docs/LanguageExtensions.rst

=========================
Clang Language Extensions
=========================

.. contents::
   :local:
   :depth: 1

.. toctree::
   :hidden:

   ObjectiveCLiterals
   BlockLanguageSpec
   Block-ABI-Apple
   AutomaticReferenceCounting   

Introduction
============

This document describes the language extensions provided by Clang.  In addition
to the language extensions listed here, Clang aims to support a broad range of
GCC extensions.  Please see the `GCC manual
<http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html>`_ for more information on
these extensions.

.. _langext-feature_check:

Feature Checking Macros
=======================

Language extensions can be very useful, but only if you know you can depend on
them.  In order to allow fine-grain features checks, we support three builtin
function-like macros.  This allows you to directly test for a feature in your
code without having to resort to something like autoconf or fragile "compiler
version checks".

``__has_builtin``
-----------------

This function-like macro takes a single identifier argument that is the name of
a builtin function.  It evaluates to 1 if the builtin is supported or 0 if not.
It can be used like this:

.. code-block:: c++

  #ifndef __has_builtin         // Optional of course.
    #define __has_builtin(x) 0  // Compatibility with non-clang compilers.
  #endif

  ...
  #if __has_builtin(__builtin_trap)
    __builtin_trap();
  #else
    abort();
  #endif
  ...

.. _langext-__has_feature-__has_extension:

``__has_feature`` and ``__has_extension``
-----------------------------------------

These function-like macros take a single identifier argument that is the name
of a feature.  ``__has_feature`` evaluates to 1 if the feature is both
supported by Clang and standardized in the current language standard or 0 if
not (but see :ref:`below <langext-has-feature-back-compat>`), while
``__has_extension`` evaluates to 1 if the feature is supported by Clang in the
current language (either as a language extension or a standard language
feature) or 0 if not.  They can be used like this:

.. code-block:: c++

  #ifndef __has_feature         // Optional of course.
    #define __has_feature(x) 0  // Compatibility with non-clang compilers.
  #endif
  #ifndef __has_extension
    #define __has_extension __has_feature // Compatibility with pre-3.0 compilers.
  #endif

  ...
  #if __has_feature(cxx_rvalue_references)
  // This code will only be compiled with the -std=c++11 and -std=gnu++11
  // options, because rvalue references are only standardized in C++11.
  #endif

  #if __has_extension(cxx_rvalue_references)
  // This code will be compiled with the -std=c++11, -std=gnu++11, -std=c++98
  // and -std=gnu++98 options, because rvalue references are supported as a
  // language extension in C++98.
  #endif

.. _langext-has-feature-back-compat:

For backwards compatibility reasons, ``__has_feature`` can also be used to test
for support for non-standardized features, i.e. features not prefixed ``c_``,
``cxx_`` or ``objc_``.

Another use of ``__has_feature`` is to check for compiler features not related
to the language standard, such as e.g. :doc:`AddressSanitizer
<AddressSanitizer>`.

If the ``-pedantic-errors`` option is given, ``__has_extension`` is equivalent
to ``__has_feature``.

The feature tag is described along with the language feature below.

The feature name or extension name can also be specified with a preceding and
following ``__`` (double underscore) to avoid interference from a macro with
the same name.  For instance, ``__cxx_rvalue_references__`` can be used instead
of ``cxx_rvalue_references``.

``__has_attribute``
-------------------

This function-like macro takes a single identifier argument that is the name of
an attribute.  It evaluates to 1 if the attribute is supported or 0 if not.  It
can be used like this:

.. code-block:: c++

  #ifndef __has_attribute         // Optional of course.
    #define __has_attribute(x) 0  // Compatibility with non-clang compilers.
  #endif

  ...
  #if __has_attribute(always_inline)
  #define ALWAYS_INLINE __attribute__((always_inline))
  #else
  #define ALWAYS_INLINE
  #endif
  ...

