forked from OSchip/llvm-project
662 lines
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ReStructuredText
662 lines
26 KiB
ReStructuredText
..
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-------------------------------------------------------------------
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NOTE: This file is automatically generated by running clang-tblgen
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-gen-attr-docs. Do not edit this file by hand!!
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-------------------------------------------------------------------
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===================
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Attributes in Clang
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===================
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.. contents::
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:local:
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Introduction
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============
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This page lists the attributes currently supported by Clang.
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Function Attributes
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===================
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interrupt
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---------
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.. csv-table:: Supported Syntaxes
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:header: "GNU", "C++11", "__declspec", "Keyword"
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"X","","",""
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Clang supports the GNU style ``__attribute__((interrupt("TYPE")))`` attribute on
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ARM targets. This attribute may be attached to a function definition and
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instructs the backend to generate appropriate function entry/exit code so that
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it can be used directly as an interrupt service routine.
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The parameter passed to the interrupt attribute is optional, but if
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provided it must be a string literal with one of the following values: "IRQ",
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"FIQ", "SWI", "ABORT", "UNDEF".
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The semantics are as follows:
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- If the function is AAPCS, Clang instructs the backend to realign the stack to
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8 bytes on entry. This is a general requirement of the AAPCS at public
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interfaces, but may not hold when an exception is taken. Doing this allows
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other AAPCS functions to be called.
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- If the CPU is M-class this is all that needs to be done since the architecture
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itself is designed in such a way that functions obeying the normal AAPCS ABI
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constraints are valid exception handlers.
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- If the CPU is not M-class, the prologue and epilogue are modified to save all
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non-banked registers that are used, so that upon return the user-mode state
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will not be corrupted. Note that to avoid unnecessary overhead, only
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general-purpose (integer) registers are saved in this way. If VFP operations
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are needed, that state must be saved manually.
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Specifically, interrupt kinds other than "FIQ" will save all core registers
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except "lr" and "sp". "FIQ" interrupts will save r0-r7.
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- If the CPU is not M-class, the return instruction is changed to one of the
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canonical sequences permitted by the architecture for exception return. Where
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possible the function itself will make the necessary "lr" adjustments so that
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the "preferred return address" is selected.
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Unfortunately the compiler is unable to make this guarantee for an "UNDEF"
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handler, where the offset from "lr" to the preferred return address depends on
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the execution state of the code which generated the exception. In this case
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a sequence equivalent to "movs pc, lr" will be used.
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availability
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------------
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.. csv-table:: Supported Syntaxes
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:header: "GNU", "C++11", "__declspec", "Keyword"
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"X","","",""
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The ``availability`` attribute can be placed on declarations to describe the
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lifecycle of that declaration relative to operating system versions. Consider
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the function declaration for a hypothetical function ``f``:
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.. code-block:: c++
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void f(void) __attribute__((availability(macosx,introduced=10.4,deprecated=10.6,obsoleted=10.7)));
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The availability attribute states that ``f`` was introduced in Mac OS X 10.4,
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deprecated in Mac OS X 10.6, and obsoleted in Mac OS X 10.7. This information
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is used by Clang to determine when it is safe to use ``f``: for example, if
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Clang is instructed to compile code for Mac OS X 10.5, a call to ``f()``
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succeeds. If Clang is instructed to compile code for Mac OS X 10.6, the call
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succeeds but Clang emits a warning specifying that the function is deprecated.
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Finally, if Clang is instructed to compile code for Mac OS X 10.7, the call
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fails because ``f()`` is no longer available.
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The availability attribute is a comma-separated list starting with the
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platform name and then including clauses specifying important milestones in the
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declaration's lifetime (in any order) along with additional information. Those
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clauses can be:
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introduced=\ *version*
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The first version in which this declaration was introduced.
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deprecated=\ *version*
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The first version in which this declaration was deprecated, meaning that
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users should migrate away from this API.
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obsoleted=\ *version*
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The first version in which this declaration was obsoleted, meaning that it
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was removed completely and can no longer be used.
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unavailable
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This declaration is never available on this platform.
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message=\ *string-literal*
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Additional message text that Clang will provide when emitting a warning or
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error about use of a deprecated or obsoleted declaration. Useful to direct
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users to replacement APIs.
