-..
- -------------------------------------------------------------------
- NOTE: This file is automatically generated by running clang-tblgen
- -gen-attr-docs. Do not edit this file by hand!!
- -------------------------------------------------------------------
-
-===================
-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_depedency``
-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 generatation of more efficient code.
-
-
-enable_if
----------
-.. csv-table:: Supported Syntaxes
- :header: "GNU", "C++11", "__declspec", "Keyword"
-
- "X","","",""
-
-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)``.
-
-
-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","",""
-
-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","","",""
-
-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","","",""
-
-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.
-
-
-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{};
- ^
-
-
-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)``.
-
-
-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
-===================
-
-
-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.
-
-
-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
-
-
+..\r
+ -------------------------------------------------------------------\r
+ NOTE: This file is automatically generated by running clang-tblgen\r
+ -gen-attr-docs. Do not edit this file by hand!!\r
+ -------------------------------------------------------------------\r
+\r
+===================\r
+Attributes in Clang\r
+===================\r
+.. contents::\r
+ :local:\r
+\r
+Introduction\r
+============\r
+\r
+This page lists the attributes currently supported by Clang.\r
+\r
+Function Attributes\r
+===================\r
+\r
+\r
+interrupt\r
+---------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+Clang supports the GNU style ``__attribute__((interrupt("TYPE")))`` attribute on\r
+ARM targets. This attribute may be attached to a function definition and\r
+instructs the backend to generate appropriate function entry/exit code so that\r
+it can be used directly as an interrupt service routine.\r
+\r
+The parameter passed to the interrupt attribute is optional, but if\r
+provided it must be a string literal with one of the following values: "IRQ",\r
+"FIQ", "SWI", "ABORT", "UNDEF".\r
+\r
+The semantics are as follows:\r
+\r
+- If the function is AAPCS, Clang instructs the backend to realign the stack to\r
+ 8 bytes on entry. This is a general requirement of the AAPCS at public\r
+ interfaces, but may not hold when an exception is taken. Doing this allows\r
+ other AAPCS functions to be called.\r
+- If the CPU is M-class this is all that needs to be done since the architecture\r
+ itself is designed in such a way that functions obeying the normal AAPCS ABI\r
+ constraints are valid exception handlers.\r
+- If the CPU is not M-class, the prologue and epilogue are modified to save all\r
+ non-banked registers that are used, so that upon return the user-mode state\r
+ will not be corrupted. Note that to avoid unnecessary overhead, only\r
+ general-purpose (integer) registers are saved in this way. If VFP operations\r
+ are needed, that state must be saved manually.\r
+\r
+ Specifically, interrupt kinds other than "FIQ" will save all core registers\r
+ except "lr" and "sp". "FIQ" interrupts will save r0-r7.\r
+- If the CPU is not M-class, the return instruction is changed to one of the\r
+ canonical sequences permitted by the architecture for exception return. Where\r
+ possible the function itself will make the necessary "lr" adjustments so that\r
+ the "preferred return address" is selected.\r
+\r
+ Unfortunately the compiler is unable to make this guarantee for an "UNDEF"\r
+ handler, where the offset from "lr" to the preferred return address depends on\r
+ the execution state of the code which generated the exception. In this case\r
+ a sequence equivalent to "movs pc, lr" will be used.\r
+\r
+\r
+acquire_capability (acquire_shared_capability, clang::acquire_capability, clang::acquire_shared_capability)\r
+-----------------------------------------------------------------------------------------------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","X","",""\r
+\r
+Marks a function as acquiring a capability.\r
+\r
+\r
+assert_capability (assert_shared_capability, clang::assert_capability, clang::assert_shared_capability)\r
+-------------------------------------------------------------------------------------------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","X","",""\r
+\r
+Marks a function that dynamically tests whether a capability is held, and halts\r
+the program if it is not held.\r
+\r
+\r
+availability\r
+------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+The ``availability`` attribute can be placed on declarations to describe the\r
