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2486 lines
92 KiB
ReStructuredText
2486 lines
92 KiB
ReStructuredText
=========================
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Clang Language Extensions
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=========================
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.. contents::
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:local:
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:depth: 1
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.. toctree::
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:hidden:
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ObjectiveCLiterals
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BlockLanguageSpec
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Block-ABI-Apple
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AutomaticReferenceCounting
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Introduction
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============
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This document describes the language extensions provided by Clang. In addition
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to the language extensions listed here, Clang aims to support a broad range of
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GCC extensions. Please see the `GCC manual
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<http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html>`_ for more information on
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these extensions.
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.. _langext-feature_check:
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Feature Checking Macros
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=======================
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Language extensions can be very useful, but only if you know you can depend on
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them. In order to allow fine-grain features checks, we support three builtin
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function-like macros. This allows you to directly test for a feature in your
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code without having to resort to something like autoconf or fragile "compiler
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version checks".
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``__has_builtin``
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-----------------
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This function-like macro takes a single identifier argument that is the name of
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a builtin function. It evaluates to 1 if the builtin is supported or 0 if not.
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It can be used like this:
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.. code-block:: c++
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#ifndef __has_builtin // Optional of course.
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#define __has_builtin(x) 0 // Compatibility with non-clang compilers.
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#endif
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...
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#if __has_builtin(__builtin_trap)
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__builtin_trap();
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#else
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abort();
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#endif
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...
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.. _langext-__has_feature-__has_extension:
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``__has_feature`` and ``__has_extension``
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-----------------------------------------
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These function-like macros take a single identifier argument that is the name
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of a feature. ``__has_feature`` evaluates to 1 if the feature is both
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supported by Clang and standardized in the current language standard or 0 if
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not (but see :ref:`below <langext-has-feature-back-compat>`), while
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``__has_extension`` evaluates to 1 if the feature is supported by Clang in the
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current language (either as a language extension or a standard language
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feature) or 0 if not. They can be used like this:
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.. code-block:: c++
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#ifndef __has_feature // Optional of course.
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#define __has_feature(x) 0 // Compatibility with non-clang compilers.
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#endif
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#ifndef __has_extension
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#define __has_extension __has_feature // Compatibility with pre-3.0 compilers.
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#endif
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...
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#if __has_feature(cxx_rvalue_references)
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// This code will only be compiled with the -std=c++11 and -std=gnu++11
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// options, because rvalue references are only standardized in C++11.
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#endif
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#if __has_extension(cxx_rvalue_references)
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// This code will be compiled with the -std=c++11, -std=gnu++11, -std=c++98
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// and -std=gnu++98 options, because rvalue references are supported as a
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// language extension in C++98.
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#endif
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.. _langext-has-feature-back-compat:
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For backwards compatibility reasons, ``__has_feature`` can also be used to test
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for support for non-standardized features, i.e. features not prefixed ``c_``,
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``cxx_`` or ``objc_``.
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Another use of ``__has_feature`` is to check for compiler features not related
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to the language standard, such as e.g. :doc:`AddressSanitizer
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<AddressSanitizer>`.
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If the ``-pedantic-errors`` option is given, ``__has_extension`` is equivalent
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to ``__has_feature``.
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The feature tag is described along with the language feature below.
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The feature name or extension name can also be specified with a preceding and
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following ``__`` (double underscore) to avoid interference from a macro with
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the same name. For instance, ``__cxx_rvalue_references__`` can be used instead
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of ``cxx_rvalue_references``.
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``__has_attribute``
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-------------------
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This function-like macro takes a single identifier argument that is the name of
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an attribute. It evaluates to 1 if the attribute is supported by the current
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compilation target, or 0 if not. It can be used like this:
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.. code-block:: c++
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#ifndef __has_attribute // Optional of course.
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#define __has_attribute(x) 0 // Compatibility with non-clang compilers.
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#endif
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...
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#if __has_attribute(always_inline)
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#define ALWAYS_INLINE __attribute__((always_inline))
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#else
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#define ALWAYS_INLINE
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#endif
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...
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The attribute name can also be specified with a preceding and following ``__``
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(double underscore) to avoid interference from a macro with the same name. For
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instance, ``__always_inline__`` can be used instead of ``always_inline``.
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Include File Checking Macros
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============================
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Not all developments systems have the same include files. The
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:ref:`langext-__has_include` and :ref:`langext-__has_include_next` macros allow
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you to check for the existence of an include file before doing a possibly
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failing ``#include`` directive. Include file checking macros must be used
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as expressions in ``#if`` or ``#elif`` preprocessing directives.
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.. _langext-__has_include:
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``__has_include``
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-----------------
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This function-like macro takes a single file name string argument that is the
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name of an include file. It evaluates to 1 if the file can be found using the
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include paths, or 0 otherwise:
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.. code-block:: c++
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// Note the two possible file name string formats.
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#if __has_include("myinclude.h") && __has_include(<stdint.h>)
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# include "myinclude.h"
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#endif
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To test for this feature, use ``#if defined(__has_include)``:
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.. code-block:: c++
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// To avoid problem with non-clang compilers not having this macro.
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#if defined(__has_include)
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#if __has_include("myinclude.h")
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# include "myinclude.h"
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#endif
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#endif
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.. _langext-__has_include_next:
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``__has_include_next``
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----------------------
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This function-like macro takes a single file name string argument that is the
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name of an include file. It is like ``__has_include`` except that it looks for
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the second instance of the given file found in the include paths. It evaluates
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to 1 if the second instance of the file can be found using the include paths,
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or 0 otherwise:
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.. code-block:: c++
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// Note the two possible file name string formats.
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#if __has_include_next("myinclude.h") && __has_include_next(<stdint.h>)
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# include_next "myinclude.h"
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#endif
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// To avoid problem with non-clang compilers not having this macro.
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#if defined(__has_include_next)
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#if __has_include_next("myinclude.h")
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# include_next "myinclude.h"
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#endif
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#endif
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Note that ``__has_include_next``, like the GNU extension ``#include_next``
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directive, is intended for use in headers only, and will issue a warning if
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used in the top-level compilation file. A warning will also be issued if an
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absolute path is used in the file argument.
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``__has_warning``
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-----------------
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This function-like macro takes a string literal that represents a command line
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option for a warning and returns true if that is a valid warning option.
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.. code-block:: c++
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#if __has_warning("-Wformat")
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...
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#endif
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Builtin Macros
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==============
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``__BASE_FILE__``
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Defined to a string that contains the name of the main input file passed to
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Clang.
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``__COUNTER__``
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Defined to an integer value that starts at zero and is incremented each time
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the ``__COUNTER__`` macro is expanded.
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``__INCLUDE_LEVEL__``
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Defined to an integral value that is the include depth of the file currently
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being translated. For the main file, this value is zero.
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``__TIMESTAMP__``
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Defined to the date and time of the last modification of the current source
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file.
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``__clang__``
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Defined when compiling with Clang
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``__clang_major__``
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Defined to the major marketing version number of Clang (e.g., the 2 in
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2.0.1). Note that marketing version numbers should not be used to check for
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language features, as different vendors use different numbering schemes.
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Instead, use the :ref:`langext-feature_check`.
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``__clang_minor__``
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Defined to the minor version number of Clang (e.g., the 0 in 2.0.1). Note
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that marketing version numbers should not be used to check for language
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features, as different vendors use different numbering schemes. Instead, use
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the :ref:`langext-feature_check`.
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``__clang_patchlevel__``
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Defined to the marketing patch level of Clang (e.g., the 1 in 2.0.1).
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``__clang_version__``
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Defined to a string that captures the Clang marketing version, including the
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Subversion tag or revision number, e.g., "``1.5 (trunk 102332)``".
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.. _langext-vectors:
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Vectors and Extended Vectors
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============================
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Supports the GCC, OpenCL, AltiVec and NEON vector extensions.
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OpenCL vector types are created using ``ext_vector_type`` attribute. It
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support for ``V.xyzw`` syntax and other tidbits as seen in OpenCL. An example
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is:
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.. code-block:: c++
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typedef float float4 __attribute__((ext_vector_type(4)));
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typedef float float2 __attribute__((ext_vector_type(2)));
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float4 foo(float2 a, float2 b) {
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float4 c;
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c.xz = a;
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c.yw = b;
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return c;
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}
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Query for this feature with ``__has_extension(attribute_ext_vector_type)``.
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Giving ``-faltivec`` option to clang enables support for AltiVec vector syntax
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and functions. For example:
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.. code-block:: c++
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vector float foo(vector int a) {
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vector int b;
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b = vec_add(a, a) + a;
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return (vector float)b;
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}
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NEON vector types are created using ``neon_vector_type`` and
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``neon_polyvector_type`` attributes. For example:
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.. code-block:: c++
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typedef __attribute__((neon_vector_type(8))) int8_t int8x8_t;
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typedef __attribute__((neon_polyvector_type(16))) poly8_t poly8x16_t;
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int8x8_t foo(int8x8_t a) {
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int8x8_t v;
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v = a;
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return v;
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}
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Vector Literals
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---------------
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Vector literals can be used to create vectors from a set of scalars, or
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vectors. Either parentheses or braces form can be used. In the parentheses
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form the number of literal values specified must be one, i.e. referring to a
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scalar value, or must match the size of the vector type being created. If a
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single scalar literal value is specified, the scalar literal value will be
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replicated to all the components of the vector type. In the brackets form any
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number of literals can be specified. For example:
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.. code-block:: c++
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typedef int v4si __attribute__((__vector_size__(16)));
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typedef float float4 __attribute__((ext_vector_type(4)));
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typedef float float2 __attribute__((ext_vector_type(2)));
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v4si vsi = (v4si){1, 2, 3, 4};
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float4 vf = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
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vector int vi1 = (vector int)(1); // vi1 will be (1, 1, 1, 1).
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vector int vi2 = (vector int){1}; // vi2 will be (1, 0, 0, 0).
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vector int vi3 = (vector int)(1, 2); // error
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vector int vi4 = (vector int){1, 2}; // vi4 will be (1, 2, 0, 0).
