docs/AttributeReference.rst

..
  -------------------------------------------------------------------
  NOTE: This file is automatically generated by running clang-tblgen
  -gen-attr-docs. Do not edit this file by hand!!
  -------------------------------------------------------------------

===================
Attributes in Clang
===================
.. contents::
   :local:

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

This page lists the attributes currently supported by Clang.

Function Attributes
===================


interrupt
---------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

Clang supports the GNU style ``__attribute__((interrupt("TYPE")))`` attribute on
ARM targets. This attribute may be attached to a function definition and
instructs the backend to generate appropriate function entry/exit code so that
it can be used directly as an interrupt service routine.

The parameter passed to the interrupt attribute is optional, but if
provided it must be a string literal with one of the following values: "IRQ",
"FIQ", "SWI", "ABORT", "UNDEF".

The semantics are as follows:

- If the function is AAPCS, Clang instructs the backend to realign the stack to
  8 bytes on entry. This is a general requirement of the AAPCS at public
  interfaces, but may not hold when an exception is taken. Doing this allows
  other AAPCS functions to be called.
- If the CPU is M-class this is all that needs to be done since the architecture
  itself is designed in such a way that functions obeying the normal AAPCS ABI
  constraints are valid exception handlers.
- If the CPU is not M-class, the prologue and epilogue are modified to save all
  non-banked registers that are used, so that upon return the user-mode state
  will not be corrupted. Note that to avoid unnecessary overhead, only
  general-purpose (integer) registers are saved in this way. If VFP operations
  are needed, that state must be saved manually.

  Specifically, interrupt kinds other than "FIQ" will save all core registers
  except "lr" and "sp". "FIQ" interrupts will save r0-r7.
- If the CPU is not M-class, the return instruction is changed to one of the
  canonical sequences permitted by the architecture for exception return. Where
  possible the function itself will make the necessary "lr" adjustments so that
  the "preferred return address" is selected.

  Unfortunately the compiler is unable to make this guarantee for an "UNDEF"
  handler, where the offset from "lr" to the preferred return address depends on
  the execution state of the code which generated the exception. In this case
  a sequence equivalent to "movs pc, lr" will be used.


acquire_capability (acquire_shared_capability, clang::acquire_capability, clang::acquire_shared_capability)
-----------------------------------------------------------------------------------------------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","X","",""

Marks a function as acquiring a capability.


assert_capability (assert_shared_capability, clang::assert_capability, clang::assert_shared_capability)
-------------------------------------------------------------------------------------------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","X","",""

Marks a function that dynamically tests whether a capability is held, and halts
the program if it is not held.


availability
------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

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

.. code-block:: c++

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

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

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

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

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

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

unavailable
  This declaration is never available on this platform.

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

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

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

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

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

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

.. code-block:: c

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

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

.. code-block:: objc

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

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


_Noreturn
---------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "","","","X"

A function declared as ``_Noreturn`` shall not return to its caller. The
compiler will generate a diagnostic for a function declared as ``_Noreturn``
that appears to be capable of returning to its caller.


noreturn
--------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "","X","",""

A function declared as ``[[noreturn]]`` shall not return to its caller. The
compiler will generate a diagnostic for a function declared as ``[[noreturn]]``
that appears to be capable of returning to its caller.


carries_dependency
------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","X","",""

The ``carries_dependency`` attribute specifies dependency propagation into and
out of functions.

When specified on a function or Objective-C method, the ``carries_depedency``
attribute means that the return value carries a dependency out of the function, 
so that the implementation need not constrain ordering upon return from that
function. Implementations of the function and its caller may choose to preserve
dependencies instead of emitting memory ordering instructions such as fences.

Note, this attribute does not change the meaning of the program, but may result
in generatation of more efficient code.


enable_if
---------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

The ``enable_if`` attribute can be placed on function declarations to control
which overload is selected based on the values of the function's arguments.
When combined with the ``overloadable`` attribute, this feature is also
available in C.

.. code-block:: c++

  int isdigit(int c);
  int isdigit(int c) __attribute__((enable_if(c <= -1 || c > 255, "chosen when 'c' is out of range"))) __attribute__((unavailable("'c' must have the value of an unsigned char or EOF")));
  
  void foo(char c) {
    isdigit(c);
    isdigit(10);
    isdigit(-10);  // results in a compile-time error.
  }

The enable_if attribute takes two arguments, the first is an expression written
in terms of the function parameters, the second is a string explaining why this
overload candidate could not be selected to be displayed in diagnostics. The
expression is part of the function signature for the purposes of determining
whether it is a redeclaration (following the rules used when determining
whether a C++ template specialization is ODR-equivalent), but is not part of
the type.