+lifecycle of that declaration relative to operating system versions. Consider\r
+the function declaration for a hypothetical function ``f``:\r
+\r
+.. code-block:: c++\r
+\r
+ void f(void) __attribute__((availability(macosx,introduced=10.4,deprecated=10.6,obsoleted=10.7)));\r
+\r
+The availability attribute states that ``f`` was introduced in Mac OS X 10.4,\r
+deprecated in Mac OS X 10.6, and obsoleted in Mac OS X 10.7. This information\r
+is used by Clang to determine when it is safe to use ``f``: for example, if\r
+Clang is instructed to compile code for Mac OS X 10.5, a call to ``f()``\r
+succeeds. If Clang is instructed to compile code for Mac OS X 10.6, the call\r
+succeeds but Clang emits a warning specifying that the function is deprecated.\r
+Finally, if Clang is instructed to compile code for Mac OS X 10.7, the call\r
+fails because ``f()`` is no longer available.\r
+\r
+The availability attribute is a comma-separated list starting with the\r
+platform name and then including clauses specifying important milestones in the\r
+declaration's lifetime (in any order) along with additional information. Those\r
+clauses can be:\r
+\r
+introduced=\ *version*\r
+ The first version in which this declaration was introduced.\r
+\r
+deprecated=\ *version*\r
+ The first version in which this declaration was deprecated, meaning that\r
+ users should migrate away from this API.\r
+\r
+obsoleted=\ *version*\r
+ The first version in which this declaration was obsoleted, meaning that it\r
+ was removed completely and can no longer be used.\r
+\r
+unavailable\r
+ This declaration is never available on this platform.\r
+\r
+message=\ *string-literal*\r
+ Additional message text that Clang will provide when emitting a warning or\r
+ error about use of a deprecated or obsoleted declaration. Useful to direct\r
+ users to replacement APIs.\r
+\r
+Multiple availability attributes can be placed on a declaration, which may\r
+correspond to different platforms. Only the availability attribute with the\r
+platform corresponding to the target platform will be used; any others will be\r
+ignored. If no availability attribute specifies availability for the current\r
+target platform, the availability attributes are ignored. Supported platforms\r
+are:\r
+\r
+``ios``\r
+ Apple's iOS operating system. The minimum deployment target is specified by\r
+ the ``-mios-version-min=*version*`` or ``-miphoneos-version-min=*version*``\r
+ command-line arguments.\r
+\r
+``macosx``\r
+ Apple's Mac OS X operating system. The minimum deployment target is\r
+ specified by the ``-mmacosx-version-min=*version*`` command-line argument.\r
+\r
+A declaration can be used even when deploying back to a platform version prior\r
+to when the declaration was introduced. When this happens, the declaration is\r
+`weakly linked\r
+<https://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html>`_,\r
+as if the ``weak_import`` attribute were added to the declaration. A\r
+weakly-linked declaration may or may not be present a run-time, and a program\r
+can determine whether the declaration is present by checking whether the\r
+address of that declaration is non-NULL.\r
+\r
+If there are multiple declarations of the same entity, the availability\r
+attributes must either match on a per-platform basis or later\r
+declarations must not have availability attributes for that\r
+platform. For example:\r
+\r
+.. code-block:: c\r
+\r
+ void g(void) __attribute__((availability(macosx,introduced=10.4)));\r
+ void g(void) __attribute__((availability(macosx,introduced=10.4))); // okay, matches\r
+ void g(void) __attribute__((availability(ios,introduced=4.0))); // okay, adds a new platform\r
+ void g(void); // okay, inherits both macosx and ios availability from above.\r
+ void g(void) __attribute__((availability(macosx,introduced=10.5))); // error: mismatch\r
+\r
+When one method overrides another, the overriding method can be more widely available than the overridden method, e.g.,:\r
+\r
+.. code-block:: objc\r
+\r
+ @interface A\r
+ - (id)method __attribute__((availability(macosx,introduced=10.4)));\r
+ - (id)method2 __attribute__((availability(macosx,introduced=10.4)));\r
+ @end\r
+\r
+ @interface B : A\r
+ - (id)method __attribute__((availability(macosx,introduced=10.3))); // okay: method moved into base class later\r
+ - (id)method __attribute__((availability(macosx,introduced=10.5))); // error: this method was available via the base class in 10.4\r
+ @end\r
+\r
+\r
+_Noreturn\r
+---------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "","","","X"\r
+\r
+A function declared as ``_Noreturn`` shall not return to its caller. The\r
+compiler will generate a diagnostic for a function declared as ``_Noreturn``\r
+that appears to be capable of returning to its caller.\r
+\r
+\r
+noreturn\r
+--------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "","X","",""\r
+\r