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vector int vi5 = (vector int)(1, 2, 3, 4);
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float4 vf = (float4)((float2)(1.0f, 2.0f), (float2)(3.0f, 4.0f));
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Vector Operations
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-----------------
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The table below shows the support for each operation by vector extension. A
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dash indicates that an operation is not accepted according to a corresponding
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specification.
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============================== ====== ======= === ====
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Opeator OpenCL AltiVec GCC NEON
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============================== ====== ======= === ====
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[] yes yes yes --
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unary operators +, -- yes yes yes --
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++, -- -- yes yes yes --
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+,--,*,/,% yes yes yes --
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bitwise operators &,|,^,~ yes yes yes --
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>>,<< yes yes yes --
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!, &&, || no -- -- --
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==, !=, >, <, >=, <= yes yes -- --
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= yes yes yes yes
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:? yes -- -- --
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sizeof yes yes yes yes
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============================== ====== ======= === ====
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See also :ref:`langext-__builtin_shufflevector`.
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Messages on ``deprecated`` and ``unavailable`` Attributes
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=========================================================
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An optional string message can be added to the ``deprecated`` and
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``unavailable`` attributes. For example:
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.. code-block:: c++
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void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));
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If the deprecated or unavailable declaration is used, the message will be
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incorporated into the appropriate diagnostic:
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.. code-block:: c++
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harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!!
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[-Wdeprecated-declarations]
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explode();
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^
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Query for this feature with
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``__has_extension(attribute_deprecated_with_message)`` and
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``__has_extension(attribute_unavailable_with_message)``.
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Attributes on Enumerators
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=========================
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Clang allows attributes to be written on individual enumerators. This allows
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enumerators to be deprecated, made unavailable, etc. The attribute must appear
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after the enumerator name and before any initializer, like so:
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.. code-block:: c++
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enum OperationMode {
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OM_Invalid,
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OM_Normal,
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OM_Terrified __attribute__((deprecated)),
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OM_AbortOnError __attribute__((deprecated)) = 4
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};
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Attributes on the ``enum`` declaration do not apply to individual enumerators.
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Query for this feature with ``__has_extension(enumerator_attributes)``.
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'User-Specified' System Frameworks
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==================================
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Clang provides a mechanism by which frameworks can be built in such a way that
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they will always be treated as being "system frameworks", even if they are not
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present in a system framework directory. This can be useful to system
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framework developers who want to be able to test building other applications
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with development builds of their framework, including the manner in which the
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compiler changes warning behavior for system headers.
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Framework developers can opt-in to this mechanism by creating a
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"``.system_framework``" file at the top-level of their framework. That is, the
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framework should have contents like:
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.. code-block:: none
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.../TestFramework.framework
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.../TestFramework.framework/.system_framework
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.../TestFramework.framework/Headers
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.../TestFramework.framework/Headers/TestFramework.h
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...
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Clang will treat the presence of this file as an indicator that the framework
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should be treated as a system framework, regardless of how it was found in the
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framework search path. For consistency, we recommend that such files never be
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included in installed versions of the framework.
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Availability attribute
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======================
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Clang introduces the ``availability`` attribute, which can be placed on
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declarations to describe the lifecycle of that declaration relative to
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operating system versions. Consider the function declaration for a
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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.,:
|
|
|
|
.. code-block:: objc
|
|
|
|
@interface A
|
|
- (id)method __attribute__((availability(macosx,introduced=10.4)));
|
|
- (id)method2 __attribute__((availability(macosx,introduced=10.4)));
|
|
@end
|
|
|
|
@interface B : A
|
|
- (id)method __attribute__((availability(macosx,introduced=10.3))); // okay: method moved into base class later
|
|
- (id)method __attribute__((availability(macosx,introduced=10.5))); // error: this method was available via the base class in 10.4
|
|
@end
|
|
|
|
Checks for Standard Language Features
|
|
=====================================
|
|
|
|
The ``__has_feature`` macro can be used to query if certain standard language
|
|
features are enabled. The ``__has_extension`` macro can be used to query if
|
|
language features are available as an extension when compiling for a standard
|
|
which does not provide them. The features which can be tested are listed here.
|
|
|
|
C++98
|
|
-----
|
|
|
|
The features listed below are part of the C++98 standard. These features are
|
|
enabled by default when compiling C++ code.
|
|
|
|
C++ exceptions
|
|
^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_exceptions)`` to determine if C++ exceptions have been
|
|
enabled. For example, compiling code with ``-fno-exceptions`` disables C++
|
|
exceptions.
|
|
|
|
C++ RTTI
|
|
^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_rtti)`` to determine if C++ RTTI has been enabled. For
|
|
example, compiling code with ``-fno-rtti`` disables the use of RTTI.
|
|
|
|
C++11
|
|
-----
|
|
|
|
The features listed below are part of the C++11 standard. As a result, all
|
|
these features are enabled with the ``-std=c++11`` or ``-std=gnu++11`` option
|
|
when compiling C++ code.
|
|
|
|
C++11 SFINAE includes access control
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_access_control_sfinae)`` or
|
|
``__has_extension(cxx_access_control_sfinae)`` to determine whether
|
|
access-control errors (e.g., calling a private constructor) are considered to
|
|
be template argument deduction errors (aka SFINAE errors), per `C++ DR1170
|
|
<http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1170>`_.
|
|
|
|
C++11 alias templates
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_alias_templates)`` or
|
|
``__has_extension(cxx_alias_templates)`` to determine if support for C++11's
|
|
alias declarations and alias templates is enabled.
|
|
|
|
C++11 alignment specifiers
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_alignas)`` or ``__has_extension(cxx_alignas)`` to
|
|
determine if support for alignment specifiers using ``alignas`` is enabled.
|
|
|
|
C++11 attributes
|
|
^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_attributes)`` or ``__has_extension(cxx_attributes)`` to
|
|
determine if support for attribute parsing with C++11's square bracket notation
|
|
is enabled.
|
|
|
|
C++11 generalized constant expressions
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_constexpr)`` to determine if support for generalized
|
|
constant expressions (e.g., ``constexpr``) is enabled.
|
|
|
|
C++11 ``decltype()``
|
|
^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_decltype)`` or ``__has_extension(cxx_decltype)`` to
|
|
determine if support for the ``decltype()`` specifier is enabled. C++11's
|
|
``decltype`` does not require type-completeness of a function call expression.
|
|
Use ``__has_feature(cxx_decltype_incomplete_return_types)`` or
|
|
``__has_extension(cxx_decltype_incomplete_return_types)`` to determine if
|
|
support for this feature is enabled.
|
|
|
|
C++11 default template arguments in function templates
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_default_function_template_args)`` or
|
|
``__has_extension(cxx_default_function_template_args)`` to determine if support
|
|
for default template arguments in function templates is enabled.
|
|
|
|
C++11 ``default``\ ed functions
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_defaulted_functions)`` or
|
|
``__has_extension(cxx_defaulted_functions)`` to determine if support for
|
|
defaulted function definitions (with ``= default``) is enabled.
|
|
|
|
C++11 delegating constructors
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_delegating_constructors)`` to determine if support for
|
|
delegating constructors is enabled.
|
|
|
|
C++11 ``deleted`` functions
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_deleted_functions)`` or
|
|
``__has_extension(cxx_deleted_functions)`` to determine if support for deleted
|
|
function definitions (with ``= delete``) is enabled.
|
|
|
|
C++11 explicit conversion functions
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_explicit_conversions)`` to determine if support for
|
|
``explicit`` conversion functions is enabled.
|
|
|
|
C++11 generalized initializers
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_generalized_initializers)`` to determine if support for
|
|
generalized initializers (using braced lists and ``std::initializer_list``) is
|
|
enabled.
|
|
|
|
C++11 implicit move constructors/assignment operators
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_implicit_moves)`` to determine if Clang will implicitly
|
|
generate move constructors and move assignment operators where needed.
|
|
|
|
C++11 inheriting constructors
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_inheriting_constructors)`` to determine if support for
|
|
inheriting constructors is enabled.
|
|
|
|
C++11 inline namespaces
|
|
^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_inline_namespaces)`` or
|
|
``__has_extension(cxx_inline_namespaces)`` to determine if support for inline
|
|
namespaces is enabled.
|
|
|
|
C++11 lambdas
|
|
^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_lambdas)`` or ``__has_extension(cxx_lambdas)`` to
|
|
determine if support for lambdas is enabled.
|
|
|
|
C++11 local and unnamed types as template arguments
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_local_type_template_args)`` or
|
|
``__has_extension(cxx_local_type_template_args)`` to determine if support for
|
|
local and unnamed types as template arguments is enabled.
|
|
|
|
C++11 noexcept
|
|
^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_noexcept)`` or ``__has_extension(cxx_noexcept)`` to
|
|
determine if support for noexcept exception specifications is enabled.
|
|
|
|
C++11 in-class non-static data member initialization
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_nonstatic_member_init)`` to determine whether in-class
|
|
initialization of non-static data members is enabled.
|
|
|
|
C++11 ``nullptr``
|
|
^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_nullptr)`` or ``__has_extension(cxx_nullptr)`` to
|
|
determine if support for ``nullptr`` is enabled.
|
|
|
|
C++11 ``override control``
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_override_control)`` or
|
|
``__has_extension(cxx_override_control)`` to determine if support for the
|
|
override control keywords is enabled.
|
|
|
|
C++11 reference-qualified functions
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_reference_qualified_functions)`` or
|
|
``__has_extension(cxx_reference_qualified_functions)`` to determine if support
|
|
for reference-qualified functions (e.g., member functions with ``&`` or ``&&``
|
|
applied to ``*this``) is enabled.
|
|
|
|
C++11 range-based ``for`` loop
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_range_for)`` or ``__has_extension(cxx_range_for)`` to
|
|
determine if support for the range-based for loop is enabled.
|
|
|
|
C++11 raw string literals
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_raw_string_literals)`` to determine if support for raw
|
|
string literals (e.g., ``R"x(foo\bar)x"``) is enabled.
|
|
|
|
C++11 rvalue references
|
|
^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_rvalue_references)`` or
|
|
``__has_extension(cxx_rvalue_references)`` to determine if support for rvalue
|
|
references is enabled.