The attribute name can also be specified with a preceding and following ``__``
(double underscore) to avoid interference from a macro with the same name.  For
instance, ``__always_inline__`` can be used instead of ``always_inline``.

Include File Checking Macros
============================

Not all developments systems have the same include files.  The
:ref:`langext-__has_include` and :ref:`langext-__has_include_next` macros allow
you to check for the existence of an include file before doing a possibly
failing ``#include`` directive.  Include file checking macros must be used
as expressions in ``#if`` or ``#elif`` preprocessing directives.

.. _langext-__has_include:

``__has_include``
-----------------

This function-like macro takes a single file name string argument that is the
name of an include file.  It evaluates to 1 if the file can be found using the
include paths, or 0 otherwise:

.. code-block:: c++

  // Note the two possible file name string formats.
  #if __has_include("myinclude.h") && __has_include(<stdint.h>)
  # include "myinclude.h"
  #endif

  // To avoid problem with non-clang compilers not having this macro.
  #if defined(__has_include) && __has_include("myinclude.h")
  # include "myinclude.h"
  #endif

To test for this feature, use ``#if defined(__has_include)``.

.. _langext-__has_include_next:

``__has_include_next``
----------------------

This function-like macro takes a single file name string argument that is the
name of an include file.  It is like ``__has_include`` except that it looks for
the second instance of the given file found in the include paths.  It evaluates
to 1 if the second instance of the file can be found using the include paths,
or 0 otherwise:

.. code-block:: c++

  // Note the two possible file name string formats.
  #if __has_include_next("myinclude.h") && __has_include_next(<stdint.h>)
  # include_next "myinclude.h"
  #endif

  // To avoid problem with non-clang compilers not having this macro.
  #if defined(__has_include_next) && __has_include_next("myinclude.h")
  # include_next "myinclude.h"
  #endif

Note that ``__has_include_next``, like the GNU extension ``#include_next``
directive, is intended for use in headers only, and will issue a warning if
used in the top-level compilation file.  A warning will also be issued if an
absolute path is used in the file argument.

``__has_warning``
-----------------

This function-like macro takes a string literal that represents a command line
option for a warning and returns true if that is a valid warning option.

.. code-block:: c++

  #if __has_warning("-Wformat")
  ...
  #endif

Builtin Macros
==============

``__BASE_FILE__``
  Defined to a string that contains the name of the main input file passed to
  Clang.

``__COUNTER__``
  Defined to an integer value that starts at zero and is incremented each time
  the ``__COUNTER__`` macro is expanded.

``__INCLUDE_LEVEL__``
  Defined to an integral value that is the include depth of the file currently
  being translated.  For the main file, this value is zero.

``__TIMESTAMP__``
  Defined to the date and time of the last modification of the current source
  file.

``__clang__``
  Defined when compiling with Clang

``__clang_major__``
  Defined to the major marketing version number of Clang (e.g., the 2 in
  2.0.1).  Note that marketing version numbers should not be used to check for
  language features, as different vendors use different numbering schemes.
  Instead, use the :ref:`langext-feature_check`.

``__clang_minor__``
  Defined to the minor version number of Clang (e.g., the 0 in 2.0.1).  Note
  that marketing version numbers should not be used to check for language
  features, as different vendors use different numbering schemes.  Instead, use
  the :ref:`langext-feature_check`.

``__clang_patchlevel__``
  Defined to the marketing patch level of Clang (e.g., the 1 in 2.0.1).

``__clang_version__``
  Defined to a string that captures the Clang marketing version, including the
  Subversion tag or revision number, e.g., "``1.5 (trunk 102332)``".

.. _langext-vectors:

Vectors and Extended Vectors
============================

Supports the GCC, OpenCL, AltiVec and NEON vector extensions.

