - - -consumable ----------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -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``. - - -param_typestate ---------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -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. - - -return_typestate ----------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -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. - - -set_typestate -------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -Annotate methods that transition an object into a new state with -``__attribute__((set_typestate(new_state)))``. The new new state must be -unconsumed, consumed, or unknown. - - -test_typestate --------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -Use ``__attribute__((test_typestate(tested_state)))`` to indicate that a method -returns true if the object is in the specified state.. - - -Type Safety Checking -==================== -Clang supports additional attributes to enable checking type safety properties -that can't be enforced by the C type system. Use cases include: - -* MPI library implementations, where these attributes enable checking that - the buffer type matches the passed ``MPI_Datatype``; -* for HDF5 library there is a similar use case to MPI; -* checking types of variadic functions' arguments for functions like - ``fcntl()`` and ``ioctl()``. - -You can detect support for these attributes with ``__has_attribute()``. For -example: - -.. code-block:: c++ - - #if defined(__has_attribute) - # if __has_attribute(argument_with_type_tag) && \ - __has_attribute(pointer_with_type_tag) && \ - __has_attribute(type_tag_for_datatype) - # define ATTR_MPI_PWT(buffer_idx, type_idx) __attribute__((pointer_with_type_tag(mpi,buffer_idx,type_idx))) - /* ... other macros ... */ - # endif - #endif - - #if !defined(ATTR_MPI_PWT) - # define ATTR_MPI_PWT(buffer_idx, type_idx) - #endif - - int MPI_Send(void *buf, int count, MPI_Datatype datatype /*, other args omitted */) - ATTR_MPI_PWT(1,3); - -argument_with_type_tag ----------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -Use ``__attribute__((argument_with_type_tag(arg_kind, arg_idx, -type_tag_idx)))`` on a function declaration to specify that the function -accepts a type tag that determines the type of some other argument. -``arg_kind`` is an identifier that should be used when annotating all -applicable type tags. - -This attribute is primarily useful for checking arguments of variadic functions -(``pointer_with_type_tag`` can be used in most non-variadic cases). - -For example: - -.. code-block:: c++ - - int fcntl(int fd, int cmd, ...) - __attribute__(( argument_with_type_tag(fcntl,3,2) )); - - -pointer_with_type_tag ---------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -Use ``__attribute__((pointer_with_type_tag(ptr_kind, ptr_idx, type_tag_idx)))`` -on a function declaration to specify that the function accepts a type tag that -determines the pointee type of some other pointer argument. - -For example: - -.. code-block:: c++ - - int MPI_Send(void *buf, int count, MPI_Datatype datatype /*, other args omitted */) - __attribute__(( pointer_with_type_tag(mpi,1,3) )); - - -type_tag_for_datatype ---------------------- -.. csv-table:: Supported Syntaxes - :header: "GNU", "C++11", "__declspec", "Keyword" - - "X","","","" - -Clang supports annotating type tags of two forms. - -* **Type tag that is an expression containing a reference to some declared - identifier.** Use ``__attribute__((type_tag_for_datatype(kind, type)))`` on a - declaration with that identifier: - - .. code-block:: c++ - - extern struct mpi_datatype mpi_datatype_int - __attribute__(( type_tag_for_datatype(mpi,int) )); - #define MPI_INT ((MPI_Datatype) &mpi_datatype_int) - -* **Type tag that is an integral literal.** Introduce a ``static const`` - variable with a corresponding initializer value and attach - ``__attribute__((type_tag_for_datatype(kind, type)))`` on that declaration, - for example: - - .. code-block:: c++ - - #define MPI_INT ((MPI_Datatype) 42) - static const MPI_Datatype mpi_datatype_int - __attribute__(( type_tag_for_datatype(mpi,int) )) = 42 - -The attribute also accepts an optional third argument that determines how the -expression is compared to the type tag. There are two supported flags: - -* ``layout_compatible`` will cause types to be compared according to - layout-compatibility rules (C++11 [class.mem] p 17, 18). This is - implemented to support annotating types like ``MPI_DOUBLE_INT``. - - For example: - - .. code-block:: c++ - - /* In mpi.h */ - struct internal_mpi_double_int { double d; int i; }; - extern struct mpi_datatype mpi_datatype_double_int - __attribute__(( type_tag_for_datatype(mpi, struct internal_mpi_double_int, layout_compatible) )); - - #define MPI_DOUBLE_INT ((MPI_Datatype) &mpi_datatype_double_int) - - /* In user code */ - struct my_pair { double a; int b; }; - struct my_pair *buffer; - MPI_Send(buffer, 1, MPI_DOUBLE_INT /*, ... */); // no warning - - struct my_int_pair { int a; int b; } - struct my_int_pair *buffer2; - MPI_Send(buffer2, 1, MPI_DOUBLE_INT /*, ... */); // warning: actual buffer element - // type 'struct my_int_pair' - // doesn't match specified MPI_Datatype - -* ``must_be_null`` specifies that the expression should be a null pointer - constant, for example: - - .. code-block:: c++ - - /* In mpi.h */ - extern struct mpi_datatype mpi_datatype_null - __attribute__(( type_tag_for_datatype(mpi, void, must_be_null) )); - - #define MPI_DATATYPE_NULL ((MPI_Datatype) &mpi_datatype_null) - - /* In user code */ - MPI_Send(buffer, 1, MPI_DATATYPE_NULL /*, ... */); // warning: MPI_DATATYPE_NULL - // was specified but buffer - // is not a null pointer - - +===================
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