|
|
|
|
C++11 ``static_assert()``
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_static_assert)`` or
|
|
``__has_extension(cxx_static_assert)`` to determine if support for compile-time
|
|
assertions using ``static_assert`` is enabled.
|
|
|
|
C++11 ``thread_local``
|
|
^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_thread_local)`` to determine if support for
|
|
``thread_local`` variables is enabled.
|
|
|
|
C++11 type inference
|
|
^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_auto_type)`` or ``__has_extension(cxx_auto_type)`` to
|
|
determine C++11 type inference is supported using the ``auto`` specifier. If
|
|
this is disabled, ``auto`` will instead be a storage class specifier, as in C
|
|
or C++98.
|
|
|
|
C++11 strongly typed enumerations
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_strong_enums)`` or
|
|
``__has_extension(cxx_strong_enums)`` to determine if support for strongly
|
|
typed, scoped enumerations is enabled.
|
|
|
|
C++11 trailing return type
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_trailing_return)`` or
|
|
``__has_extension(cxx_trailing_return)`` to determine if support for the
|
|
alternate function declaration syntax with trailing return type is enabled.
|
|
|
|
C++11 Unicode string literals
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_unicode_literals)`` to determine if support for Unicode
|
|
string literals is enabled.
|
|
|
|
C++11 unrestricted unions
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_unrestricted_unions)`` to determine if support for
|
|
unrestricted unions is enabled.
|
|
|
|
C++11 user-defined literals
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_user_literals)`` to determine if support for
|
|
user-defined literals is enabled.
|
|
|
|
C++11 variadic templates
|
|
^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_variadic_templates)`` or
|
|
``__has_extension(cxx_variadic_templates)`` to determine if support for
|
|
variadic templates is enabled.
|
|
|
|
C++1y
|
|
-----
|
|
|
|
The features listed below are part of the committee draft for the C++1y
|
|
standard. As a result, all these features are enabled with the ``-std=c++1y``
|
|
or ``-std=gnu++1y`` option when compiling C++ code.
|
|
|
|
C++1y binary literals
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_binary_literals)`` or
|
|
``__has_extension(cxx_binary_literals)`` to determine whether
|
|
binary literals (for instance, ``0b10010``) are recognized. Clang supports this
|
|
feature as an extension in all language modes.
|
|
|
|
C++1y contextual conversions
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_contextual_conversions)`` or
|
|
``__has_extension(cxx_contextual_conversions)`` to determine if the C++1y rules
|
|
are used when performing an implicit conversion for an array bound in a
|
|
*new-expression*, the operand of a *delete-expression*, an integral constant
|
|
expression, or a condition in a ``switch`` statement.
|
|
|
|
C++1y decltype(auto)
|
|
^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_decltype_auto)`` or
|
|
``__has_extension(cxx_decltype_auto)`` to determine if support
|
|
for the ``decltype(auto)`` placeholder type is enabled.
|
|
|
|
C++1y default initializers for aggregates
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_aggregate_nsdmi)`` or
|
|
``__has_extension(cxx_aggregate_nsdmi)`` to determine if support
|
|
for default initializers in aggregate members is enabled.
|
|
|
|
C++1y generalized lambda capture
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_init_capture)`` or
|
|
``__has_extension(cxx_init_capture)`` to determine if support for
|
|
lambda captures with explicit initializers is enabled
|
|
(for instance, ``[n(0)] { return ++n; }``).
|
|
Clang does not yet support this feature.
|
|
|
|
C++1y generic lambdas
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_generic_lambda)`` or
|
|
``__has_extension(cxx_generic_lambda)`` to determine if support for generic
|
|
(polymorphic) lambdas is enabled
|
|
(for instance, ``[] (auto x) { return x + 1; }``).
|
|
Clang does not yet support this feature.
|
|
|
|
C++1y relaxed constexpr
|
|
^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_relaxed_constexpr)`` or
|
|
``__has_extension(cxx_relaxed_constexpr)`` to determine if variable
|
|
declarations, local variable modification, and control flow constructs
|
|
are permitted in ``constexpr`` functions.
|
|
|
|
C++1y return type deduction
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_return_type_deduction)`` or
|
|
``__has_extension(cxx_return_type_deduction)`` to determine if support
|
|
for return type deduction for functions (using ``auto`` as a return type)
|
|
is enabled.
|
|
|
|
C++1y runtime-sized arrays
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_runtime_array)`` or
|
|
``__has_extension(cxx_runtime_array)`` to determine if support
|
|
for arrays of runtime bound (a restricted form of variable-length arrays)
|
|
is enabled.
|
|
Clang's implementation of this feature is incomplete.
|
|
|
|
C++1y variable templates
|
|
^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(cxx_variable_templates)`` or
|
|
``__has_extension(cxx_variable_templates)`` to determine if support for
|
|
templated variable declarations is enabled.
|
|
Clang does not yet support this feature.
|
|
|
|
C11
|
|
---
|
|
|
|
The features listed below are part of the C11 standard. As a result, all these
|
|
features are enabled with the ``-std=c11`` or ``-std=gnu11`` option when
|
|
compiling C code. Additionally, because these features are all
|
|
backward-compatible, they are available as extensions in all language modes.
|
|
|
|
C11 alignment specifiers
|
|
^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(c_alignas)`` or ``__has_extension(c_alignas)`` to determine
|
|
if support for alignment specifiers using ``_Alignas`` is enabled.
|
|
|
|
C11 atomic operations
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(c_atomic)`` or ``__has_extension(c_atomic)`` to determine
|
|
if support for atomic types using ``_Atomic`` is enabled. Clang also provides
|
|
:ref:`a set of builtins <langext-__c11_atomic>` which can be used to implement
|
|
the ``<stdatomic.h>`` operations on ``_Atomic`` types.
|
|
|
|
C11 generic selections
|
|
^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(c_generic_selections)`` or
|
|
``__has_extension(c_generic_selections)`` to determine if support for generic
|
|
selections is enabled.
|
|
|
|
As an extension, the C11 generic selection expression is available in all
|
|
languages supported by Clang. The syntax is the same as that given in the C11
|
|
standard.
|
|
|
|
In C, type compatibility is decided according to the rules given in the
|
|
appropriate standard, but in C++, which lacks the type compatibility rules used
|
|
in C, types are considered compatible only if they are equivalent.
|
|
|
|
C11 ``_Static_assert()``
|
|
^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(c_static_assert)`` or ``__has_extension(c_static_assert)``
|
|
to determine if support for compile-time assertions using ``_Static_assert`` is
|
|
enabled.
|
|
|
|
C11 ``_Thread_local``
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Use ``__has_feature(c_thread_local)`` or ``__has_extension(c_thread_local)``
|
|
to determine if support for ``_Thread_local`` variables is enabled.
|
|
|
|
Checks for Type Traits
|
|
======================
|
|
|
|
Clang supports the `GNU C++ type traits
|
|
<http://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html>`_ and a subset of the
|
|
`Microsoft Visual C++ Type traits
|
|
<http://msdn.microsoft.com/en-us/library/ms177194(v=VS.100).aspx>`_. For each
|
|
supported type trait ``__X``, ``__has_extension(X)`` indicates the presence of
|
|
the type trait. For example:
|
|
|
|
.. code-block:: c++
|
|
|
|
#if __has_extension(is_convertible_to)
|
|
template<typename From, typename To>
|
|
struct is_convertible_to {
|
|
static const bool value = __is_convertible_to(From, To);
|
|
};
|
|
#else
|
|
// Emulate type trait
|
|
#endif
|
|
|
|
The following type traits are supported by Clang:
|
|
|
|
* ``__has_nothrow_assign`` (GNU, Microsoft)
|
|
* ``__has_nothrow_copy`` (GNU, Microsoft)
|
|
* ``__has_nothrow_constructor`` (GNU, Microsoft)
|
|
* ``__has_trivial_assign`` (GNU, Microsoft)
|
|
* ``__has_trivial_copy`` (GNU, Microsoft)
|
|
* ``__has_trivial_constructor`` (GNU, Microsoft)
|
|
* ``__has_trivial_destructor`` (GNU, Microsoft)
|
|
* ``__has_virtual_destructor`` (GNU, Microsoft)
|
|
* ``__is_abstract`` (GNU, Microsoft)
|
|
* ``__is_base_of`` (GNU, Microsoft)
|
|
* ``__is_class`` (GNU, Microsoft)
|
|
* ``__is_convertible_to`` (Microsoft)
|
|
* ``__is_empty`` (GNU, Microsoft)
|
|
* ``__is_enum`` (GNU, Microsoft)
|
|
* ``__is_interface_class`` (Microsoft)
|
|
* ``__is_pod`` (GNU, Microsoft)
|
|
* ``__is_polymorphic`` (GNU, Microsoft)
|
|
* ``__is_union`` (GNU, Microsoft)
|
|
* ``__is_literal(type)``: Determines whether the given type is a literal type
|
|
* ``__is_final``: Determines whether the given type is declared with a
|
|
``final`` class-virt-specifier.
|
|
* ``__underlying_type(type)``: Retrieves the underlying type for a given
|
|
``enum`` type. This trait is required to implement the C++11 standard
|
|
library.
|
|
* ``__is_trivially_assignable(totype, fromtype)``: Determines whether a value
|
|
of type ``totype`` can be assigned to from a value of type ``fromtype`` such
|
|
that no non-trivial functions are called as part of that assignment. This
|
|
trait is required to implement the C++11 standard library.
|
|
* ``__is_trivially_constructible(type, argtypes...)``: Determines whether a
|
|
value of type ``type`` can be direct-initialized with arguments of types
|
|
``argtypes...`` such that no non-trivial functions are called as part of
|
|
that initialization. This trait is required to implement the C++11 standard
|
|
library.
|
|
|
|
Blocks
|
|
======
|
|
|
|
The syntax and high level language feature description is in
|
|
:doc:`BlockLanguageSpec<BlockLanguageSpec>`. Implementation and ABI details for
|
|
the clang implementation are in :doc:`Block-ABI-Apple<Block-ABI-Apple>`.
|
|
|
|
Query for this feature with ``__has_extension(blocks)``.
|
|
|
|
Objective-C Features
|
|
====================
|
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Related result types
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--------------------
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According to Cocoa conventions, Objective-C methods with certain names
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("``init``", "``alloc``", etc.) always return objects that are an instance of
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the receiving class's type. Such methods are said to have a "related result
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type", meaning that a message send to one of these methods will have the same
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static type as an instance of the receiver class. For example, given the
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following classes:
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.. code-block:: objc
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@interface NSObject
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+ (id)alloc;
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- (id)init;
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@end
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@interface NSArray : NSObject
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@end
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and this common initialization pattern
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.. code-block:: objc
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NSArray *array = [[NSArray alloc] init];
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the type of the expression ``[NSArray alloc]`` is ``NSArray*`` because
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``alloc`` implicitly has a related result type. Similarly, the type of the
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expression ``[[NSArray alloc] init]`` is ``NSArray*``, since ``init`` has a
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related result type and its receiver is known to have the type ``NSArray *``.