+A function declared as ``[[noreturn]]`` shall not return to its caller. The\r
+compiler will generate a diagnostic for a function declared as ``[[noreturn]]``\r
+that appears to be capable of returning to its caller.\r
+\r
+\r
+carries_dependency\r
+------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","X","",""\r
+\r
+The ``carries_dependency`` attribute specifies dependency propagation into and\r
+out of functions.\r
+\r
+When specified on a function or Objective-C method, the ``carries_depedency``\r
+attribute means that the return value carries a dependency out of the function, \r
+so that the implementation need not constrain ordering upon return from that\r
+function. Implementations of the function and its caller may choose to preserve\r
+dependencies instead of emitting memory ordering instructions such as fences.\r
+\r
+Note, this attribute does not change the meaning of the program, but may result\r
+in generatation of more efficient code.\r
+\r
+\r
+enable_if\r
+---------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+The ``enable_if`` attribute can be placed on function declarations to control\r
+which overload is selected based on the values of the function's arguments.\r
+When combined with the ``overloadable`` attribute, this feature is also\r
+available in C.\r
+\r
+.. code-block:: c++\r
+\r
+ int isdigit(int c);\r
+ 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")));\r
+ \r
+ void foo(char c) {\r
+ isdigit(c);\r
+ isdigit(10);\r
+ isdigit(-10); // results in a compile-time error.\r
+ }\r
+\r
+The enable_if attribute takes two arguments, the first is an expression written\r
+in terms of the function parameters, the second is a string explaining why this\r
+overload candidate could not be selected to be displayed in diagnostics. The\r
+expression is part of the function signature for the purposes of determining\r
+whether it is a redeclaration (following the rules used when determining\r
+whether a C++ template specialization is ODR-equivalent), but is not part of\r
+the type.\r
+\r
+The enable_if expression is evaluated as if it were the body of a\r
+bool-returning constexpr function declared with the arguments of the function\r
+it is being applied to, then called with the parameters at the callsite. If the\r
+result is false or could not be determined through constant expression\r
+evaluation, then this overload will not be chosen and the provided string may\r
+be used in a diagnostic if the compile fails as a result.\r
+\r
+Because the enable_if expression is an unevaluated context, there are no global\r
+state changes, nor the ability to pass information from the enable_if\r
+expression to the function body. For example, suppose we want calls to\r
+strnlen(strbuf, maxlen) to resolve to strnlen_chk(strbuf, maxlen, size of\r
+strbuf) only if the size of strbuf can be determined:\r
+\r
+.. code-block:: c++\r
+\r
+ __attribute__((always_inline))\r
+ static inline size_t strnlen(const char *s, size_t maxlen)\r
+ __attribute__((overloadable))\r
+ __attribute__((enable_if(__builtin_object_size(s, 0) != -1))),\r
+ "chosen when the buffer size is known but 'maxlen' is not")))\r
+ {\r
+ return strnlen_chk(s, maxlen, __builtin_object_size(s, 0));\r
+ }\r
+\r
+Multiple enable_if attributes may be applied to a single declaration. In this\r
+case, the enable_if expressions are evaluated from left to right in the\r
+following manner. First, the candidates whose enable_if expressions evaluate to\r
+false or cannot be evaluated are discarded. If the remaining candidates do not\r
+share ODR-equivalent enable_if expressions, the overload resolution is\r
+ambiguous. Otherwise, enable_if overload resolution continues with the next\r
+enable_if attribute on the candidates that have not been discarded and have\r
+remaining enable_if attributes. In this way, we pick the most specific\r
+overload out of a number of viable overloads using enable_if.\r
+\r
+.. code-block:: c++\r
+\r
+ void f() __attribute__((enable_if(true, ""))); // #1\r
+ void f() __attribute__((enable_if(true, ""))) __attribute__((enable_if(true, ""))); // #2\r
+ \r
+ void g(int i, int j) __attribute__((enable_if(i, ""))); // #1\r
+ void g(int i, int j) __attribute__((enable_if(j, ""))) __attribute__((enable_if(true))); // #2\r
+\r
+In this example, a call to f() is always resolved to #2, as the first enable_if\r
+expression is ODR-equivalent for both declarations, but #1 does not have another\r
+enable_if expression to continue evaluating, so the next round of evaluation has\r
+only a single candidate. In a call to g(1, 1), the call is ambiguous even though\r
+#2 has more enable_if attributes, because the first enable_if expressions are\r
+not ODR-equivalent.\r
+\r
+Query for this feature with ``__has_attribute(enable_if)``.\r
+\r
+\r
+format (gnu::format)\r
+--------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","X","",""\r
+\r