OpenCL vector types are created using ``ext_vector_type`` attribute.  It
support for ``V.xyzw`` syntax and other tidbits as seen in OpenCL.  An example
is:

.. code-block:: c++

  typedef float float4 __attribute__((ext_vector_type(4)));
  typedef float float2 __attribute__((ext_vector_type(2)));

  float4 foo(float2 a, float2 b) {
    float4 c;
    c.xz = a;
    c.yw = b;
    return c;
  }

Query for this feature with ``__has_extension(attribute_ext_vector_type)``.

Giving ``-faltivec`` option to clang enables support for AltiVec vector syntax
and functions.  For example:

.. code-block:: c++

  vector float foo(vector int a) {
    vector int b;
    b = vec_add(a, a) + a;
    return (vector float)b;
  }

NEON vector types are created using ``neon_vector_type`` and
``neon_polyvector_type`` attributes.  For example:

.. code-block:: c++

  typedef __attribute__((neon_vector_type(8))) int8_t int8x8_t;
  typedef __attribute__((neon_polyvector_type(16))) poly8_t poly8x16_t;

  int8x8_t foo(int8x8_t a) {
    int8x8_t v;
    v = a;
    return v;
  }

Vector Literals
---------------

Vector literals can be used to create vectors from a set of scalars, or
vectors.  Either parentheses or braces form can be used.  In the parentheses
form the number of literal values specified must be one, i.e. referring to a
scalar value, or must match the size of the vector type being created.  If a
single scalar literal value is specified, the scalar literal value will be
replicated to all the components of the vector type.  In the brackets form any
number of literals can be specified.  For example:

.. code-block:: c++

  typedef int v4si __attribute__((__vector_size__(16)));
  typedef float float4 __attribute__((ext_vector_type(4)));
  typedef float float2 __attribute__((ext_vector_type(2)));

  v4si vsi = (v4si){1, 2, 3, 4};
  float4 vf = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
  vector int vi1 = (vector int)(1);    // vi1 will be (1, 1, 1, 1).
  vector int vi2 = (vector int){1};    // vi2 will be (1, 0, 0, 0).
  vector int vi3 = (vector int)(1, 2); // error
  vector int vi4 = (vector int){1, 2}; // vi4 will be (1, 2, 0, 0).
  vector int vi5 = (vector int)(1, 2, 3, 4);
  float4 vf = (float4)((float2)(1.0f, 2.0f), (float2)(3.0f, 4.0f));

Vector Operations
-----------------

The table below shows the support for each operation by vector extension.  A
dash indicates that an operation is not accepted according to a corresponding
specification.

============================== ====== ======= === ====
         Opeator               OpenCL AltiVec GCC NEON
============================== ====== ======= === ====
[]                              yes     yes   yes  --
unary operators +, --           yes     yes   yes  --
++, -- --                       yes     yes   yes  --
+,--,*,/,%                      yes     yes   yes  --
bitwise operators &,|,^,~       yes     yes   yes  --
>>,<<                           yes     yes   yes  --
!, &&, ||                       no      --    --   --
==, !=, >, <, >=, <=            yes     yes   --   --
=                               yes     yes   yes yes
:?                              yes     --    --   --
sizeof                          yes     yes   yes yes
============================== ====== ======= === ====

See also :ref:`langext-__builtin_shufflevector`.

Messages on ``deprecated`` and ``unavailable`` Attributes
=========================================================

An optional string message can be added to the ``deprecated`` and
``unavailable`` attributes.  For example:

.. code-block:: c++

  void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));

If the deprecated or unavailable declaration is used, the message will be
incorporated into the appropriate diagnostic:

.. code-block:: c++

  harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!!
        [-Wdeprecated-declarations]
    explode();
    ^

Query for this feature with
``__has_extension(attribute_deprecated_with_message)`` and
``__has_extension(attribute_unavailable_with_message)``.

Attributes on Enumerators
=========================

Clang allows attributes to be written on individual enumerators.  This allows
enumerators to be deprecated, made unavailable, etc.  The attribute must appear
after the enumerator name and before any initializer, like so:

.. code-block:: c++

  enum OperationMode {
    OM_Invalid,
    OM_Normal,
    OM_Terrified __attribute__((deprecated)),
    OM_AbortOnError __attribute__((deprecated)) = 4
  };

Attributes on the ``enum`` declaration do not apply to individual enumerators.