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If neither ``alloc`` nor ``init`` had a related result type, the expressions
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would have had type ``id``, as declared in the method signature.
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A method with a related result type can be declared by using the type
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``instancetype`` as its result type. ``instancetype`` is a contextual keyword
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that is only permitted in the result type of an Objective-C method, e.g.
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.. code-block:: objc
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@interface A
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+ (instancetype)constructAnA;
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@end
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The related result type can also be inferred for some methods. To determine
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whether a method has an inferred related result type, the first word in the
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camel-case selector (e.g., "``init``" in "``initWithObjects``") is considered,
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and the method will have a related result type if its return type is compatible
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with the type of its class and if:
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* the first word is "``alloc``" or "``new``", and the method is a class method,
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or
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* the first word is "``autorelease``", "``init``", "``retain``", or "``self``",
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and the method is an instance method.
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If a method with a related result type is overridden by a subclass method, the
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subclass method must also return a type that is compatible with the subclass
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type. For example:
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.. code-block:: objc
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@interface NSString : NSObject
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- (NSUnrelated *)init; // incorrect usage: NSUnrelated is not NSString or a superclass of NSString
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@end
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Related result types only affect the type of a message send or property access
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via the given method. In all other respects, a method with a related result
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type is treated the same way as method that returns ``id``.
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Use ``__has_feature(objc_instancetype)`` to determine whether the
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``instancetype`` contextual keyword is available.
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Automatic reference counting
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----------------------------
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Clang provides support for :doc:`automated reference counting
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<AutomaticReferenceCounting>` in Objective-C, which eliminates the need
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for manual ``retain``/``release``/``autorelease`` message sends. There are two
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feature macros associated with automatic reference counting:
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``__has_feature(objc_arc)`` indicates the availability of automated reference
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counting in general, while ``__has_feature(objc_arc_weak)`` indicates that
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automated reference counting also includes support for ``__weak`` pointers to
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Objective-C objects.
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.. _objc-fixed-enum:
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Enumerations with a fixed underlying type
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-----------------------------------------
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Clang provides support for C++11 enumerations with a fixed underlying type
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within Objective-C. For example, one can write an enumeration type as:
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.. code-block:: c++
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typedef enum : unsigned char { Red, Green, Blue } Color;
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This specifies that the underlying type, which is used to store the enumeration
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value, is ``unsigned char``.
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Use ``__has_feature(objc_fixed_enum)`` to determine whether support for fixed
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underlying types is available in Objective-C.
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Interoperability with C++11 lambdas
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-----------------------------------
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Clang provides interoperability between C++11 lambdas and blocks-based APIs, by
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permitting a lambda to be implicitly converted to a block pointer with the
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corresponding signature. For example, consider an API such as ``NSArray``'s
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array-sorting method:
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.. code-block:: objc
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- (NSArray *)sortedArrayUsingComparator:(NSComparator)cmptr;
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``NSComparator`` is simply a typedef for the block pointer ``NSComparisonResult
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(^)(id, id)``, and parameters of this type are generally provided with block
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literals as arguments. However, one can also use a C++11 lambda so long as it
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provides the same signature (in this case, accepting two parameters of type
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``id`` and returning an ``NSComparisonResult``):
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.. code-block:: objc
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NSArray *array = @[@"string 1", @"string 21", @"string 12", @"String 11",
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@"String 02"];
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const NSStringCompareOptions comparisonOptions
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= NSCaseInsensitiveSearch | NSNumericSearch |
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NSWidthInsensitiveSearch | NSForcedOrderingSearch;
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NSLocale *currentLocale = [NSLocale currentLocale];
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NSArray *sorted
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= [array sortedArrayUsingComparator:[=](id s1, id s2) -> NSComparisonResult {
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NSRange string1Range = NSMakeRange(0, [s1 length]);
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return [s1 compare:s2 options:comparisonOptions
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range:string1Range locale:currentLocale];
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}];
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NSLog(@"sorted: %@", sorted);
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This code relies on an implicit conversion from the type of the lambda
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expression (an unnamed, local class type called the *closure type*) to the
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corresponding block pointer type. The conversion itself is expressed by a
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conversion operator in that closure type that produces a block pointer with the
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same signature as the lambda itself, e.g.,
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.. code-block:: objc
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operator NSComparisonResult (^)(id, id)() const;
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This conversion function returns a new block that simply forwards the two
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parameters to the lambda object (which it captures by copy), then returns the
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result. The returned block is first copied (with ``Block_copy``) and then
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autoreleased. As an optimization, if a lambda expression is immediately
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converted to a block pointer (as in the first example, above), then the block
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is not copied and autoreleased: rather, it is given the same lifetime as a
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block literal written at that point in the program, which avoids the overhead
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of copying a block to the heap in the common case.
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The conversion from a lambda to a block pointer is only available in
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Objective-C++, and not in C++ with blocks, due to its use of Objective-C memory
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management (autorelease).
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Object Literals and Subscripting
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--------------------------------
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Clang provides support for :doc:`Object Literals and Subscripting
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<ObjectiveCLiterals>` in Objective-C, which simplifies common Objective-C
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programming patterns, makes programs more concise, and improves the safety of
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container creation. There are several feature macros associated with object
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literals and subscripting: ``__has_feature(objc_array_literals)`` tests the
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availability of array literals; ``__has_feature(objc_dictionary_literals)``
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tests the availability of dictionary literals;
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``__has_feature(objc_subscripting)`` tests the availability of object
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subscripting.
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Objective-C Autosynthesis of Properties
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---------------------------------------
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Clang provides support for autosynthesis of declared properties. Using this
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feature, clang provides default synthesis of those properties not declared
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@dynamic and not having user provided backing getter and setter methods.
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``__has_feature(objc_default_synthesize_properties)`` checks for availability
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of this feature in version of clang being used.
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.. _langext-objc_method_family:
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Objective-C requiring a call to ``super`` in an override
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--------------------------------------------------------
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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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Objective-C Method Families
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---------------------------
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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 :ref:`retaining behavior attributes <langext-objc-retain-release>`
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described below.
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Query for this feature with ``__has_attribute(objc_method_family)``.
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.. _langext-objc-retain-release:
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Objective-C retaining behavior attributes
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-----------------------------------------
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In Objective-C, functions and methods are generally assumed to follow the
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`Cocoa Memory Management
|
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<http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html>`_
|
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conventions for ownership of object arguments and
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return values. However, there are exceptions, and so Clang provides attributes
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to allow these exceptions to be documented. This are used by ARC and the
|
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`static analyzer <http://clang-analyzer.llvm.org>`_ Some exceptions may be
|
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better described using the :ref:`objc_method_family
|
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<langext-objc_method_family>` attribute instead.
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**Usage**: The ``ns_returns_retained``, ``ns_returns_not_retained``,
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``ns_returns_autoreleased``, ``cf_returns_retained``, and
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``cf_returns_not_retained`` attributes can be placed on methods and functions
|
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that return Objective-C or CoreFoundation objects. They are commonly placed at
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the end of a function prototype or method declaration:
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.. code-block:: objc
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id foo() __attribute__((ns_returns_retained));
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- (NSString *)bar:(int)x __attribute__((ns_returns_retained));
|
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The ``*_returns_retained`` attributes specify that the returned object has a +1
|
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retain count. The ``*_returns_not_retained`` attributes specify that the return
|
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object has a +0 retain count, even if the normal convention for its selector
|
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would be +1. ``ns_returns_autoreleased`` specifies that the returned object is
|
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+0, but is guaranteed to live at least as long as the next flush of an
|
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autorelease pool.
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|
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**Usage**: The ``ns_consumed`` and ``cf_consumed`` attributes can be placed on
|
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an parameter declaration; they specify that the argument is expected to have a
|
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+1 retain count, which will be balanced in some way by the function or method.
|
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The ``ns_consumes_self`` attribute can only be placed on an Objective-C
|
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method; it specifies that the method expects its ``self`` parameter to have a
|
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+1 retain count, which it will balance in some way.
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.. code-block:: objc
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|
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void foo(__attribute__((ns_consumed)) NSString *string);
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|
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- (void) bar __attribute__((ns_consumes_self));
|
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- (void) baz:(id) __attribute__((ns_consumed)) x;
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|
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Further examples of these attributes are available in the static analyzer's `list of annotations for analysis
|
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<http://clang-analyzer.llvm.org/annotations.html#cocoa_mem>`_.
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Query for these features with ``__has_attribute(ns_consumed)``,
|
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``__has_attribute(ns_returns_retained)``, etc.
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|
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Objective-C++ ABI: protocol-qualifier mangling of parameters
|
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------------------------------------------------------------
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Starting with LLVM 3.4, Clang produces a new mangling for parameters whose
|
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type is a qualified-``id`` (e.g., ``id<Foo>``). This mangling allows such
|
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parameters to be differentiated from those with the regular unqualified ``id``
|
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type.
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This was a non-backward compatible mangling change to the ABI. This change
|
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allows proper overloading, and also prevents mangling conflicts with template
|
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parameters of protocol-qualified type.
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Query the presence of this new mangling with
|
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``__has_feature(objc_protocol_qualifier_mangling)``.
|
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|
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.. _langext-overloading:
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|
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Function Overloading in C
|
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=========================
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|
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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
|
|
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``,
|
|
``double``, or ``long double`` precision:
|
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|
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.. code-block:: c
|
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|
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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
|
|
to pick the best overload given the call arguments, with a few C-specific
|
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semantics:
|
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|
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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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|
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* 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.