The enable_if expression is evaluated as if it were the body of a
bool-returning constexpr function declared with the arguments of the function
it is being applied to, then called with the parameters at the callsite. If the
result is false or could not be determined through constant expression
evaluation, then this overload will not be chosen and the provided string may
be used in a diagnostic if the compile fails as a result.

Because the enable_if expression is an unevaluated context, there are no global
state changes, nor the ability to pass information from the enable_if
expression to the function body. For example, suppose we want calls to
strnlen(strbuf, maxlen) to resolve to strnlen_chk(strbuf, maxlen, size of
strbuf) only if the size of strbuf can be determined:

.. code-block:: c++

  __attribute__((always_inline))
  static inline size_t strnlen(const char *s, size_t maxlen)
    __attribute__((overloadable))
    __attribute__((enable_if(__builtin_object_size(s, 0) != -1))),
                             "chosen when the buffer size is known but 'maxlen' is not")))
  {
    return strnlen_chk(s, maxlen, __builtin_object_size(s, 0));
  }

Multiple enable_if attributes may be applied to a single declaration. In this
case, the enable_if expressions are evaluated from left to right in the
following manner. First, the candidates whose enable_if expressions evaluate to
false or cannot be evaluated are discarded. If the remaining candidates do not
share ODR-equivalent enable_if expressions, the overload resolution is
ambiguous. Otherwise, enable_if overload resolution continues with the next
enable_if attribute on the candidates that have not been discarded and have
remaining enable_if attributes. In this way, we pick the most specific
overload out of a number of viable overloads using enable_if.

.. code-block:: c++

  void f() __attribute__((enable_if(true, "")));  // #1
  void f() __attribute__((enable_if(true, ""))) __attribute__((enable_if(true, "")));  // #2
  
  void g(int i, int j) __attribute__((enable_if(i, "")));  // #1
  void g(int i, int j) __attribute__((enable_if(j, ""))) __attribute__((enable_if(true)));  // #2

In this example, a call to f() is always resolved to #2, as the first enable_if
expression is ODR-equivalent for both declarations, but #1 does not have another
enable_if expression to continue evaluating, so the next round of evaluation has
only a single candidate. In a call to g(1, 1), the call is ambiguous even though
#2 has more enable_if attributes, because the first enable_if expressions are
not ODR-equivalent.

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


format (gnu::format)
--------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","X","",""

Clang supports the ``format`` attribute, which indicates that the function
accepts a ``printf`` or ``scanf``-like format string and corresponding
arguments or a ``va_list`` that contains these arguments.

Please see `GCC documentation about format attribute
<http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_ to find details
about attribute syntax.

Clang implements two kinds of checks with this attribute.

#. Clang checks that the function with the ``format`` attribute is called with
   a format string that uses format specifiers that are allowed, and that
   arguments match the format string.  This is the ``-Wformat`` warning, it is
   on by default.

#. Clang checks that the format string argument is a literal string.  This is
   the ``-Wformat-nonliteral`` warning, it is off by default.

   Clang implements this mostly the same way as GCC, but there is a difference
   for functions that accept a ``va_list`` argument (for example, ``vprintf``).
   GCC does not emit ``-Wformat-nonliteral`` warning for calls to such
   fuctions.  Clang does not warn if the format string comes from a function
   parameter, where the function is annotated with a compatible attribute,
   otherwise it warns.  For example:

   .. code-block:: c

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

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

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

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

   Another example:

   .. code-block:: c

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

       vprintf(s, ap); // warning
     }

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


noduplicate (clang::noduplicate)
--------------------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","X","",""

The ``noduplicate`` attribute can be placed on function declarations to control
whether function calls to this function can be duplicated or not as a result of
optimizations. This is required for the implementation of functions with
certain special requirements, like the OpenCL "barrier" function, that might
need to be run concurrently by all the threads that are executing in lockstep
on the hardware. For example this attribute applied on the function
"nodupfunc" in the code below avoids that:

.. code-block:: c

  void nodupfunc() __attribute__((noduplicate));
  // Setting it as a C++11 attribute is also valid
  // void nodupfunc() [[clang::noduplicate]];
  void foo();
  void bar();

  nodupfunc();
  if (a > n) {
    foo();
  } else {
    bar();
  }

gets possibly modified by some optimizations into code similar to this:

.. code-block:: c

  if (a > n) {
    nodupfunc();
    foo();
  } else {
    nodupfunc();
    bar();
  }

where the call to "nodupfunc" is duplicated and sunk into the two branches
of the condition.


no_sanitize_address (no_address_safety_analysis, gnu::no_address_safety_analysis, gnu::no_sanitize_address)
-----------------------------------------------------------------------------------------------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","X","",""

.. _langext-address_sanitizer:

Use ``__attribute__((no_sanitize_address))`` on a function declaration to
specify that address safety instrumentation (e.g. AddressSanitizer) should
not be applied to that function.


no_sanitize_memory
------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

.. _langext-memory_sanitizer:

Use ``__attribute__((no_sanitize_memory))`` on a function declaration to
specify that checks for uninitialized memory should not be inserted 
(e.g. by MemorySanitizer). The function may still be instrumented by the tool
to avoid false positives in other places.


no_sanitize_thread
------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

.. _langext-thread_sanitizer:

Use ``__attribute__((no_sanitize_thread))`` on a function declaration to
specify that checks for data races on plain (non-atomic) memory accesses should
not be inserted by ThreadSanitizer. The function is still instrumented by the
tool to avoid false positives and provide meaningful stack traces.


objc_method_family
------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

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

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

.. code-block:: objc

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

Users who do not wish to change the conventional meaning of a method, and who
merely want to document its non-standard retain and release semantics, should
use the retaining behavior attributes (``ns_returns_retained``,
``ns_returns_not_retained``, etc).

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


objc_requires_super
-------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

Some Objective-C classes allow a subclass to override a particular method in a
parent class but expect that the overriding method also calls the overridden
method in the parent class. For these cases, we provide an attribute to
designate that a method requires a "call to ``super``" in the overriding
method in the subclass.

**Usage**: ``__attribute__((objc_requires_super))``.  This attribute can only
be placed at the end of a method declaration:

.. code-block:: objc

  - (void)foo __attribute__((objc_requires_super));

This attribute can only be applied the method declarations within a class, and
not a protocol.  Currently this attribute does not enforce any placement of
where the call occurs in the overriding method (such as in the case of
``-dealloc`` where the call must appear at the end).  It checks only that it
exists.

Note that on both OS X and iOS that the Foundation framework provides a
convenience macro ``NS_REQUIRES_SUPER`` that provides syntactic sugar for this
attribute:

.. code-block:: objc

  - (void)foo NS_REQUIRES_SUPER;

This macro is conditionally defined depending on the compiler's support for
this attribute.  If the compiler does not support the attribute the macro
expands to nothing.

Operationally, when a method has this annotation the compiler will warn if the
implementation of an override in a subclass does not call super.  For example:

.. code-block:: objc

   warning: method possibly missing a [super AnnotMeth] call
   - (void) AnnotMeth{};
                      ^


overloadable
------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

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

.. code-block:: c

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

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

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

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

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

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

.. code-block:: c

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

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

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

.. code-block:: c

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

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

.. code-block:: c++

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

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

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

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

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


release_capability (release_shared_capability, clang::release_capability, clang::release_shared_capability)
-----------------------------------------------------------------------------------------------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","X","",""

Marks a function as releasing a capability.


try_acquire_capability (try_acquire_shared_capability, clang::try_acquire_capability, clang::try_acquire_shared_capability)
---------------------------------------------------------------------------------------------------------------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","X","",""

Marks a function that attempts to acquire a capability. This function may fail to
actually acquire the capability; they accept a Boolean value determining
whether acquiring the capability means success (true), or failing to acquire
the capability means success (false).


Variable Attributes
===================


tls_model (gnu::tls_model)
--------------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","X","",""

The ``tls_model`` attribute allows you to specify which thread-local storage
model to use. It accepts the following strings:

* global-dynamic
* local-dynamic
* initial-exec
* local-exec

TLS models are mutually exclusive.


Type Attributes
===============


__single_inhertiance, __multiple_inheritance, __virtual_inheritance
-------------------------------------------------------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "","","","X"

This collection of keywords is enabled under ``-fms-extensions`` and controls
the pointer-to-member representation used on ``*-*-win32`` targets.

The ``*-*-win32`` targets utilize a pointer-to-member representation which
varies in size and alignment depending on the definition of the underlying
class.

However, this is problematic when a forward declaration is only available and
no definition has been made yet.  In such cases, Clang is forced to utilize the
most general representation that is available to it.

These keywords make it possible to use a pointer-to-member representation other
than the most general one regardless of whether or not the definition will ever
be present in the current translation unit.