Query for this feature with ``__has_extension(enumerator_attributes)``.

'User-Specified' System Frameworks
==================================

Clang provides a mechanism by which frameworks can be built in such a way that
they will always be treated as being "system frameworks", even if they are not
present in a system framework directory.  This can be useful to system
framework developers who want to be able to test building other applications
with development builds of their framework, including the manner in which the
compiler changes warning behavior for system headers.

Framework developers can opt-in to this mechanism by creating a
"``.system_framework``" file at the top-level of their framework.  That is, the
framework should have contents like:

.. code-block:: none

  .../TestFramework.framework
  .../TestFramework.framework/.system_framework
  .../TestFramework.framework/Headers
  .../TestFramework.framework/Headers/TestFramework.h
  ...

Clang will treat the presence of this file as an indicator that the framework
should be treated as a system framework, regardless of how it was found in the
framework search path.  For consistency, we recommend that such files never be
included in installed versions of the framework.

Availability attribute
======================

Clang introduces the ``availability`` attribute, which can be placed on
declarations to describe the lifecycle of that declaration relative to
operating system versions.  Consider the function declaration for a
hypothetical function ``f``:

.. code-block:: c++

  void f(void) __attribute__((availability(macosx,introduced=10.4,deprecated=10.6,obsoleted=10.7)));

The availability attribute states that ``f`` was introduced in Mac OS X 10.4,
deprecated in Mac OS X 10.6, and obsoleted in Mac OS X 10.7.  This information
is used by Clang to determine when it is safe to use ``f``: for example, if
Clang is instructed to compile code for Mac OS X 10.5, a call to ``f()``
succeeds.  If Clang is instructed to compile code for Mac OS X 10.6, the call
succeeds but Clang emits a warning specifying that the function is deprecated.
Finally, if Clang is instructed to compile code for Mac OS X 10.7, the call
fails because ``f()`` is no longer available.

The availability attribute is a comma-separated list starting with the
platform name and then including clauses specifying important milestones in the
declaration's lifetime (in any order) along with additional information.  Those
clauses can be:

introduced=\ *version*
  The first version in which this declaration was introduced.

deprecated=\ *version*
  The first version in which this declaration was deprecated, meaning that
  users should migrate away from this API.

obsoleted=\ *version*
  The first version in which this declaration was obsoleted, meaning that it
  was removed completely and can no longer be used.

unavailable
  This declaration is never available on this platform.

message=\ *string-literal*
  Additional message text that Clang will provide when emitting a warning or
  error about use of a deprecated or obsoleted declaration.  Useful to direct
  users to replacement APIs.

Multiple availability attributes can be placed on a declaration, which may
correspond to different platforms.  Only the availability attribute with the
platform corresponding to the target platform will be used; any others will be
ignored.  If no availability attribute specifies availability for the current
target platform, the availability attributes are ignored.  Supported platforms
are:

``ios``
  Apple's iOS operating system.  The minimum deployment target is specified by
  the ``-mios-version-min=*version*`` or ``-miphoneos-version-min=*version*``
  command-line arguments.

``macosx``
  Apple's Mac OS X operating system.  The minimum deployment target is
  specified by the ``-mmacosx-version-min=*version*`` command-line argument.

A declaration can be used even when deploying back to a platform version prior
to when the declaration was introduced.  When this happens, the declaration is
`weakly linked
<https://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html>`_,
as if the ``weak_import`` attribute were added to the declaration.  A
weakly-linked declaration may or may not be present a run-time, and a program
can determine whether the declaration is present by checking whether the
address of that declaration is non-NULL.