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|
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* 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.
|
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|
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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
|
|
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.,
|
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|
|
.. code-block:: c
|
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|
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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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|
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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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|
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Functions marked ``overloadable`` must have prototypes. Therefore, the
|
|
following code is ill-formed:
|
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|
|
.. code-block:: c
|
|
|
|
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
|
|
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
|
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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)``.
|
|
|
|
Controlling Overload Resolution
|
|
===============================
|
|
|
|
Clang introduces the ``enable_if`` attribute, which can be placed on function
|
|
declarations to control which overload is selected based on the values of the
|
|
function's arguments. When combined with the
|
|
:ref:``overloadable<langext-overloading>`` attribute, this feature is also
|
|
available in C.
|
|
|
|
.. code-block:: c++
|
|
|
|
int isdigit(int c) __attribute__((enable_if(c >= -1 && c <= 255, "'c' must have the value of an unsigned char or EOF")));
|
|
|
|
void foo(char c) {
|
|
isdigit(10);
|
|
isdigit(-10); // results in a compile-time error.
|
|
}
|
|
|
|
The enable_if attribute takes two arguments, the first is an expression written
|
|
in terms of the function parameters, the second is a string explaining why this
|
|
overload candidate could not be selected to be displayed in diagnostics. The
|
|
expression is part of the function signature for the purposes of determining
|
|
whether it is a redeclaration (following the rules used when determining
|
|
whether a C++ template specialization is ODR-equivalent), but is not part of
|
|
the type.
|
|
|
|
An enable_if expression will be evaluated by substituting the values of the
|
|
parameters from the call site into the arguments in the expression and
|
|
determining whether the result is true. If the result is false or could not be
|
|
determined through constant expression evaluation, then this overload will not
|
|
be chosen and the reason supplied in the string will be given to the user if
|
|
their code does not compile as a result.
|
|
|
|
Because the enable_if expression is an unevaluated context, there are no global
|
|
state changes, nor the ability to pass information from the enable_if
|
|
expression to the function body. For example, suppose we want calls to
|
|
strnlen(strbuf, maxlen) to resolve to strnlen_chk(strbuf, maxlen, size of
|
|
strbuf) only if the size of strbuf can be determined:
|
|
|
|
.. code-block:: c++
|
|
|
|
__attribute__((always_inline))
|
|
static inline size_t strnlen(const char *s, size_t maxlen)
|
|
__attribute__((overloadable))
|
|
__attribute__((enable_if(__builtin_object_size(s, 0) != -1))),
|
|
"chosen when the buffer size is known but 'maxlen' is not")))
|
|
{
|
|
return strnlen_chk(s, maxlen, __builtin_object_size(s, 0));
|
|
}
|
|
|
|
Multiple enable_if attributes may be applied to a single declaration. In this
|
|
case, the enable_if expressions are evaluated from left to right in the
|
|
following manner. First, the candidates whose enable_if expressions evaluate to
|
|
false or cannot be evaluated are discarded. If the remaining candidates do not
|
|
share ODR-equivalent enable_if expressions, the overload resolution is
|
|
ambiguous. Otherwise, enable_if overload resolution continues with the next
|
|
enable_if attribute on the candidates that have not been discarded and have
|
|
remaining enable_if attributes. In this way, we pick the most specific
|
|
overload out of a number of viable overloads using enable_if.
|
|
|
|
.. code-block:: c++
|
|
void f() __attribute__((enable_if(true, ""))); // #1
|
|
void f() __attribute__((enable_if(true, ""))) __attribute__((enable_if(true, ""))); // #2
|
|
|
|
void g(int i, int j) __attribute__((enable_if(i, ""))); // #1
|
|
void g(int i, int j) __attribute__((enable_if(j, ""))) __attribute__((enable_if(true))); // #2
|
|
|
|
In this example, a call to f() is always resolved to #2, as the first enable_if
|
|
expression is ODR-equivalent for both declarations, but #1 does not have another
|
|
enable_if expression to continue evaluating, so the next round of evaluation has
|
|
only a single candidate. In a call to g(1, 1), the call is ambiguous even though
|
|
#2 has more enable_if attributes, because the first enable_if expressions are
|
|
not ODR-equivalent.
|
|
|
|
Query for this feature with ``__has_attribute(enable_if)``.
|
|
|
|
Initializer lists for complex numbers in C
|
|
==========================================
|
|
|
|
clang supports an extension which allows the following in C:
|
|
|
|
.. code-block:: c++
|
|
|
|
#include <math.h>
|
|
#include <complex.h>
|
|
complex float x = { 1.0f, INFINITY }; // Init to (1, Inf)
|
|
|
|
This construct is useful because there is no way to separately initialize the
|
|
real and imaginary parts of a complex variable in standard C, given that clang
|
|
does not support ``_Imaginary``. (Clang also supports the ``__real__`` and
|
|
``__imag__`` extensions from gcc, which help in some cases, but are not usable
|
|
in static initializers.)
|
|
|
|
Note that this extension does not allow eliding the braces; the meaning of the
|
|
following two lines is different:
|
|
|
|
.. code-block:: c++
|
|
|
|
complex float x[] = { { 1.0f, 1.0f } }; // [0] = (1, 1)
|
|
complex float x[] = { 1.0f, 1.0f }; // [0] = (1, 0), [1] = (1, 0)
|
|
|
|
This extension also works in C++ mode, as far as that goes, but does not apply
|
|
to the C++ ``std::complex``. (In C++11, list initialization allows the same
|
|
syntax to be used with ``std::complex`` with the same meaning.)
|
|
|
|
Builtin Functions
|
|
=================
|
|
|
|
Clang supports a number of builtin library functions with the same syntax as
|
|
GCC, including things like ``__builtin_nan``, ``__builtin_constant_p``,
|
|
``__builtin_choose_expr``, ``__builtin_types_compatible_p``,
|
|
``__sync_fetch_and_add``, etc. In addition to the GCC builtins, Clang supports
|
|
a number of builtins that GCC does not, which are listed here.
|
|
|
|
Please note that Clang does not and will not support all of the GCC builtins
|
|
for vector operations. Instead of using builtins, you should use the functions
|
|
defined in target-specific header files like ``<xmmintrin.h>``, which define
|
|
portable wrappers for these. Many of the Clang versions of these functions are
|
|
implemented directly in terms of :ref:`extended vector support
|
|
<langext-vectors>` instead of builtins, in order to reduce the number of
|
|
builtins that we need to implement.
|
|
|
|
``__builtin_readcyclecounter``
|
|
------------------------------
|
|
|
|
``__builtin_readcyclecounter`` is used to access the cycle counter register (or
|
|
a similar low-latency, high-accuracy clock) on those targets that support it.
|
|
|
|
**Syntax**:
|
|
|
|
.. code-block:: c++
|
|
|
|
__builtin_readcyclecounter()
|
|
|
|
**Example of Use**:
|
|
|
|
.. code-block:: c++
|
|
|
|
unsigned long long t0 = __builtin_readcyclecounter();
|
|
do_something();
|
|
unsigned long long t1 = __builtin_readcyclecounter();
|
|
unsigned long long cycles_to_do_something = t1 - t0; // assuming no overflow
|
|
|
|
**Description**:
|
|
|
|
The ``__builtin_readcyclecounter()`` builtin returns the cycle counter value,
|
|
which may be either global or process/thread-specific depending on the target.
|
|
As the backing counters often overflow quickly (on the order of seconds) this
|
|
should only be used for timing small intervals. When not supported by the
|
|
target, the return value is always zero. This builtin takes no arguments and
|
|
produces an unsigned long long result.
|
|
|
|
Query for this feature with ``__has_builtin(__builtin_readcyclecounter)``. Note
|
|
that even if present, its use may depend on run-time privilege or other OS
|
|
controlled state.
|
|
|
|
.. _langext-__builtin_shufflevector:
|
|
|
|
``__builtin_shufflevector``
|
|
---------------------------
|
|
|
|
``__builtin_shufflevector`` is used to express generic vector
|
|
permutation/shuffle/swizzle operations. This builtin is also very important
|
|
for the implementation of various target-specific header files like
|
|
``<xmmintrin.h>``.
|
|
|
|
**Syntax**:
|
|
|
|
.. code-block:: c++
|
|
|
|
__builtin_shufflevector(vec1, vec2, index1, index2, ...)
|
|
|
|
**Examples**:
|
|
|
|
.. code-block:: c++
|
|
|
|
// identity operation - return 4-element vector v1.
|
|
__builtin_shufflevector(v1, v1, 0, 1, 2, 3)
|
|
|
|
// "Splat" element 0 of V1 into a 4-element result.
|
|
__builtin_shufflevector(V1, V1, 0, 0, 0, 0)
|
|
|
|
// Reverse 4-element vector V1.
|
|
__builtin_shufflevector(V1, V1, 3, 2, 1, 0)
|
|
|
|
// Concatenate every other element of 4-element vectors V1 and V2.
|
|
__builtin_shufflevector(V1, V2, 0, 2, 4, 6)
|
|
|
|
// Concatenate every other element of 8-element vectors V1 and V2.
|
|
__builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)
|
|
|
|
// Shuffle v1 with some elements being undefined
|
|
__builtin_shufflevector(v1, v1, 3, -1, 1, -1)
|
|
|
|
**Description**:
|
|
|
|
The first two arguments to ``__builtin_shufflevector`` are vectors that have
|
|
the same element type. The remaining arguments are a list of integers that
|
|
specify the elements indices of the first two vectors that should be extracted
|
|
and returned in a new vector. These element indices are numbered sequentially
|
|
starting with the first vector, continuing into the second vector. Thus, if
|
|
``vec1`` is a 4-element vector, index 5 would refer to the second element of
|
|
``vec2``. An index of -1 can be used to indicate that the corresponding element
|
|
in the returned vector is a don't care and can be optimized by the backend.
|
|
|
|
The result of ``__builtin_shufflevector`` is a vector with the same element
|
|
type as ``vec1``/``vec2`` but that has an element count equal to the number of
|
|
indices specified.