+Clang supports the ``format`` attribute, which indicates that the function\r
+accepts a ``printf`` or ``scanf``-like format string and corresponding\r
+arguments or a ``va_list`` that contains these arguments.\r
+\r
+Please see `GCC documentation about format attribute\r
+<http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_ to find details\r
+about attribute syntax.\r
+\r
+Clang implements two kinds of checks with this attribute.\r
+\r
+#. Clang checks that the function with the ``format`` attribute is called with\r
+ a format string that uses format specifiers that are allowed, and that\r
+ arguments match the format string. This is the ``-Wformat`` warning, it is\r
+ on by default.\r
+\r
+#. Clang checks that the format string argument is a literal string. This is\r
+ the ``-Wformat-nonliteral`` warning, it is off by default.\r
+\r
+ Clang implements this mostly the same way as GCC, but there is a difference\r
+ for functions that accept a ``va_list`` argument (for example, ``vprintf``).\r
+ GCC does not emit ``-Wformat-nonliteral`` warning for calls to such\r
+ fuctions. Clang does not warn if the format string comes from a function\r
+ parameter, where the function is annotated with a compatible attribute,\r
+ otherwise it warns. For example:\r
+\r
+ .. code-block:: c\r
+\r
+ __attribute__((__format__ (__scanf__, 1, 3)))\r
+ void foo(const char* s, char *buf, ...) {\r
+ va_list ap;\r
+ va_start(ap, buf);\r
+\r
+ vprintf(s, ap); // warning: format string is not a string literal\r
+ }\r
+\r
+ In this case we warn because ``s`` contains a format string for a\r
+ ``scanf``-like function, but it is passed to a ``printf``-like function.\r
+\r
+ If the attribute is removed, clang still warns, because the format string is\r
+ not a string literal.\r
+\r
+ Another example:\r
+\r
+ .. code-block:: c\r
+\r
+ __attribute__((__format__ (__printf__, 1, 3)))\r
+ void foo(const char* s, char *buf, ...) {\r
+ va_list ap;\r
+ va_start(ap, buf);\r
+\r
+ vprintf(s, ap); // warning\r
+ }\r
+\r
+ In this case Clang does not warn because the format string ``s`` and\r
+ the corresponding arguments are annotated. If the arguments are\r
+ incorrect, the caller of ``foo`` will receive a warning.\r
+\r
+\r
+noduplicate (clang::noduplicate)\r
+--------------------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","X","",""\r
+\r
+The ``noduplicate`` attribute can be placed on function declarations to control\r
+whether function calls to this function can be duplicated or not as a result of\r
+optimizations. This is required for the implementation of functions with\r
+certain special requirements, like the OpenCL "barrier" function, that might\r
+need to be run concurrently by all the threads that are executing in lockstep\r
+on the hardware. For example this attribute applied on the function\r
+"nodupfunc" in the code below avoids that:\r
+\r
+.. code-block:: c\r
+\r
+ void nodupfunc() __attribute__((noduplicate));\r
+ // Setting it as a C++11 attribute is also valid\r
+ // void nodupfunc() [[clang::noduplicate]];\r
+ void foo();\r
+ void bar();\r
+\r
+ nodupfunc();\r
+ if (a > n) {\r
+ foo();\r
+ } else {\r
+ bar();\r
+ }\r
+\r
+gets possibly modified by some optimizations into code similar to this:\r
+\r
+.. code-block:: c\r
+\r
+ if (a > n) {\r
+ nodupfunc();\r
+ foo();\r
+ } else {\r
+ nodupfunc();\r
+ bar();\r
+ }\r
+\r
+where the call to "nodupfunc" is duplicated and sunk into the two branches\r
+of the condition.\r
+\r
+\r
+no_sanitize_address (no_address_safety_analysis, gnu::no_address_safety_analysis, gnu::no_sanitize_address)\r
+-----------------------------------------------------------------------------------------------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","X","",""\r
+\r
+.. _langext-address_sanitizer:\r
+\r
+Use ``__attribute__((no_sanitize_address))`` on a function declaration to\r
+specify that address safety instrumentation (e.g. AddressSanitizer) should\r
+not be applied to that function.\r
+\r
+\r
+no_sanitize_memory\r
+------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+.. _langext-memory_sanitizer:\r
+\r
+Use ``__attribute__((no_sanitize_memory))`` on a function declaration to\r
+specify that checks for uninitialized memory should not be inserted \r
+(e.g. by MemorySanitizer). The function may still be instrumented by the tool\r
+to avoid false positives in other places.\r
+\r
+\r
+no_sanitize_thread\r
+------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+.. _langext-thread_sanitizer:\r
+\r
+Use ``__attribute__((no_sanitize_thread))`` on a function declaration to\r
+specify that checks for data races on plain (non-atomic) memory accesses should\r
+not be inserted by ThreadSanitizer. The function is still instrumented by the\r
+tool to avoid false positives and provide meaningful stack traces.\r
+\r
+\r
+objc_method_family\r