If there are multiple declarations of the same entity, the availability
attributes must either match on a per-platform basis or later
declarations must not have availability attributes for that
platform. For example:

.. code-block:: c

  void g(void) __attribute__((availability(macosx,introduced=10.4)));
  void g(void) __attribute__((availability(macosx,introduced=10.4))); // okay, matches
  void g(void) __attribute__((availability(ios,introduced=4.0))); // okay, adds a new platform
  void g(void); // okay, inherits both macosx and ios availability from above.
  void g(void) __attribute__((availability(macosx,introduced=10.5))); // error: mismatch

When one method overrides another, the overriding method can be more widely available than the overridden method, e.g.,:

.. code-block:: objc

  @interface A
  - (id)method __attribute__((availability(macosx,introduced=10.4)));
  - (id)method2 __attribute__((availability(macosx,introduced=10.4)));
  @end

  @interface B : A
  - (id)method __attribute__((availability(macosx,introduced=10.3))); // okay: method moved into base class later
  - (id)method __attribute__((availability(macosx,introduced=10.5))); // error: this method was available via the base class in 10.4
  @end

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. Clang does not yet
support this feature.

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_generalized_capture)`` or
``__has_extension(cxx_generalized_capture`` to determine if support for
generalized lambda captures 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.
Clang's implementation of this feature is incomplete.

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)`` 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
====================

Related result types
--------------------

According to Cocoa conventions, Objective-C methods with certain names
("``init``", "``alloc``", etc.) always return objects that are an instance of
the receiving class's type.  Such methods are said to have a "related result
type", meaning that a message send to one of these methods will have the same
static type as an instance of the receiver class.  For example, given the
following classes:

.. code-block:: objc

  @interface NSObject
  + (id)alloc;
  - (id)init;
  @end

  @interface NSArray : NSObject
  @end

and this common initialization pattern

.. code-block:: objc

  NSArray *array = [[NSArray alloc] init];

the type of the expression ``[NSArray alloc]`` is ``NSArray*`` because
``alloc`` implicitly has a related result type.  Similarly, the type of the
expression ``[[NSArray alloc] init]`` is ``NSArray*``, since ``init`` has a
related result type and its receiver is known to have the type ``NSArray *``.
If neither ``alloc`` nor ``init`` had a related result type, the expressions
would have had type ``id``, as declared in the method signature.

A method with a related result type can be declared by using the type
``instancetype`` as its result type.  ``instancetype`` is a contextual keyword
that is only permitted in the result type of an Objective-C method, e.g.

.. code-block:: objc

  @interface A
  + (instancetype)constructAnA;
  @end

The related result type can also be inferred for some methods.  To determine
whether a method has an inferred related result type, the first word in the
camel-case selector (e.g., "``init``" in "``initWithObjects``") is considered,
and the method will have a related result type if its return type is compatible
with the type of its class and if:

* the first word is "``alloc``" or "``new``", and the method is a class method,
  or

* the first word is "``autorelease``", "``init``", "``retain``", or "``self``",
  and the method is an instance method.

If a method with a related result type is overridden by a subclass method, the
subclass method must also return a type that is compatible with the subclass
type.  For example:

.. code-block:: objc

  @interface NSString : NSObject
  - (NSUnrelated *)init; // incorrect usage: NSUnrelated is not NSString or a superclass of NSString
  @end

Related result types only affect the type of a message send or property access
via the given method.  In all other respects, a method with a related result
type is treated the same way as method that returns ``id``.

Use ``__has_feature(objc_instancetype)`` to determine whether the
``instancetype`` contextual keyword is available.

Automatic reference counting
----------------------------

Clang provides support for :doc:`automated reference counting
<AutomaticReferenceCounting>` in Objective-C, which eliminates the need
for manual ``retain``/``release``/``autorelease`` message sends.  There are two
feature macros associated with automatic reference counting:
``__has_feature(objc_arc)`` indicates the availability of automated reference
counting in general, while ``__has_feature(objc_arc_weak)`` indicates that
automated reference counting also includes support for ``__weak`` pointers to
Objective-C objects.