|
|
|
|
Query for this feature with ``__has_builtin(__builtin_shufflevector)``.
|
|
|
|
``__builtin_convertvector``
|
|
---------------------------
|
|
|
|
``__builtin_convertvector`` is used to express generic vector
|
|
type-conversion operations. The input vector and the output vector
|
|
type must have the same number of elements.
|
|
|
|
**Syntax**:
|
|
|
|
.. code-block:: c++
|
|
|
|
__builtin_convertvector(src_vec, dst_vec_type)
|
|
|
|
**Examples**:
|
|
|
|
.. code-block:: c++
|
|
|
|
typedef double vector4double __attribute__((__vector_size__(32)));
|
|
typedef float vector4float __attribute__((__vector_size__(16)));
|
|
typedef short vector4short __attribute__((__vector_size__(8)));
|
|
vector4float vf; vector4short vs;
|
|
|
|
// convert from a vector of 4 floats to a vector of 4 doubles.
|
|
__builtin_convertvector(vf, vector4double)
|
|
// equivalent to:
|
|
(vector4double) { (double) vf[0], (double) vf[1], (double) vf[2], (double) vf[3] }
|
|
|
|
// convert from a vector of 4 shorts to a vector of 4 floats.
|
|
__builtin_convertvector(vs, vector4float)
|
|
// equivalent to:
|
|
(vector4float) { (float) vf[0], (float) vf[1], (float) vf[2], (float) vf[3] }
|
|
|
|
**Description**:
|
|
|
|
The first argument to ``__builtin_convertvector`` is a vector, and the second
|
|
argument is a vector type with the same number of elements as the first
|
|
argument.
|
|
|
|
The result of ``__builtin_convertvector`` is a vector with the same element
|
|
type as the second argument, with a value defined in terms of the action of a
|
|
C-style cast applied to each element of the first argument.
|
|
|
|
Query for this feature with ``__has_builtin(__builtin_convertvector)``.
|
|
|
|
``__builtin_unreachable``
|
|
-------------------------
|
|
|
|
``__builtin_unreachable`` is used to indicate that a specific point in the
|
|
program cannot be reached, even if the compiler might otherwise think it can.
|
|
This is useful to improve optimization and eliminates certain warnings. For
|
|
example, without the ``__builtin_unreachable`` in the example below, the
|
|
compiler assumes that the inline asm can fall through and prints a "function
|
|
declared '``noreturn``' should not return" warning.
|
|
|
|
**Syntax**:
|
|
|
|
.. code-block:: c++
|
|
|
|
__builtin_unreachable()
|
|
|
|
**Example of use**:
|
|
|
|
.. code-block:: c++
|
|
|
|
void myabort(void) __attribute__((noreturn));
|
|
void myabort(void) {
|
|
asm("int3");
|
|
__builtin_unreachable();
|
|
}
|
|
|
|
**Description**:
|
|
|
|
The ``__builtin_unreachable()`` builtin has completely undefined behavior.
|
|
Since it has undefined behavior, it is a statement that it is never reached and
|
|
the optimizer can take advantage of this to produce better code. This builtin
|
|
takes no arguments and produces a void result.
|
|
|
|
Query for this feature with ``__has_builtin(__builtin_unreachable)``.
|
|
|
|
``__sync_swap``
|
|
---------------
|
|
|
|
``__sync_swap`` is used to atomically swap integers or pointers in memory.
|
|
|
|
**Syntax**:
|
|
|
|
.. code-block:: c++
|
|
|
|
type __sync_swap(type *ptr, type value, ...)
|
|
|
|
**Example of Use**:
|
|
|
|
.. code-block:: c++
|
|
|
|
int old_value = __sync_swap(&value, new_value);
|
|
|
|
**Description**:
|
|
|
|
The ``__sync_swap()`` builtin extends the existing ``__sync_*()`` family of
|
|
atomic intrinsics to allow code to atomically swap the current value with the
|
|
new value. More importantly, it helps developers write more efficient and
|
|
correct code by avoiding expensive loops around
|
|
``__sync_bool_compare_and_swap()`` or relying on the platform specific
|
|
implementation details of ``__sync_lock_test_and_set()``. The
|
|
``__sync_swap()`` builtin is a full barrier.
|
|
|
|
``__builtin_addressof``
|
|
-----------------------
|
|
|
|
``__builtin_addressof`` performs the functionality of the built-in ``&``
|
|
operator, ignoring any ``operator&`` overload. This is useful in constant
|
|
expressions in C++11, where there is no other way to take the address of an
|
|
object that overloads ``operator&``.
|
|
|
|
**Example of use**:
|
|
|
|
.. code-block:: c++
|
|
|
|
template<typename T> constexpr T *addressof(T &value) {
|
|
return __builtin_addressof(value);
|
|
}
|
|
|
|
Multiprecision Arithmetic Builtins
|
|
----------------------------------
|
|
|
|
Clang provides a set of builtins which expose multiprecision arithmetic in a
|
|
manner amenable to C. They all have the following form:
|
|
|
|
.. code-block:: c
|
|
|
|
unsigned x = ..., y = ..., carryin = ..., carryout;
|
|
unsigned sum = __builtin_addc(x, y, carryin, &carryout);
|
|
|
|
Thus one can form a multiprecision addition chain in the following manner:
|
|
|
|
.. code-block:: c
|
|
|
|
unsigned *x, *y, *z, carryin=0, carryout;
|
|
z[0] = __builtin_addc(x[0], y[0], carryin, &carryout);
|
|
carryin = carryout;
|
|
z[1] = __builtin_addc(x[1], y[1], carryin, &carryout);
|
|
carryin = carryout;
|
|
z[2] = __builtin_addc(x[2], y[2], carryin, &carryout);
|
|
carryin = carryout;
|
|
z[3] = __builtin_addc(x[3], y[3], carryin, &carryout);
|
|
|
|
The complete list of builtins are:
|
|
|
|
.. code-block:: c
|
|
|
|
unsigned char __builtin_addcb (unsigned char x, unsigned char y, unsigned char carryin, unsigned char *carryout);
|
|
unsigned short __builtin_addcs (unsigned short x, unsigned short y, unsigned short carryin, unsigned short *carryout);
|
|
unsigned __builtin_addc (unsigned x, unsigned y, unsigned carryin, unsigned *carryout);
|
|
unsigned long __builtin_addcl (unsigned long x, unsigned long y, unsigned long carryin, unsigned long *carryout);
|
|
unsigned long long __builtin_addcll(unsigned long long x, unsigned long long y, unsigned long long carryin, unsigned long long *carryout);
|
|
unsigned char __builtin_subcb (unsigned char x, unsigned char y, unsigned char carryin, unsigned char *carryout);
|
|
unsigned short __builtin_subcs (unsigned short x, unsigned short y, unsigned short carryin, unsigned short *carryout);
|
|
unsigned __builtin_subc (unsigned x, unsigned y, unsigned carryin, unsigned *carryout);
|
|
unsigned long __builtin_subcl (unsigned long x, unsigned long y, unsigned long carryin, unsigned long *carryout);
|
|
unsigned long long __builtin_subcll(unsigned long long x, unsigned long long y, unsigned long long carryin, unsigned long long *carryout);
|
|
|
|
Checked Arithmetic Builtins
|
|
---------------------------
|
|
|
|
Clang provides a set of builtins that implement checked arithmetic for security
|
|
critical applications in a manner that is fast and easily expressable in C. As
|
|
an example of their usage:
|
|
|
|
.. code-block:: c
|
|
|
|
errorcode_t security_critical_application(...) {
|
|
unsigned x, y, result;
|
|
...
|
|
if (__builtin_umul_overflow(x, y, &result))
|
|
return kErrorCodeHackers;
|
|
...
|
|
use_multiply(result);
|
|
...
|
|
}
|
|
|
|
A complete enumeration of the builtins are:
|
|
|
|
.. code-block:: c
|
|
|
|
bool __builtin_uadd_overflow (unsigned x, unsigned y, unsigned *sum);
|
|
bool __builtin_uaddl_overflow (unsigned long x, unsigned long y, unsigned long *sum);
|
|
bool __builtin_uaddll_overflow(unsigned long long x, unsigned long long y, unsigned long long *sum);
|
|
bool __builtin_usub_overflow (unsigned x, unsigned y, unsigned *diff);
|
|
bool __builtin_usubl_overflow (unsigned long x, unsigned long y, unsigned long *diff);
|
|
bool __builtin_usubll_overflow(unsigned long long x, unsigned long long y, unsigned long long *diff);
|
|
bool __builtin_umul_overflow (unsigned x, unsigned y, unsigned *prod);
|
|
bool __builtin_umull_overflow (unsigned long x, unsigned long y, unsigned long *prod);
|
|
bool __builtin_umulll_overflow(unsigned long long x, unsigned long long y, unsigned long long *prod);
|
|
bool __builtin_sadd_overflow (int x, int y, int *sum);
|
|
bool __builtin_saddl_overflow (long x, long y, long *sum);
|
|
bool __builtin_saddll_overflow(long long x, long long y, long long *sum);
|
|
bool __builtin_ssub_overflow (int x, int y, int *diff);
|
|
bool __builtin_ssubl_overflow (long x, long y, long *diff);
|
|
bool __builtin_ssubll_overflow(long long x, long long y, long long *diff);
|
|
bool __builtin_smul_overflow (int x, int y, int *prod);
|
|
bool __builtin_smull_overflow (long x, long y, long *prod);
|
|
bool __builtin_smulll_overflow(long long x, long long y, long long *prod);
|
|
|
|
|
|
.. _langext-__c11_atomic:
|
|
|
|
__c11_atomic builtins
|
|
---------------------
|
|
|
|
Clang provides a set of builtins which are intended to be used to implement
|
|
C11's ``<stdatomic.h>`` header. These builtins provide the semantics of the
|
|
``_explicit`` form of the corresponding C11 operation, and are named with a
|
|
``__c11_`` prefix. The supported operations are:
|
|
|
|
* ``__c11_atomic_init``
|
|
* ``__c11_atomic_thread_fence``
|
|
* ``__c11_atomic_signal_fence``
|
|
* ``__c11_atomic_is_lock_free``
|
|
* ``__c11_atomic_store``
|
|
* ``__c11_atomic_load``
|
|
* ``__c11_atomic_exchange``
|
|
* ``__c11_atomic_compare_exchange_strong``
|
|
* ``__c11_atomic_compare_exchange_weak``
|
|
* ``__c11_atomic_fetch_add``
|
|
* ``__c11_atomic_fetch_sub``
|
|
* ``__c11_atomic_fetch_and``
|
|
* ``__c11_atomic_fetch_or``
|
|
* ``__c11_atomic_fetch_xor``
|
|
|
|
Low-level ARM exclusive memory builtins
|
|
---------------------------------------
|
|
|
|
Clang provides overloaded builtins giving direct access to the three key ARM
|
|
instructions for implementing atomic operations.