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Multiple availability attributes can be placed on a declaration, which may
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correspond to different platforms. Only the availability attribute with the
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platform corresponding to the target platform will be used; any others will be
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ignored. If no availability attribute specifies availability for the current
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target platform, the availability attributes are ignored. Supported platforms
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are:
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``ios``
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Apple's iOS operating system. The minimum deployment target is specified by
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the ``-mios-version-min=*version*`` or ``-miphoneos-version-min=*version*``
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command-line arguments.
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``macosx``
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Apple's Mac OS X operating system. The minimum deployment target is
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specified by the ``-mmacosx-version-min=*version*`` command-line argument.
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A declaration can be used even when deploying back to a platform version prior
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to when the declaration was introduced. When this happens, the declaration is
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`weakly linked
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<https://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html>`_,
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as if the ``weak_import`` attribute were added to the declaration. A
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weakly-linked declaration may or may not be present a run-time, and a program
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can determine whether the declaration is present by checking whether the
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address of that declaration is non-NULL.
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If there are multiple declarations of the same entity, the availability
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attributes must either match on a per-platform basis or later
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declarations must not have availability attributes for that
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platform. For example:
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.. code-block:: c
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void g(void) __attribute__((availability(macosx,introduced=10.4)));
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void g(void) __attribute__((availability(macosx,introduced=10.4))); // okay, matches
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void g(void) __attribute__((availability(ios,introduced=4.0))); // okay, adds a new platform
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void g(void); // okay, inherits both macosx and ios availability from above.
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void g(void) __attribute__((availability(macosx,introduced=10.5))); // error: mismatch
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When one method overrides another, the overriding method can be more widely available than the overridden method, e.g.,:
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.. code-block:: objc
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@interface A
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- (id)method __attribute__((availability(macosx,introduced=10.4)));
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- (id)method2 __attribute__((availability(macosx,introduced=10.4)));
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@end
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@interface B : A
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- (id)method __attribute__((availability(macosx,introduced=10.3))); // okay: method moved into base class later
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- (id)method __attribute__((availability(macosx,introduced=10.5))); // error: this method was available via the base class in 10.4
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@end
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_Noreturn
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---------
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.. csv-table:: Supported Syntaxes
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:header: "GNU", "C++11", "__declspec", "Keyword"
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"","","","X"
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A function declared as ``_Noreturn`` shall not return to its caller. The
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compiler will generate a diagnostic for a function declared as ``_Noreturn``
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that appears to be capable of returning to its caller.
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noreturn
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--------
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.. csv-table:: Supported Syntaxes
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:header: "GNU", "C++11", "__declspec", "Keyword"
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"","X","",""
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A function declared as ``[[noreturn]]`` shall not return to its caller. The
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compiler will generate a diagnostic for a function declared as ``[[noreturn]]``
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that appears to be capable of returning to its caller.
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carries_dependency
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------------------
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.. csv-table:: Supported Syntaxes
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:header: "GNU", "C++11", "__declspec", "Keyword"
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"X","X","",""
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The ``carries_dependency`` attribute specifies dependency propagation into and
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out of functions.
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When specified on a function or Objective-C method, the ``carries_depedency``
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attribute means that the return value carries a dependency out of the function,
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so that the implementation need not constrain ordering upon return from that
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function. Implementations of the function and its caller may choose to preserve
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dependencies instead of emitting memory ordering instructions such as fences.
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Note, this attribute does not change the meaning of the program, but may result
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in generatation of more efficient code.
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enable_if
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---------
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.. csv-table:: Supported Syntaxes
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:header: "GNU", "C++11", "__declspec", "Keyword"
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"X","","",""
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The ``enable_if`` attribute can be placed on function declarations to control
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which overload is selected based on the values of the function's arguments.
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When combined with the ``overloadable`` attribute, this feature is also
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available in C.
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.. code-block:: c++
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int isdigit(int c);
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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")));
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void foo(char c) {
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isdigit(c);
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isdigit(10);
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isdigit(-10); // results in a compile-time error.
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}
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The enable_if attribute takes two arguments, the first is an expression written
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in terms of the function parameters, the second is a string explaining why this
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overload candidate could not be selected to be displayed in diagnostics. The
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expression is part of the function signature for the purposes of determining
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whether it is a redeclaration (following the rules used when determining
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whether a C++ template specialization is ODR-equivalent), but is not part of
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the type.
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The enable_if expression is evaluated as if it were the body of a
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bool-returning constexpr function declared with the arguments of the function
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it is being applied to, then called with the parameters at the callsite. If the
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result is false or could not be determined through constant expression
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evaluation, then this overload will not be chosen and the provided string may
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be used in a diagnostic if the compile fails as a result.