.. _objc-fixed-enum:

Enumerations with a fixed underlying type
-----------------------------------------

Clang provides support for C++11 enumerations with a fixed underlying type
within Objective-C.  For example, one can write an enumeration type as:

.. code-block:: c++

  typedef enum : unsigned char { Red, Green, Blue } Color;

This specifies that the underlying type, which is used to store the enumeration
value, is ``unsigned char``.

Use ``__has_feature(objc_fixed_enum)`` to determine whether support for fixed
underlying types is available in Objective-C.

Interoperability with C++11 lambdas
-----------------------------------

Clang provides interoperability between C++11 lambdas and blocks-based APIs, by
permitting a lambda to be implicitly converted to a block pointer with the
corresponding signature.  For example, consider an API such as ``NSArray``'s
array-sorting method:

.. code-block:: objc

  - (NSArray *)sortedArrayUsingComparator:(NSComparator)cmptr;

``NSComparator`` is simply a typedef for the block pointer ``NSComparisonResult
(^)(id, id)``, and parameters of this type are generally provided with block
literals as arguments.  However, one can also use a C++11 lambda so long as it
provides the same signature (in this case, accepting two parameters of type
``id`` and returning an ``NSComparisonResult``):

.. code-block:: objc

  NSArray *array = @[@"string 1", @"string 21", @"string 12", @"String 11",
                     @"String 02"];
  const NSStringCompareOptions comparisonOptions
    = NSCaseInsensitiveSearch | NSNumericSearch |
      NSWidthInsensitiveSearch | NSForcedOrderingSearch;
  NSLocale *currentLocale = [NSLocale currentLocale];
  NSArray *sorted
    = [array sortedArrayUsingComparator:[=](id s1, id s2) -> NSComparisonResult {
               NSRange string1Range = NSMakeRange(0, [s1 length]);
               return [s1 compare:s2 options:comparisonOptions
               range:string1Range locale:currentLocale];
       }];
  NSLog(@"sorted: %@", sorted);

This code relies on an implicit conversion from the type of the lambda
expression (an unnamed, local class type called the *closure type*) to the
corresponding block pointer type.  The conversion itself is expressed by a
conversion operator in that closure type that produces a block pointer with the
same signature as the lambda itself, e.g.,

.. code-block:: objc

  operator NSComparisonResult (^)(id, id)() const;

This conversion function returns a new block that simply forwards the two
parameters to the lambda object (which it captures by copy), then returns the
result.  The returned block is first copied (with ``Block_copy``) and then
autoreleased.  As an optimization, if a lambda expression is immediately
converted to a block pointer (as in the first example, above), then the block
is not copied and autoreleased: rather, it is given the same lifetime as a
block literal written at that point in the program, which avoids the overhead
of copying a block to the heap in the common case.

The conversion from a lambda to a block pointer is only available in
Objective-C++, and not in C++ with blocks, due to its use of Objective-C memory
management (autorelease).

Object Literals and Subscripting
--------------------------------

Clang provides support for :doc:`Object Literals and Subscripting
<ObjectiveCLiterals>` in Objective-C, which simplifies common Objective-C
programming patterns, makes programs more concise, and improves the safety of
container creation.  There are several feature macros associated with object
literals and subscripting: ``__has_feature(objc_array_literals)`` tests the
availability of array literals; ``__has_feature(objc_dictionary_literals)``
tests the availability of dictionary literals;
``__has_feature(objc_subscripting)`` tests the availability of object
subscripting.

Objective-C Autosynthesis of Properties
---------------------------------------

Clang provides support for autosynthesis of declared properties.  Using this
feature, clang provides default synthesis of those properties not declared
@dynamic and not having user provided backing getter and setter methods.
``__has_feature(objc_default_synthesize_properties)`` checks for availability
of this feature in version of clang being used.