|
|
|
|
.. code-block:: c
|
|
|
|
T __builtin_arm_ldrex(const volatile T *addr);
|
|
int __builtin_arm_strex(T val, volatile T *addr);
|
|
void __builtin_arm_clrex(void);
|
|
|
|
The types ``T`` currently supported are:
|
|
* Integer types with width at most 64 bits.
|
|
* Floating-point types
|
|
* Pointer types.
|
|
|
|
Note that the compiler does not guarantee it will not insert stores which clear
|
|
the exclusive monitor in between an ``ldrex`` and its paired ``strex``. In
|
|
practice this is only usually a risk when the extra store is on the same cache
|
|
line as the variable being modified and Clang will only insert stack stores on
|
|
its own, so it is best not to use these operations on variables with automatic
|
|
storage duration.
|
|
|
|
Also, loads and stores may be implicit in code written between the ``ldrex`` and
|
|
``strex``. Clang will not necessarily mitigate the effects of these either, so
|
|
care should be exercised.
|
|
|
|
For these reasons the higher level atomic primitives should be preferred where
|
|
possible.
|
|
|
|
Non-standard C++11 Attributes
|
|
=============================
|
|
|
|
Clang's non-standard C++11 attributes live in the ``clang`` attribute
|
|
namespace.
|
|
|
|
The ``clang::fallthrough`` attribute
|
|
------------------------------------
|
|
|
|
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();
|
|
}
|
|
|
|
``gnu::`` attributes
|
|
--------------------
|
|
|
|
Clang also supports GCC's ``gnu`` attribute namespace. All GCC attributes which
|
|
are accepted with the ``__attribute__((foo))`` syntax are also accepted as
|
|
``[[gnu::foo]]``. This only extends to attributes which are specified by GCC
|
|
(see the list of `GCC function attributes
|
|
<http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_, `GCC variable
|
|
attributes <http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html>`_, and
|
|
`GCC type attributes
|
|
<http://gcc.gnu.org/onlinedocs/gcc/Type-Attributes.html>`_). As with the GCC
|
|
implementation, these attributes must appertain to the *declarator-id* in a
|
|
declaration, which means they must go either at the start of the declaration or
|
|
immediately after the name being declared.
|
|
|
|
For example, this applies the GNU ``unused`` attribute to ``a`` and ``f``, and
|
|
also applies the GNU ``noreturn`` attribute to ``f``.
|
|
|
|
.. code-block:: c++
|
|
|
|
[[gnu::unused]] int a, f [[gnu::noreturn]] ();
|
|
|
|
Target-Specific Extensions
|
|
==========================
|
|
|
|
Clang supports some language features conditionally on some targets.
|
|
|
|
X86/X86-64 Language Extensions
|
|
------------------------------
|
|
|
|
The X86 backend has these language extensions:
|
|
|
|
Memory references off the GS segment
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Annotating a pointer with address space #256 causes it to be code generated
|
|
relative to the X86 GS segment register, and address space #257 causes it to be
|
|
relative to the X86 FS segment. Note that this is a very very low-level
|
|
feature that should only be used if you know what you're doing (for example in
|
|
an OS kernel).
|
|
|
|
Here is an example:
|
|
|
|
.. code-block:: c++
|
|
|
|
#define GS_RELATIVE __attribute__((address_space(256)))
|
|
int foo(int GS_RELATIVE *P) {
|
|
return *P;
|
|
}
|
|
|
|
Which compiles to (on X86-32):
|
|
|
|
.. code-block:: gas
|
|
|
|
_foo:
|
|
movl 4(%esp), %eax
|
|
movl %gs:(%eax), %eax
|
|
ret
|
|
|
|
ARM Language Extensions
|
|
-----------------------
|
|
|
|
Interrupt attribute
|
|
^^^^^^^^^^^^^^^^^^^
|
|
|
|
Clang supports the GNU style ``__attribute__((interrupt("TYPE")))`` attribute on
|
|
ARM targets. This attribute may be attached to a function definition and
|
|
instructs the backend to generate appropriate function entry/exit code so that
|
|
it can be used directly as an interrupt service routine.
|
|
|
|
The parameter passed to the interrupt attribute is optional, but if
|
|
provided it must be a string literal with one of the following values: "IRQ",
|
|
"FIQ", "SWI", "ABORT", "UNDEF".
|
|
|
|
The semantics are as follows:
|
|
|
|
- If the function is AAPCS, Clang instructs the backend to realign the stack to
|
|
8 bytes on entry. This is a general requirement of the AAPCS at public
|
|
interfaces, but may not hold when an exception is taken. Doing this allows
|
|
other AAPCS functions to be called.
|
|
- If the CPU is M-class this is all that needs to be done since the architecture
|
|
itself is designed in such a way that functions obeying the normal AAPCS ABI
|
|
constraints are valid exception handlers.
|
|
- If the CPU is not M-class, the prologue and epilogue are modified to save all
|
|
non-banked registers that are used, so that upon return the user-mode state
|
|
will not be corrupted. Note that to avoid unnecessary overhead, only
|
|
general-purpose (integer) registers are saved in this way. If VFP operations
|
|
are needed, that state must be saved manually.
|
|
|
|
Specifically, interrupt kinds other than "FIQ" will save all core registers
|
|
except "lr" and "sp". "FIQ" interrupts will save r0-r7.
|
|
- If the CPU is not M-class, the return instruction is changed to one of the
|
|
canonical sequences permitted by the architecture for exception return. Where
|
|
possible the function itself will make the necessary "lr" adjustments so that
|
|
the "preferred return address" is selected.
|
|
|
|
Unfortunately the compiler is unable to make this guarantee for an "UNDEF"
|
|
handler, where the offset from "lr" to the preferred return address depends on
|
|
the execution state of the code which generated the exception. In this case
|
|
a sequence equivalent to "movs pc, lr" will be used.
|
|
|
|
Extensions for Static Analysis
|
|
==============================
|
|
|
|
Clang supports additional attributes that are useful for documenting program
|
|
invariants and rules for static analysis tools, such as the `Clang Static
|
|
Analyzer <http://clang-analyzer.llvm.org/>`_. These attributes are documented
|
|
in the analyzer's `list of source-level annotations
|
|
<http://clang-analyzer.llvm.org/annotations.html>`_.
|
|
|
|
|
|
Extensions for Dynamic Analysis
|
|
===============================
|
|
|
|
.. _langext-address_sanitizer:
|
|
|
|
AddressSanitizer
|
|
----------------
|
|
|
|
Use ``__has_feature(address_sanitizer)`` to check if the code is being built
|
|
with :doc:`AddressSanitizer`.
|
|
|
|
Use ``__attribute__((no_sanitize_address))``
|
|
on a function declaration
|
|
to specify that address safety instrumentation (e.g. AddressSanitizer) should
|
|
not be applied to that function.
|
|
|
|
.. _langext-thread_sanitizer:
|
|
|
|
ThreadSanitizer
|
|
----------------
|
|
|
|
Use ``__has_feature(thread_sanitizer)`` to check if the code is being built
|
|
with :doc:`ThreadSanitizer`.
|
|
|
|
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.
|
|
|
|
.. _langext-memory_sanitizer:
|
|
|
|
MemorySanitizer
|
|
----------------
|
|
Use ``__has_feature(memory_sanitizer)`` to check if the code is being built
|
|
with :doc:`MemorySanitizer`.
|
|
|
|
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.
|
|
|
|
|
|
Thread-Safety Annotation Checking
|
|
=================================
|
|
|
|
Clang supports additional attributes for checking basic locking policies in
|
|
multithreaded programs. Clang currently parses the following list of
|
|
attributes, although **the implementation for these annotations is currently in
|
|
development.** For more details, see the `GCC implementation
|
|
<http://gcc.gnu.org/wiki/ThreadSafetyAnnotation>`_.
|
|
|
|
``no_thread_safety_analysis``
|
|
-----------------------------
|
|
|
|
Use ``__attribute__((no_thread_safety_analysis))`` on a function declaration to
|
|
specify that the thread safety analysis should not be run on that function.
|
|
This attribute provides an escape hatch (e.g. for situations when it is
|
|
difficult to annotate the locking policy).
|
|
|
|
``lockable``
|
|
------------
|
|
|
|
Use ``__attribute__((lockable))`` on a class definition to specify that it has
|
|
a lockable type (e.g. a Mutex class). This annotation is primarily used to
|
|
check consistency.
|
|
|
|
``scoped_lockable``
|
|
-------------------
|
|
|
|
Use ``__attribute__((scoped_lockable))`` on a class definition to specify that
|
|
it has a "scoped" lockable type. Objects of this type will acquire the lock
|
|
upon construction and release it upon going out of scope. This annotation is
|
|
primarily used to check consistency.
|
|
|
|
``guarded_var``
|
|
---------------
|
|
|
|
Use ``__attribute__((guarded_var))`` on a variable declaration to specify that
|
|
the variable must be accessed while holding some lock.
|
|
|
|
``pt_guarded_var``
|
|
------------------
|
|
|
|
Use ``__attribute__((pt_guarded_var))`` on a pointer declaration to specify
|
|
that the pointer must be dereferenced while holding some lock.
|
|
|
|
``guarded_by(l)``
|
|
-----------------
|
|
|
|
Use ``__attribute__((guarded_by(l)))`` on a variable declaration to specify
|
|
that the variable must be accessed while holding lock ``l``.