+------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+Many methods in Objective-C have conventional meanings determined by their\r
+selectors. It is sometimes useful to be able to mark a method as having a\r
+particular conventional meaning despite not having the right selector, or as\r
+not having the conventional meaning that its selector would suggest. For these\r
+use cases, we provide an attribute to specifically describe the "method family"\r
+that a method belongs to.\r
+\r
+**Usage**: ``__attribute__((objc_method_family(X)))``, where ``X`` is one of\r
+``none``, ``alloc``, ``copy``, ``init``, ``mutableCopy``, or ``new``. This\r
+attribute can only be placed at the end of a method declaration:\r
+\r
+.. code-block:: objc\r
+\r
+ - (NSString *)initMyStringValue __attribute__((objc_method_family(none)));\r
+\r
+Users who do not wish to change the conventional meaning of a method, and who\r
+merely want to document its non-standard retain and release semantics, should\r
+use the retaining behavior attributes (``ns_returns_retained``,\r
+``ns_returns_not_retained``, etc).\r
+\r
+Query for this feature with ``__has_attribute(objc_method_family)``.\r
+\r
+\r
+objc_requires_super\r
+-------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+Some Objective-C classes allow a subclass to override a particular method in a\r
+parent class but expect that the overriding method also calls the overridden\r
+method in the parent class. For these cases, we provide an attribute to\r
+designate that a method requires a "call to ``super``" in the overriding\r
+method in the subclass.\r
+\r
+**Usage**: ``__attribute__((objc_requires_super))``. This attribute can only\r
+be placed at the end of a method declaration:\r
+\r
+.. code-block:: objc\r
+\r
+ - (void)foo __attribute__((objc_requires_super));\r
+\r
+This attribute can only be applied the method declarations within a class, and\r
+not a protocol. Currently this attribute does not enforce any placement of\r
+where the call occurs in the overriding method (such as in the case of\r
+``-dealloc`` where the call must appear at the end). It checks only that it\r
+exists.\r
+\r
+Note that on both OS X and iOS that the Foundation framework provides a\r
+convenience macro ``NS_REQUIRES_SUPER`` that provides syntactic sugar for this\r
+attribute:\r
+\r
+.. code-block:: objc\r
+\r
+ - (void)foo NS_REQUIRES_SUPER;\r
+\r
+This macro is conditionally defined depending on the compiler's support for\r
+this attribute. If the compiler does not support the attribute the macro\r
+expands to nothing.\r
+\r
+Operationally, when a method has this annotation the compiler will warn if the\r
+implementation of an override in a subclass does not call super. For example:\r
+\r
+.. code-block:: objc\r
+\r
+ warning: method possibly missing a [super AnnotMeth] call\r
+ - (void) AnnotMeth{};\r
+ ^\r
+\r
+\r
+overloadable\r
+------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+Clang provides support for C++ function overloading in C. Function overloading\r
+in C is introduced using the ``overloadable`` attribute. For example, one\r
+might provide several overloaded versions of a ``tgsin`` function that invokes\r
+the appropriate standard function computing the sine of a value with ``float``,\r
+``double``, or ``long double`` precision:\r
+\r
+.. code-block:: c\r
+\r
+ #include <math.h>\r
+ float __attribute__((overloadable)) tgsin(float x) { return sinf(x); }\r
+ double __attribute__((overloadable)) tgsin(double x) { return sin(x); }\r
+ long double __attribute__((overloadable)) tgsin(long double x) { return sinl(x); }\r
+\r
+Given these declarations, one can call ``tgsin`` with a ``float`` value to\r
+receive a ``float`` result, with a ``double`` to receive a ``double`` result,\r
+etc. Function overloading in C follows the rules of C++ function overloading\r
+to pick the best overload given the call arguments, with a few C-specific\r
+semantics:\r
+\r
+* Conversion from ``float`` or ``double`` to ``long double`` is ranked as a\r
+ floating-point promotion (per C99) rather than as a floating-point conversion\r
+ (as in C++).\r
+\r
+* A conversion from a pointer of type ``T*`` to a pointer of type ``U*`` is\r
+ considered a pointer conversion (with conversion rank) if ``T`` and ``U`` are\r
+ compatible types.\r
+\r
+* A conversion from type ``T`` to a value of type ``U`` is permitted if ``T``\r
+ and ``U`` are compatible types. This conversion is given "conversion" rank.\r
+\r
+The declaration of ``overloadable`` functions is restricted to function\r
+declarations and definitions. Most importantly, if any function with a given\r
+name is given the ``overloadable`` attribute, then all function declarations\r
+and definitions with that name (and in that scope) must have the\r
+``overloadable`` attribute. This rule even applies to redeclarations of\r
+functions whose original declaration had the ``overloadable`` attribute, e.g.,\r
+\r
+.. code-block:: c\r