This family of keywords belong between the ``class-key`` and ``class-name``:

.. code-block:: c++

  struct __single_inheritance S;
  int S::*i;
  struct S {};

This keyword can be applied to class templates but only has an effect when used
on full specializations:

.. code-block:: c++

  template <typename T, typename U> struct __single_inheritance A; // warning: inheritance model ignored on primary template
  template <typename T> struct __multiple_inheritance A<T, T>; // warning: inheritance model ignored on partial specialization
  template <> struct __single_inheritance A<int, float>;

Note that choosing an inheritance model less general than strictly necessary is
an error:

.. code-block:: c++

  struct __multiple_inheritance S; // error: inheritance model does not match definition
  int S::*i;
  struct S {};


Statement Attributes
====================


fallthrough (clang::fallthrough)
--------------------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "","X","",""

The ``clang::fallthrough`` attribute is used along with the
``-Wimplicit-fallthrough`` argument to annotate intentional fall-through
between switch labels.  It can only be applied to a null statement placed at a
point of execution between any statement and the next switch label.  It is
common to mark these places with a specific comment, but this attribute is
meant to replace comments with a more strict annotation, which can be checked
by the compiler.  This attribute doesn't change semantics of the code and can
be used wherever an intended fall-through occurs.  It is designed to mimic
control-flow statements like ``break;``, so it can be placed in most places
where ``break;`` can, but only if there are no statements on the execution path
between it and the next switch label.

Here is an example:

.. code-block:: c++

  // compile with -Wimplicit-fallthrough
  switch (n) {
  case 22:
  case 33:  // no warning: no statements between case labels
    f();
  case 44:  // warning: unannotated fall-through
    g();
    [[clang::fallthrough]];
  case 55:  // no warning
    if (x) {
      h();
      break;
    }
    else {
      i();
      [[clang::fallthrough]];
    }
  case 66:  // no warning
    p();
    [[clang::fallthrough]]; // warning: fallthrough annotation does not
                            //          directly precede case label
    q();
  case 77:  // warning: unannotated fall-through
    r();
  }


Consumed Annotation Checking
============================
Clang supports additional attributes for checking basic resource management
properties, specifically for unique objects that have a single owning reference.
The following attributes are currently supported, although **the implementation
for these annotations is currently in development and are subject to change.**

callable_when
-------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

Use ``__attribute__((callable_when(...)))`` to indicate what states a method
may be called in.  Valid states are unconsumed, consumed, or unknown.  Each
argument to this attribute must be a quoted string.  E.g.:

``__attribute__((callable_when("unconsumed", "unknown")))``


consumable
----------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

Each ``class`` that uses any of the typestate annotations must first be marked
using the ``consumable`` attribute.  Failure to do so will result in a warning.

This attribute accepts a single parameter that must be one of the following:
``unknown``, ``consumed``, or ``unconsumed``.


param_typestate
---------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

This attribute specifies expectations about function parameters.  Calls to an
function with annotated parameters will issue a warning if the corresponding
argument isn't in the expected state.  The attribute is also used to set the
initial state of the parameter when analyzing the function's body.


return_typestate
----------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

The ``return_typestate`` attribute can be applied to functions or parameters.
When applied to a function the attribute specifies the state of the returned
value.  The function's body is checked to ensure that it always returns a value
in the specified state.  On the caller side, values returned by the annotated
function are initialized to the given state.

When applied to a function parameter it modifies the state of an argument after
a call to the function returns.  The function's body is checked to ensure that
the parameter is in the expected state before returning.


set_typestate
-------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

Annotate methods that transition an object into a new state with
``__attribute__((set_typestate(new_state)))``.  The new new state must be
unconsumed, consumed, or unknown.


test_typestate
--------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

Use ``__attribute__((test_typestate(tested_state)))`` to indicate that a method
returns true if the object is in the specified state..


Type Safety Checking
====================
Clang supports additional attributes to enable checking type safety properties
that can't be enforced by the C type system.  Use cases include:

* MPI library implementations, where these attributes enable checking that
  the buffer type matches the passed ``MPI_Datatype``;
* for HDF5 library there is a similar use case to MPI;
* checking types of variadic functions' arguments for functions like
  ``fcntl()`` and ``ioctl()``.