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Because the enable_if expression is an unevaluated context, there are no global
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state changes, nor the ability to pass information from the enable_if
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expression to the function body. For example, suppose we want calls to
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strnlen(strbuf, maxlen) to resolve to strnlen_chk(strbuf, maxlen, size of
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strbuf) only if the size of strbuf can be determined:
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.. code-block:: c++
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__attribute__((always_inline))
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static inline size_t strnlen(const char *s, size_t maxlen)
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__attribute__((overloadable))
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__attribute__((enable_if(__builtin_object_size(s, 0) != -1))),
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"chosen when the buffer size is known but 'maxlen' is not")))
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{
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return strnlen_chk(s, maxlen, __builtin_object_size(s, 0));
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}
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Multiple enable_if attributes may be applied to a single declaration. In this
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case, the enable_if expressions are evaluated from left to right in the
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following manner. First, the candidates whose enable_if expressions evaluate to
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false or cannot be evaluated are discarded. If the remaining candidates do not
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share ODR-equivalent enable_if expressions, the overload resolution is
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ambiguous. Otherwise, enable_if overload resolution continues with the next
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enable_if attribute on the candidates that have not been discarded and have
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remaining enable_if attributes. In this way, we pick the most specific
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overload out of a number of viable overloads using enable_if.
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.. code-block:: c++
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void f() __attribute__((enable_if(true, ""))); // #1
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void f() __attribute__((enable_if(true, ""))) __attribute__((enable_if(true, ""))); // #2
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void g(int i, int j) __attribute__((enable_if(i, ""))); // #1
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void g(int i, int j) __attribute__((enable_if(j, ""))) __attribute__((enable_if(true))); // #2
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In this example, a call to f() is always resolved to #2, as the first enable_if
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expression is ODR-equivalent for both declarations, but #1 does not have another
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enable_if expression to continue evaluating, so the next round of evaluation has
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only a single candidate. In a call to g(1, 1), the call is ambiguous even though
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#2 has more enable_if attributes, because the first enable_if expressions are
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not ODR-equivalent.
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Query for this feature with ``__has_attribute(enable_if)``.
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no_sanitize_address (no_address_safety_analysis, gnu::no_address_safety_analysis, gnu::no_sanitize_address)
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-----------------------------------------------------------------------------------------------------------
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.. csv-table:: Supported Syntaxes
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:header: "GNU", "C++11", "__declspec", "Keyword"
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"X","X","",""
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Use ``__attribute__((no_sanitize_address))`` on a function declaration to
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specify that address safety instrumentation (e.g. AddressSanitizer) should
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not be applied to that function.
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no_sanitize_memory
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------------------
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.. csv-table:: Supported Syntaxes
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:header: "GNU", "C++11", "__declspec", "Keyword"
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"X","","",""
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Use ``__attribute__((no_sanitize_memory))`` on a function declaration to
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specify that checks for uninitialized memory should not be inserted
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(e.g. by MemorySanitizer). The function may still be instrumented by the tool
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to avoid false positives in other places.
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no_sanitize_thread
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------------------
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.. csv-table:: Supported Syntaxes
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:header: "GNU", "C++11", "__declspec", "Keyword"
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"X","","",""
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Use ``__attribute__((no_sanitize_thread))`` on a function declaration to
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specify that checks for data races on plain (non-atomic) memory accesses should
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not be inserted by ThreadSanitizer. The function is still instrumented by the
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tool to avoid false positives and provide meaningful stack traces.
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objc_method_family
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------------------
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.. csv-table:: Supported Syntaxes
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:header: "GNU", "C++11", "__declspec", "Keyword"
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"X","","",""
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Many methods in Objective-C have conventional meanings determined by their
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selectors. It is sometimes useful to be able to mark a method as having a
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particular conventional meaning despite not having the right selector, or as
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not having the conventional meaning that its selector would suggest. For these
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use cases, we provide an attribute to specifically describe the "method family"
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that a method belongs to.
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**Usage**: ``__attribute__((objc_method_family(X)))``, where ``X`` is one of
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``none``, ``alloc``, ``copy``, ``init``, ``mutableCopy``, or ``new``. This
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attribute can only be placed at the end of a method declaration:
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.. code-block:: objc
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- (NSString *)initMyStringValue __attribute__((objc_method_family(none)));
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Users who do not wish to change the conventional meaning of a method, and who
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merely want to document its non-standard retain and release semantics, should
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use the retaining behavior attributes (``ns_returns_retained``,
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``ns_returns_not_retained``, etc).