+\r
+ int f(int) __attribute__((overloadable));\r
+ float f(float); // error: declaration of "f" must have the "overloadable" attribute\r
+\r
+ int g(int) __attribute__((overloadable));\r
+ int g(int) { } // error: redeclaration of "g" must also have the "overloadable" attribute\r
+\r
+Functions marked ``overloadable`` must have prototypes. Therefore, the\r
+following code is ill-formed:\r
+\r
+.. code-block:: c\r
+\r
+ int h() __attribute__((overloadable)); // error: h does not have a prototype\r
+\r
+However, ``overloadable`` functions are allowed to use a ellipsis even if there\r
+are no named parameters (as is permitted in C++). This feature is particularly\r
+useful when combined with the ``unavailable`` attribute:\r
+\r
+.. code-block:: c++\r
+\r
+ void honeypot(...) __attribute__((overloadable, unavailable)); // calling me is an error\r
+\r
+Functions declared with the ``overloadable`` attribute have their names mangled\r
+according to the same rules as C++ function names. For example, the three\r
+``tgsin`` functions in our motivating example get the mangled names\r
+``_Z5tgsinf``, ``_Z5tgsind``, and ``_Z5tgsine``, respectively. There are two\r
+caveats to this use of name mangling:\r
+\r
+* Future versions of Clang may change the name mangling of functions overloaded\r
+ in C, so you should not depend on an specific mangling. To be completely\r
+ safe, we strongly urge the use of ``static inline`` with ``overloadable``\r
+ functions.\r
+\r
+* The ``overloadable`` attribute has almost no meaning when used in C++,\r
+ because names will already be mangled and functions are already overloadable.\r
+ However, when an ``overloadable`` function occurs within an ``extern "C"``\r
+ linkage specification, it's name *will* be mangled in the same way as it\r
+ would in C.\r
+\r
+Query for this feature with ``__has_extension(attribute_overloadable)``.\r
+\r
+\r
+release_capability (release_shared_capability, clang::release_capability, clang::release_shared_capability)\r
+-----------------------------------------------------------------------------------------------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","X","",""\r
+\r
+Marks a function as releasing a capability.\r
+\r
+\r
+try_acquire_capability (try_acquire_shared_capability, clang::try_acquire_capability, clang::try_acquire_shared_capability)\r
+---------------------------------------------------------------------------------------------------------------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","X","",""\r
+\r
+Marks a function that attempts to acquire a capability. This function may fail to\r
+actually acquire the capability; they accept a Boolean value determining\r
+whether acquiring the capability means success (true), or failing to acquire\r
+the capability means success (false).\r
+\r
+\r
+Variable Attributes\r
+===================\r
+\r
+\r
+tls_model (gnu::tls_model)\r
+--------------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","X","",""\r
+\r
+The ``tls_model`` attribute allows you to specify which thread-local storage\r
+model to use. It accepts the following strings:\r
+\r
+* global-dynamic\r
+* local-dynamic\r
+* initial-exec\r
+* local-exec\r
+\r
+TLS models are mutually exclusive.\r
+\r
+\r
+Type Attributes\r
+===============\r
+\r
+\r
+__single_inhertiance, __multiple_inheritance, __virtual_inheritance\r
+-------------------------------------------------------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "","","","X"\r
+\r
+This collection of keywords is enabled under ``-fms-extensions`` and controls\r
+the pointer-to-member representation used on ``*-*-win32`` targets.\r
+\r
+The ``*-*-win32`` targets utilize a pointer-to-member representation which\r
+varies in size and alignment depending on the definition of the underlying\r
+class.\r
+\r
+However, this is problematic when a forward declaration is only available and\r
+no definition has been made yet. In such cases, Clang is forced to utilize the\r
+most general representation that is available to it.\r
+\r
+These keywords make it possible to use a pointer-to-member representation other\r
+than the most general one regardless of whether or not the definition will ever\r
+be present in the current translation unit.\r
+\r
+This family of keywords belong between the ``class-key`` and ``class-name``:\r
+\r
+.. code-block:: c++\r
+\r
+ struct __single_inheritance S;\r
+ int S::*i;\r
+ struct S {};\r
+\r
+This keyword can be applied to class templates but only has an effect when used\r
+on full specializations:\r
+\r
+.. code-block:: c++\r
+\r
+ template <typename T, typename U> struct __single_inheritance A; // warning: inheritance model ignored on primary template\r
+ template <typename T> struct __multiple_inheritance A<T, T>; // warning: inheritance model ignored on partial specialization\r
+ template <> struct __single_inheritance A<int, float>;\r