You can detect support for these attributes with ``__has_attribute()``.  For
example:

.. code-block:: c++

  #if defined(__has_attribute)
  #  if __has_attribute(argument_with_type_tag) && \
        __has_attribute(pointer_with_type_tag) && \
        __has_attribute(type_tag_for_datatype)
  #    define ATTR_MPI_PWT(buffer_idx, type_idx) __attribute__((pointer_with_type_tag(mpi,buffer_idx,type_idx)))
  /* ... other macros ...  */
  #  endif
  #endif

  #if !defined(ATTR_MPI_PWT)
  # define ATTR_MPI_PWT(buffer_idx, type_idx)
  #endif

  int MPI_Send(void *buf, int count, MPI_Datatype datatype /*, other args omitted */)
      ATTR_MPI_PWT(1,3);

argument_with_type_tag
----------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

Use ``__attribute__((argument_with_type_tag(arg_kind, arg_idx,
type_tag_idx)))`` on a function declaration to specify that the function
accepts a type tag that determines the type of some other argument.
``arg_kind`` is an identifier that should be used when annotating all
applicable type tags.

This attribute is primarily useful for checking arguments of variadic functions
(``pointer_with_type_tag`` can be used in most non-variadic cases).

For example:

.. code-block:: c++

  int fcntl(int fd, int cmd, ...)
      __attribute__(( argument_with_type_tag(fcntl,3,2) ));


pointer_with_type_tag
---------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

Use ``__attribute__((pointer_with_type_tag(ptr_kind, ptr_idx, type_tag_idx)))``
on a function declaration to specify that the function accepts a type tag that
determines the pointee type of some other pointer argument.

For example:

.. code-block:: c++

  int MPI_Send(void *buf, int count, MPI_Datatype datatype /*, other args omitted */)
      __attribute__(( pointer_with_type_tag(mpi,1,3) ));


type_tag_for_datatype
---------------------
.. csv-table:: Supported Syntaxes
   :header: "GNU", "C++11", "__declspec", "Keyword"

   "X","","",""

Clang supports annotating type tags of two forms.

* **Type tag that is an expression containing a reference to some declared
  identifier.** Use ``__attribute__((type_tag_for_datatype(kind, type)))`` on a
  declaration with that identifier:

  .. code-block:: c++

    extern struct mpi_datatype mpi_datatype_int
        __attribute__(( type_tag_for_datatype(mpi,int) ));
    #define MPI_INT ((MPI_Datatype) &mpi_datatype_int)

* **Type tag that is an integral literal.** Introduce a ``static const``
  variable with a corresponding initializer value and attach
  ``__attribute__((type_tag_for_datatype(kind, type)))`` on that declaration,
  for example:

  .. code-block:: c++

    #define MPI_INT ((MPI_Datatype) 42)
    static const MPI_Datatype mpi_datatype_int
        __attribute__(( type_tag_for_datatype(mpi,int) )) = 42

The attribute also accepts an optional third argument that determines how the
expression is compared to the type tag.  There are two supported flags:

* ``layout_compatible`` will cause types to be compared according to
  layout-compatibility rules (C++11 [class.mem] p 17, 18).  This is
  implemented to support annotating types like ``MPI_DOUBLE_INT``.

  For example:

  .. code-block:: c++

    /* In mpi.h */
    struct internal_mpi_double_int { double d; int i; };
    extern struct mpi_datatype mpi_datatype_double_int
        __attribute__(( type_tag_for_datatype(mpi, struct internal_mpi_double_int, layout_compatible) ));

    #define MPI_DOUBLE_INT ((MPI_Datatype) &mpi_datatype_double_int)

    /* In user code */
    struct my_pair { double a; int b; };
    struct my_pair *buffer;
    MPI_Send(buffer, 1, MPI_DOUBLE_INT /*, ...  */); // no warning

    struct my_int_pair { int a; int b; }
    struct my_int_pair *buffer2;
    MPI_Send(buffer2, 1, MPI_DOUBLE_INT /*, ...  */); // warning: actual buffer element
                                                      // type 'struct my_int_pair'
                                                      // doesn't match specified MPI_Datatype

* ``must_be_null`` specifies that the expression should be a null pointer
  constant, for example:

  .. code-block:: c++

    /* In mpi.h */
    extern struct mpi_datatype mpi_datatype_null
        __attribute__(( type_tag_for_datatype(mpi, void, must_be_null) ));

    #define MPI_DATATYPE_NULL ((MPI_Datatype) &mpi_datatype_null)

    /* In user code */
    MPI_Send(buffer, 1, MPI_DATATYPE_NULL /*, ...  */); // warning: MPI_DATATYPE_NULL
                                                        // was specified but buffer
                                                        // is not a null pointer