forked from OSchip/llvm-project
1027 lines
37 KiB
ReStructuredText
1027 lines
37 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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acquire_capability (acquire_shared_capability, clang::acquire_capability, clang::acquire_shared_capability)
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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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Marks a function as acquiring a capability.
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assert_capability (assert_shared_capability, clang::assert_capability, clang::assert_shared_capability)
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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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Marks a function that dynamically tests whether a capability is held, and halts
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the program if it is not held.
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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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format (gnu::format)
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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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Clang supports the ``format`` attribute, which indicates that the function
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accepts a ``printf`` or ``scanf``-like format string and corresponding
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arguments or a ``va_list`` that contains these arguments.
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Please see `GCC documentation about format attribute
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<http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_ to find details
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about attribute syntax.
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Clang implements two kinds of checks with this attribute.
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#. Clang checks that the function with the ``format`` attribute is called with
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a format string that uses format specifiers that are allowed, and that
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arguments match the format string. This is the ``-Wformat`` warning, it is
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on by default.
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#. Clang checks that the format string argument is a literal string. This is
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the ``-Wformat-nonliteral`` warning, it is off by default.
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Clang implements this mostly the same way as GCC, but there is a difference
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for functions that accept a ``va_list`` argument (for example, ``vprintf``).
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GCC does not emit ``-Wformat-nonliteral`` warning for calls to such
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fuctions. Clang does not warn if the format string comes from a function
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parameter, where the function is annotated with a compatible attribute,
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otherwise it warns. For example:
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.. code-block:: c
|
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__attribute__((__format__ (__scanf__, 1, 3)))
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void foo(const char* s, char *buf, ...) {
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va_list ap;
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va_start(ap, buf);
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vprintf(s, ap); // warning: format string is not a string literal
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}
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In this case we warn because ``s`` contains a format string for a
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``scanf``-like function, but it is passed to a ``printf``-like function.
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If the attribute is removed, clang still warns, because the format string is
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not a string literal.
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Another example:
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.. code-block:: c
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__attribute__((__format__ (__printf__, 1, 3)))
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void foo(const char* s, char *buf, ...) {
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va_list ap;
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va_start(ap, buf);
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vprintf(s, ap); // warning
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}
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In this case Clang does not warn because the format string ``s`` and
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the corresponding arguments are annotated. If the arguments are
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incorrect, the caller of ``foo`` will receive a warning.
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noduplicate (clang::noduplicate)
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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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|
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The ``noduplicate`` attribute can be placed on function declarations to control
|
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whether function calls to this function can be duplicated or not as a result of
|
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optimizations. This is required for the implementation of functions with
|
|
certain special requirements, like the OpenCL "barrier" function, that might
|
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need to be run concurrently by all the threads that are executing in lockstep
|
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on the hardware. For example this attribute applied on the function
|
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"nodupfunc" in the code below avoids that:
|
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|
.. code-block:: c
|
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void nodupfunc() __attribute__((noduplicate));
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// Setting it as a C++11 attribute is also valid
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// void nodupfunc() [[clang::noduplicate]];
|
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void foo();
|
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void bar();
|
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nodupfunc();
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if (a > n) {
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foo();
|
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} else {
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bar();
|
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}
|
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gets possibly modified by some optimizations into code similar to this:
|
|
|
|
.. code-block:: c
|
|
|
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if (a > n) {
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nodupfunc();
|
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foo();
|
|
} else {
|
|
nodupfunc();
|
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bar();
|
|
}
|
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|
|
where the call to "nodupfunc" is duplicated and sunk into the two branches
|
|
of the condition.
|
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|
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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"
|
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|
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"X","X","",""
|
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.. _langext-address_sanitizer:
|
|
|
|
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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------------------
|
|
.. 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"
|
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that a method belongs to.
|
|
|
|
**Usage**: ``__attribute__((objc_method_family(X)))``, where ``X`` is one of
|
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``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:
|
|
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.. code-block:: c++
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/* In mpi.h */
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extern struct mpi_datatype mpi_datatype_null
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__attribute__(( type_tag_for_datatype(mpi, void, must_be_null) ));
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#define MPI_DATATYPE_NULL ((MPI_Datatype) &mpi_datatype_null)
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/* In user code */
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MPI_Send(buffer, 1, MPI_DATATYPE_NULL /*, ... */); // warning: MPI_DATATYPE_NULL
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// was specified but buffer
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// is not a null pointer
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