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author | David Majnemer <david.majnemer@gmail.com> | 2014-03-02 18:17:08 +0000 |
---|---|---|
committer | David Majnemer <david.majnemer@gmail.com> | 2014-03-02 18:17:08 +0000 |
commit | 4dd639d7bbba233ad419dcb4399c69d4e79d7c4f (patch) | |
tree | 89314bfa241e7b05727e65c9dc3413fff755f5ab /docs/AttributeReference.rst | |
parent | 1817a104d76ba71a0581113942ffb0f7eeef72c4 (diff) | |
download | clang-4dd639d7bbba233ad419dcb4399c69d4e79d7c4f.tar.gz |
AttributeReference: Use UNIX line endings
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202656 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'docs/AttributeReference.rst')
-rw-r--r-- | docs/AttributeReference.rst | 2052 |
1 files changed, 1026 insertions, 1026 deletions
diff --git a/docs/AttributeReference.rst b/docs/AttributeReference.rst index d75674fb1d..34a812d7e7 100644 --- a/docs/AttributeReference.rst +++ b/docs/AttributeReference.rst @@ -1,1026 +1,1026 @@ -..
- -------------------------------------------------------------------
- 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","",""
-
-.. _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.
-
-
-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
-
-
+.. + ------------------------------------------------------------------- + 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","","" + +.. _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. + + +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 + + |