|
|
|
|
``pt_guarded_by(l)``
|
|
--------------------
|
|
|
|
Use ``__attribute__((pt_guarded_by(l)))`` on a pointer declaration to specify
|
|
that the pointer must be dereferenced while holding lock ``l``.
|
|
|
|
``acquired_before(...)``
|
|
------------------------
|
|
|
|
Use ``__attribute__((acquired_before(...)))`` on a declaration of a lockable
|
|
variable to specify that the lock must be acquired before all attribute
|
|
arguments. Arguments must be lockable type, and there must be at least one
|
|
argument.
|
|
|
|
``acquired_after(...)``
|
|
-----------------------
|
|
|
|
Use ``__attribute__((acquired_after(...)))`` on a declaration of a lockable
|
|
variable to specify that the lock must be acquired after all attribute
|
|
arguments. Arguments must be lockable type, and there must be at least one
|
|
argument.
|
|
|
|
``exclusive_lock_function(...)``
|
|
--------------------------------
|
|
|
|
Use ``__attribute__((exclusive_lock_function(...)))`` on a function declaration
|
|
to specify that the function acquires all listed locks exclusively. This
|
|
attribute takes zero or more arguments: either of lockable type or integers
|
|
indexing into function parameters of lockable type. If no arguments are given,
|
|
the acquired lock is implicitly ``this`` of the enclosing object.
|
|
|
|
``shared_lock_function(...)``
|
|
-----------------------------
|
|
|
|
Use ``__attribute__((shared_lock_function(...)))`` on a function declaration to
|
|
specify that the function acquires all listed locks, although the locks may be
|
|
shared (e.g. read locks). This attribute takes zero or more arguments: either
|
|
of lockable type or integers indexing into function parameters of lockable
|
|
type. If no arguments are given, the acquired lock is implicitly ``this`` of
|
|
the enclosing object.
|
|
|
|
``exclusive_trylock_function(...)``
|
|
-----------------------------------
|
|
|
|
Use ``__attribute__((exclusive_lock_function(...)))`` on a function declaration
|
|
to specify that the function will try (without blocking) to acquire all listed
|
|
locks exclusively. This attribute takes one or more arguments. The first
|
|
argument is an integer or boolean value specifying the return value of a
|
|
successful lock acquisition. The remaining arugments are either of lockable
|
|
type or integers indexing into function parameters of lockable type. If only
|
|
one argument is given, the acquired lock is implicitly ``this`` of the
|
|
enclosing object.
|
|
|
|
``shared_trylock_function(...)``
|
|
--------------------------------
|
|
|
|
Use ``__attribute__((shared_lock_function(...)))`` on a function declaration to
|
|
specify that the function will try (without blocking) to acquire all listed
|
|
locks, although the locks may be shared (e.g. read locks). This attribute
|
|
takes one or more arguments. The first argument is an integer or boolean value
|
|
specifying the return value of a successful lock acquisition. The remaining
|
|
arugments are either of lockable type or integers indexing into function
|
|
parameters of lockable type. If only one argument is given, the acquired lock
|
|
is implicitly ``this`` of the enclosing object.
|
|
|
|
``unlock_function(...)``
|
|
------------------------
|
|
|
|
Use ``__attribute__((unlock_function(...)))`` on a function declaration to
|
|
specify that the function release all listed locks. This attribute takes zero
|
|
or more arguments: either of lockable type or integers indexing into function
|
|
parameters of lockable type. If no arguments are given, the acquired lock is
|
|
implicitly ``this`` of the enclosing object.
|
|
|
|
``lock_returned(l)``
|
|
--------------------
|
|
|
|
Use ``__attribute__((lock_returned(l)))`` on a function declaration to specify
|
|
that the function returns lock ``l`` (``l`` must be of lockable type). This
|
|
annotation is used to aid in resolving lock expressions.
|
|
|
|
``locks_excluded(...)``
|
|
-----------------------
|
|
|
|
Use ``__attribute__((locks_excluded(...)))`` on a function declaration to
|
|
specify that the function must not be called with the listed locks. Arguments
|
|
must be lockable type, and there must be at least one argument.
|
|
|
|
``exclusive_locks_required(...)``
|
|
---------------------------------
|
|
|
|
Use ``__attribute__((exclusive_locks_required(...)))`` on a function
|
|
declaration to specify that the function must be called while holding the
|
|
listed exclusive locks. Arguments must be lockable type, and there must be at
|
|
least one argument.
|
|
|
|
``shared_locks_required(...)``
|
|
------------------------------
|
|
|
|
Use ``__attribute__((shared_locks_required(...)))`` on a function declaration
|
|
to specify that the function must be called while holding the listed shared
|
|
locks. Arguments must be lockable type, and there must be at least one
|
|
argument.
|
|
|
|
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.**
|
|
|
|
``consumable``
|
|
--------------
|
|
|
|
Each class that uses any of the following annotations must first be marked
|
|
using the consumable attribute. Failure to do so will result in a warning.
|
|
|
|
``set_typestate(new_state)``
|
|
----------------------------
|
|
|
|
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.
|
|
|
|
``callable_when(...)``
|
|
----------------------
|
|
|
|
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")))``
|
|
|
|
``tests_typestate(tested_state)``
|
|
---------------------------------
|
|
|
|
Use ``__attribute__((tests_typestate(tested_state)))`` to indicate that a method
|
|
returns true if the object is in the specified state..
|
|
|
|
``param_typestate(expected_state)``
|
|
-----------------------------------
|
|
|
|
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(ret_state)``
|
|
-------------------------------
|
|
|
|
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.
|
|
|
|
If the attribute is 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.
|
|
|
|
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(...)``
|
|
-------------------------------
|
|
|
|
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(...)``
|
|
------------------------------
|
|
|
|
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(...)``
|
|
------------------------------
|
|
|
|
Clang supports annotating type tags of two forms.
|
|
|
|
* **Type tag that is an expression containing a reference to some declared
|
|
identifier.** Use ``__attribute__((type_tag_for_datatype(kind, type)))`` on a
|
|
declaration with that identifier:
|
|
|
|
.. code-block:: c++
|
|
|
|
extern struct mpi_datatype mpi_datatype_int
|
|
__attribute__(( type_tag_for_datatype(mpi,int) ));
|
|
#define MPI_INT ((MPI_Datatype) &mpi_datatype_int)
|
|
|
|
* **Type tag that is an integral literal.** Introduce a ``static const``
|
|
variable with a corresponding initializer value and attach
|
|
``__attribute__((type_tag_for_datatype(kind, type)))`` on that declaration,
|
|
for example:
|
|
|
|
.. code-block:: c++
|
|
|
|
#define MPI_INT ((MPI_Datatype) 42)
|
|
static const MPI_Datatype mpi_datatype_int
|
|
__attribute__(( type_tag_for_datatype(mpi,int) )) = 42
|
|
|
|
The attribute also accepts an optional third argument that determines how the
|
|
expression is compared to the type tag. There are two supported flags:
|
|
|
|
* ``layout_compatible`` will cause types to be compared according to
|
|
layout-compatibility rules (C++11 [class.mem] p 17, 18). This is
|
|
implemented to support annotating types like ``MPI_DOUBLE_INT``.
|
|
|
|
For example:
|
|
|
|
.. code-block:: c++
|
|
|
|
/* In mpi.h */
|
|
struct internal_mpi_double_int { double d; int i; };
|
|
extern struct mpi_datatype mpi_datatype_double_int
|
|
__attribute__(( type_tag_for_datatype(mpi, struct internal_mpi_double_int, layout_compatible) ));
|
|
|
|
#define MPI_DOUBLE_INT ((MPI_Datatype) &mpi_datatype_double_int)
|
|
|
|
/* In user code */
|
|
struct my_pair { double a; int b; };
|
|
struct my_pair *buffer;
|
|
MPI_Send(buffer, 1, MPI_DOUBLE_INT /*, ... */); // no warning
|
|
|
|
struct my_int_pair { int a; int b; }
|
|
struct my_int_pair *buffer2;
|
|
MPI_Send(buffer2, 1, MPI_DOUBLE_INT /*, ... */); // warning: actual buffer element
|
|
// type 'struct my_int_pair'
|
|
// doesn't match specified MPI_Datatype
|
|
|
|
* ``must_be_null`` specifies that the expression should be a null pointer
|
|
constant, for example:
|
|
|
|
.. code-block:: c++
|
|
|
|
/* In mpi.h */
|
|
extern struct mpi_datatype mpi_datatype_null
|
|
__attribute__(( type_tag_for_datatype(mpi, void, must_be_null) ));
|
|
|
|
#define MPI_DATATYPE_NULL ((MPI_Datatype) &mpi_datatype_null)
|
|
|
|
/* In user code */
|
|
MPI_Send(buffer, 1, MPI_DATATYPE_NULL /*, ... */); // warning: MPI_DATATYPE_NULL
|
|
// was specified but buffer
|
|
// is not a null pointer
|
|
|
|
Format String Checking
|
|
======================
|
|
|
|
Clang supports the ``format`` attribute, which indicates that the function
|
|
accepts a ``printf`` or ``scanf``-like format string and corresponding
|
|
arguments or a ``va_list`` that contains these arguments.
|
|
|
|
Please see `GCC documentation about format attribute
|
|
<http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_ to find details
|
|
about attribute syntax.
|
|
|
|
Clang implements two kinds of checks with this attribute.
|
|
|
|
#. Clang checks that the function with the ``format`` attribute is called with
|
|
a format string that uses format specifiers that are allowed, and that
|
|
arguments match the format string. This is the ``-Wformat`` warning, it is
|
|
on by default.
|
|
|
|
#. Clang checks that the format string argument is a literal string. This is
|
|
the ``-Wformat-nonliteral`` warning, it is off by default.
|
|
|
|
Clang implements this mostly the same way as GCC, but there is a difference
|
|
for functions that accept a ``va_list`` argument (for example, ``vprintf``).
|
|
GCC does not emit ``-Wformat-nonliteral`` warning for calls to such
|
|
fuctions. Clang does not warn if the format string comes from a function
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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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|
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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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