.. _langext-objc_method_family:

The ``objc_method_family`` attribute
------------------------------------

Many methods in Objective-C have conventional meanings determined by their
selectors. It is sometimes useful to be able to mark a method as having a
particular conventional meaning despite not having the right selector, or as
not having the conventional meaning that its selector would suggest. For these
use cases, we provide an attribute to specifically describe the "method family"
that a method belongs to.

**Usage**: ``__attribute__((objc_method_family(X)))``, where ``X`` is one of
``none``, ``alloc``, ``copy``, ``init``, ``mutableCopy``, or ``new``.  This
attribute can only be placed at the end of a method declaration:

.. code-block:: objc

  - (NSString *)initMyStringValue __attribute__((objc_method_family(none)));

Users who do not wish to change the conventional meaning of a method, and who
merely want to document its non-standard retain and release semantics, should
use the :ref:`retaining behavior attributes <langext-objc-retain-release>`
described below.

Query for this feature with ``__has_attribute(objc_method_family)``.

.. _langext-objc-retain-release:

Objective-C retaining behavior attributes
-----------------------------------------

In Objective-C, functions and methods are generally assumed to follow the
`Cocoa Memory Management 
<http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html>`_
conventions for ownership of object arguments and
return values. However, there are exceptions, and so Clang provides attributes
to allow these exceptions to be documented. This are used by ARC and the
`static analyzer <http://clang-analyzer.llvm.org>`_ Some exceptions may be
better described using the :ref:`objc_method_family
<langext-objc_method_family>` attribute instead.

**Usage**: The ``ns_returns_retained``, ``ns_returns_not_retained``,
``ns_returns_autoreleased``, ``cf_returns_retained``, and
``cf_returns_not_retained`` attributes can be placed on methods and functions
that return Objective-C or CoreFoundation objects. They are commonly placed at
the end of a function prototype or method declaration:

.. code-block:: objc

  id foo() __attribute__((ns_returns_retained));

  - (NSString *)bar:(int)x __attribute__((ns_returns_retained));

The ``*_returns_retained`` attributes specify that the returned object has a +1
retain count.  The ``*_returns_not_retained`` attributes specify that the return
object has a +0 retain count, even if the normal convention for its selector
would be +1.  ``ns_returns_autoreleased`` specifies that the returned object is
+0, but is guaranteed to live at least as long as the next flush of an
autorelease pool.

**Usage**: The ``ns_consumed`` and ``cf_consumed`` attributes can be placed on
an parameter declaration; they specify that the argument is expected to have a
+1 retain count, which will be balanced in some way by the function or method.
The ``ns_consumes_self`` attribute can only be placed on an Objective-C
method; it specifies that the method expects its ``self`` parameter to have a
+1 retain count, which it will balance in some way.

.. code-block:: objc

  void foo(__attribute__((ns_consumed)) NSString *string);

  - (void) bar __attribute__((ns_consumes_self));
  - (void) baz:(id) __attribute__((ns_consumed)) x;

Further examples of these attributes are available in the static analyzer's `list of annotations for analysis
<http://clang-analyzer.llvm.org/annotations.html#cocoa_mem>`_.

Query for these features with ``__has_attribute(ns_consumed)``,
``__has_attribute(ns_returns_retained)``, etc.


Function Overloading in C
=========================

Clang provides support for C++ function overloading in C.  Function overloading
in C is introduced using the ``overloadable`` attribute.  For example, one
might provide several overloaded versions of a ``tgsin`` function that invokes
the appropriate standard function computing the sine of a value with ``float``,
``double``, or ``long double`` precision:

.. code-block:: c

  #include <math.h>
  float __attribute__((overloadable)) tgsin(float x) { return sinf(x); }
  double __attribute__((overloadable)) tgsin(double x) { return sin(x); }
  long double __attribute__((overloadable)) tgsin(long double x) { return sinl(x); }

Given these declarations, one can call ``tgsin`` with a ``float`` value to
receive a ``float`` result, with a ``double`` to receive a ``double`` result,
etc.  Function overloading in C follows the rules of C++ function overloading
to pick the best overload given the call arguments, with a few C-specific
semantics:

* Conversion from ``float`` or ``double`` to ``long double`` is ranked as a
  floating-point promotion (per C99) rather than as a floating-point conversion
  (as in C++).