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Query for this feature with ``__has_attribute(objc_method_family)``.
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objc_requires_super
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-------------------
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.. csv-table:: Supported Syntaxes
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:header: "GNU", "C++11", "__declspec", "Keyword"
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"X","","",""
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Some Objective-C classes allow a subclass to override a particular method in a
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parent class but expect that the overriding method also calls the overridden
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method in the parent class. For these cases, we provide an attribute to
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designate that a method requires a "call to ``super``" in the overriding
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method in the subclass.
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**Usage**: ``__attribute__((objc_requires_super))``. This attribute can only
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be placed at the end of a method declaration:
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.. code-block:: objc
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- (void)foo __attribute__((objc_requires_super));
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This attribute can only be applied the method declarations within a class, and
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not a protocol. Currently this attribute does not enforce any placement of
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where the call occurs in the overriding method (such as in the case of
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``-dealloc`` where the call must appear at the end). It checks only that it
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exists.
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Note that on both OS X and iOS that the Foundation framework provides a
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convenience macro ``NS_REQUIRES_SUPER`` that provides syntactic sugar for this
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attribute:
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.. code-block:: objc
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- (void)foo NS_REQUIRES_SUPER;
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This macro is conditionally defined depending on the compiler's support for
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this attribute. If the compiler does not support the attribute the macro
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expands to nothing.
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Operationally, when a method has this annotation the compiler will warn if the
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implementation of an override in a subclass does not call super. For example:
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.. code-block:: objc
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warning: method possibly missing a [super AnnotMeth] call
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- (void) AnnotMeth{};
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^
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overloadable
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------------
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.. csv-table:: Supported Syntaxes
|
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:header: "GNU", "C++11", "__declspec", "Keyword"
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"X","","",""
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Clang provides support for C++ function overloading in C. Function overloading
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in C is introduced using the ``overloadable`` attribute. For example, one
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might provide several overloaded versions of a ``tgsin`` function that invokes
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the appropriate standard function computing the sine of a value with ``float``,
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``double``, or ``long double`` precision:
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.. code-block:: c
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#include <math.h>
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float __attribute__((overloadable)) tgsin(float x) { return sinf(x); }
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double __attribute__((overloadable)) tgsin(double x) { return sin(x); }
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long double __attribute__((overloadable)) tgsin(long double x) { return sinl(x); }
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Given these declarations, one can call ``tgsin`` with a ``float`` value to
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receive a ``float`` result, with a ``double`` to receive a ``double`` result,
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etc. Function overloading in C follows the rules of C++ function overloading
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to pick the best overload given the call arguments, with a few C-specific
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semantics:
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* Conversion from ``float`` or ``double`` to ``long double`` is ranked as a
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floating-point promotion (per C99) rather than as a floating-point conversion
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(as in C++).
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* A conversion from a pointer of type ``T*`` to a pointer of type ``U*`` is
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considered a pointer conversion (with conversion rank) if ``T`` and ``U`` are
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compatible types.
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* A conversion from type ``T`` to a value of type ``U`` is permitted if ``T``
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and ``U`` are compatible types. This conversion is given "conversion" rank.
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The declaration of ``overloadable`` functions is restricted to function
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declarations and definitions. Most importantly, if any function with a given
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name is given the ``overloadable`` attribute, then all function declarations
|
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and definitions with that name (and in that scope) must have the
|
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``overloadable`` attribute. This rule even applies to redeclarations of
|
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functions whose original declaration had the ``overloadable`` attribute, e.g.,
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.. code-block:: c
|
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int f(int) __attribute__((overloadable));
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float f(float); // error: declaration of "f" must have the "overloadable" attribute
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int g(int) __attribute__((overloadable));
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int g(int) { } // error: redeclaration of "g" must also have the "overloadable" attribute
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Functions marked ``overloadable`` must have prototypes. Therefore, the
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following code is ill-formed:
|
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.. code-block:: c
|
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int h() __attribute__((overloadable)); // error: h does not have a prototype
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However, ``overloadable`` functions are allowed to use a ellipsis even if there
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are no named parameters (as is permitted in C++). This feature is particularly
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useful when combined with the ``unavailable`` attribute:
|
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|
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.. code-block:: c++
|
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void honeypot(...) __attribute__((overloadable, unavailable)); // calling me is an error
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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)``.
|
|
|
|
|
|
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.
|
|
|
|
|
|
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..
|
|
|
|
|