+\r
+Note that choosing an inheritance model less general than strictly necessary is\r
+an error:\r
+\r
+.. code-block:: c++\r
+\r
+ struct __multiple_inheritance S; // error: inheritance model does not match definition\r
+ int S::*i;\r
+ struct S {};\r
+\r
+\r
+Statement Attributes\r
+====================\r
+\r
+\r
+fallthrough (clang::fallthrough)\r
+--------------------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "","X","",""\r
+\r
+The ``clang::fallthrough`` attribute is used along with the\r
+``-Wimplicit-fallthrough`` argument to annotate intentional fall-through\r
+between switch labels. It can only be applied to a null statement placed at a\r
+point of execution between any statement and the next switch label. It is\r
+common to mark these places with a specific comment, but this attribute is\r
+meant to replace comments with a more strict annotation, which can be checked\r
+by the compiler. This attribute doesn't change semantics of the code and can\r
+be used wherever an intended fall-through occurs. It is designed to mimic\r
+control-flow statements like ``break;``, so it can be placed in most places\r
+where ``break;`` can, but only if there are no statements on the execution path\r
+between it and the next switch label.\r
+\r
+Here is an example:\r
+\r
+.. code-block:: c++\r
+\r
+ // compile with -Wimplicit-fallthrough\r
+ switch (n) {\r
+ case 22:\r
+ case 33: // no warning: no statements between case labels\r
+ f();\r
+ case 44: // warning: unannotated fall-through\r
+ g();\r
+ [[clang::fallthrough]];\r
+ case 55: // no warning\r
+ if (x) {\r
+ h();\r
+ break;\r
+ }\r
+ else {\r
+ i();\r
+ [[clang::fallthrough]];\r
+ }\r
+ case 66: // no warning\r
+ p();\r
+ [[clang::fallthrough]]; // warning: fallthrough annotation does not\r
+ // directly precede case label\r
+ q();\r
+ case 77: // warning: unannotated fall-through\r
+ r();\r
+ }\r
+\r
+\r
+Consumed Annotation Checking\r
+============================\r
+Clang supports additional attributes for checking basic resource management\r
+properties, specifically for unique objects that have a single owning reference.\r
+The following attributes are currently supported, although **the implementation\r
+for these annotations is currently in development and are subject to change.**\r
+\r
+callable_when\r
+-------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+Use ``__attribute__((callable_when(...)))`` to indicate what states a method\r
+may be called in. Valid states are unconsumed, consumed, or unknown. Each\r
+argument to this attribute must be a quoted string. E.g.:\r
+\r
+``__attribute__((callable_when("unconsumed", "unknown")))``\r
+\r
+\r
+consumable\r
+----------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+Each ``class`` that uses any of the typestate annotations must first be marked\r
+using the ``consumable`` attribute. Failure to do so will result in a warning.\r
+\r
+This attribute accepts a single parameter that must be one of the following:\r
+``unknown``, ``consumed``, or ``unconsumed``.\r
+\r
+\r
+param_typestate\r
+---------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+This attribute specifies expectations about function parameters. Calls to an\r
+function with annotated parameters will issue a warning if the corresponding\r
+argument isn't in the expected state. The attribute is also used to set the\r
+initial state of the parameter when analyzing the function's body.\r
+\r
+\r
+return_typestate\r
+----------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+The ``return_typestate`` attribute can be applied to functions or parameters.\r
+When applied to a function the attribute specifies the state of the returned\r
+value. The function's body is checked to ensure that it always returns a value\r
+in the specified state. On the caller side, values returned by the annotated\r
+function are initialized to the given state.\r
+\r
+When applied to a function parameter it modifies the state of an argument after\r
+a call to the function returns. The function's body is checked to ensure that\r
+the parameter is in the expected state before returning.\r
+\r
+\r
+set_typestate\r
+-------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+Annotate methods that transition an object into a new state with\r
+``__attribute__((set_typestate(new_state)))``. The new new state must be\r
+unconsumed, consumed, or unknown.\r
+\r
+\r
+test_typestate\r
+--------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+Use ``__attribute__((test_typestate(tested_state)))`` to indicate that a method\r
+returns true if the object is in the specified state..\r
+\r
+\r
+Type Safety Checking\r
+====================\r
+Clang supports additional attributes to enable checking type safety properties\r
+that can't be enforced by the C type system. Use cases include:\r
+\r
+* MPI library implementations, where these attributes enable checking that\r