* A conversion from a pointer of type ``T*`` to a pointer of type ``U*`` is
  considered a pointer conversion (with conversion rank) if ``T`` and ``U`` are
  compatible types.

* A conversion from type ``T`` to a value of type ``U`` is permitted if ``T``
  and ``U`` are compatible types.  This conversion is given "conversion" rank.

The declaration of ``overloadable`` functions is restricted to function
declarations and definitions.  Most importantly, if any function with a given
name is given the ``overloadable`` attribute, then all function declarations
and definitions with that name (and in that scope) must have the
``overloadable`` attribute.  This rule even applies to redeclarations of
functions whose original declaration had the ``overloadable`` attribute, e.g.,

.. code-block:: c

  int f(int) __attribute__((overloadable));
  float f(float); // error: declaration of "f" must have the "overloadable" attribute

  int g(int) __attribute__((overloadable));
  int g(int) { } // error: redeclaration of "g" must also have the "overloadable" attribute

Functions marked ``overloadable`` must have prototypes.  Therefore, the
following code is ill-formed:

.. code-block:: c

  int h() __attribute__((overloadable)); // error: h does not have a prototype

However, ``overloadable`` functions are allowed to use a ellipsis even if there
are no named parameters (as is permitted in C++).  This feature is particularly
useful when combined with the ``unavailable`` attribute:

.. code-block:: c++

  void honeypot(...) __attribute__((overloadable, unavailable)); // calling me is an error

Functions declared with the ``overloadable`` attribute have their names mangled
according to the same rules as C++ function names.  For example, the three
``tgsin`` functions in our motivating example get the mangled names
``_Z5tgsinf``, ``_Z5tgsind``, and ``_Z5tgsine``, respectively.  There are two
caveats to this use of name mangling:

* Future versions of Clang may change the name mangling of functions overloaded
  in C, so you should not depend on an specific mangling.  To be completely
  safe, we strongly urge the use of ``static inline`` with ``overloadable``
  functions.

* The ``overloadable`` attribute has almost no meaning when used in C++,
  because names will already be mangled and functions are already overloadable.
  However, when an ``overloadable`` function occurs within an ``extern "C"``
  linkage specification, it's name *will* be mangled in the same way as it
  would in C.

Query for this feature with ``__has_extension(attribute_overloadable)``.

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)

**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``.

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_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.

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``

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

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 may still be instrumented by the tool
to avoid false positives in other places.

.. _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.

Type Safety Checking
====================

Clang supports additional attributes to enable checking type safety properties
that can't be enforced by C type system.  Usecases include:

* MPI library implementations, where these attributes enable checking that
  buffer type matches the passed ``MPI_Datatype``;
* for HDF5 library there is a similar usecase as 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 of 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
   parameter, where the function is annotated with a compatible attribute,
   otherwise it warns.  For example:

   .. code-block:: c

     __attribute__((__format__ (__scanf__, 1, 3)))
     void foo(const char* s, char *buf, ...) {
       va_list ap;
       va_start(ap, buf);

       vprintf(s, ap); // warning: format string is not a string literal
     }

   In this case we warn because ``s`` contains a format string for a
   ``scanf``-like function, but it is passed to a ``printf``-like function.

   If the attribute is removed, clang still warns, because the format string is
   not a string literal.

   Another example:

   .. code-block:: c

     __attribute__((__format__ (__printf__, 1, 3)))
     void foo(const char* s, char *buf, ...) {
       va_list ap;
       va_start(ap, buf);

       vprintf(s, ap); // warning
     }

   In this case Clang does not warn because the format string ``s`` and
   the corresponding arguments are annotated.  If the arguments are
   incorrect, the caller of ``foo`` will receive a warning.