+ the buffer type matches the passed ``MPI_Datatype``;\r
+* for HDF5 library there is a similar use case to MPI;\r
+* checking types of variadic functions' arguments for functions like\r
+ ``fcntl()`` and ``ioctl()``.\r
+\r
+You can detect support for these attributes with ``__has_attribute()``. For\r
+example:\r
+\r
+.. code-block:: c++\r
+\r
+ #if defined(__has_attribute)\r
+ # if __has_attribute(argument_with_type_tag) && \\r
+ __has_attribute(pointer_with_type_tag) && \\r
+ __has_attribute(type_tag_for_datatype)\r
+ # define ATTR_MPI_PWT(buffer_idx, type_idx) __attribute__((pointer_with_type_tag(mpi,buffer_idx,type_idx)))\r
+ /* ... other macros ... */\r
+ # endif\r
+ #endif\r
+\r
+ #if !defined(ATTR_MPI_PWT)\r
+ # define ATTR_MPI_PWT(buffer_idx, type_idx)\r
+ #endif\r
+\r
+ int MPI_Send(void *buf, int count, MPI_Datatype datatype /*, other args omitted */)\r
+ ATTR_MPI_PWT(1,3);\r
+\r
+argument_with_type_tag\r
+----------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+Use ``__attribute__((argument_with_type_tag(arg_kind, arg_idx,\r
+type_tag_idx)))`` on a function declaration to specify that the function\r
+accepts a type tag that determines the type of some other argument.\r
+``arg_kind`` is an identifier that should be used when annotating all\r
+applicable type tags.\r
+\r
+This attribute is primarily useful for checking arguments of variadic functions\r
+(``pointer_with_type_tag`` can be used in most non-variadic cases).\r
+\r
+For example:\r
+\r
+.. code-block:: c++\r
+\r
+ int fcntl(int fd, int cmd, ...)\r
+ __attribute__(( argument_with_type_tag(fcntl,3,2) ));\r
+\r
+\r
+pointer_with_type_tag\r
+---------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+Use ``__attribute__((pointer_with_type_tag(ptr_kind, ptr_idx, type_tag_idx)))``\r
+on a function declaration to specify that the function accepts a type tag that\r
+determines the pointee type of some other pointer argument.\r
+\r
+For example:\r
+\r
+.. code-block:: c++\r
+\r
+ int MPI_Send(void *buf, int count, MPI_Datatype datatype /*, other args omitted */)\r
+ __attribute__(( pointer_with_type_tag(mpi,1,3) ));\r
+\r
+\r
+type_tag_for_datatype\r
+---------------------\r
+.. csv-table:: Supported Syntaxes\r
+ :header: "GNU", "C++11", "__declspec", "Keyword"\r
+\r
+ "X","","",""\r
+\r
+Clang supports annotating type tags of two forms.\r
+\r
+* **Type tag that is an expression containing a reference to some declared\r
+ identifier.** Use ``__attribute__((type_tag_for_datatype(kind, type)))`` on a\r
+ declaration with that identifier:\r
+\r
+ .. code-block:: c++\r
+\r
+ extern struct mpi_datatype mpi_datatype_int\r
+ __attribute__(( type_tag_for_datatype(mpi,int) ));\r
+ #define MPI_INT ((MPI_Datatype) &mpi_datatype_int)\r
+\r
+* **Type tag that is an integral literal.** Introduce a ``static const``\r
+ variable with a corresponding initializer value and attach\r
+ ``__attribute__((type_tag_for_datatype(kind, type)))`` on that declaration,\r
+ for example:\r
+\r
+ .. code-block:: c++\r
+\r
+ #define MPI_INT ((MPI_Datatype) 42)\r
+ static const MPI_Datatype mpi_datatype_int\r
+ __attribute__(( type_tag_for_datatype(mpi,int) )) = 42\r
+\r
+The attribute also accepts an optional third argument that determines how the\r
+expression is compared to the type tag. There are two supported flags:\r
+\r
+* ``layout_compatible`` will cause types to be compared according to\r
+ layout-compatibility rules (C++11 [class.mem] p 17, 18). This is\r
+ implemented to support annotating types like ``MPI_DOUBLE_INT``.\r
+\r
+ For example:\r
+\r
+ .. code-block:: c++\r
+\r
+ /* In mpi.h */\r
+ struct internal_mpi_double_int { double d; int i; };\r
+ extern struct mpi_datatype mpi_datatype_double_int\r
+ __attribute__(( type_tag_for_datatype(mpi, struct internal_mpi_double_int, layout_compatible) ));\r
+\r
+ #define MPI_DOUBLE_INT ((MPI_Datatype) &mpi_datatype_double_int)\r
+\r
+ /* In user code */\r
+ struct my_pair { double a; int b; };\r
+ struct my_pair *buffer;\r
+ MPI_Send(buffer, 1, MPI_DOUBLE_INT /*, ... */); // no warning\r
+\r
+ struct my_int_pair { int a; int b; }\r
+ struct my_int_pair *buffer2;\r
+ MPI_Send(buffer2, 1, MPI_DOUBLE_INT /*, ... */); // warning: actual buffer element\r
+ // type 'struct my_int_pair'\r
+ // doesn't match specified MPI_Datatype\r
+\r
+* ``must_be_null`` specifies that the expression should be a null pointer\r
+ constant, for example:\r
+\r
+ .. code-block:: c++\r
+\r
+ /* In mpi.h */\r
+ extern struct mpi_datatype mpi_datatype_null\r
+ __attribute__(( type_tag_for_datatype(mpi, void, must_be_null) ));\r
+\r
+ #define MPI_DATATYPE_NULL ((MPI_Datatype) &mpi_datatype_null)\r
+\r
+ /* In user code */\r
+ MPI_Send(buffer, 1, MPI_DATATYPE_NULL /*, ... */); // warning: MPI_DATATYPE_NULL\r
+ // was specified but buffer\r
+ // is not a null pointer\r
+\r
+\r
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