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<h3 class="section">6.37 Specifying Attributes of Types</h3>

<p><a name="index-attribute-of-types-2797"></a><a name="index-type-attributes-2798"></a>
The keyword <code>__attribute__</code> allows you to specify special
attributes of <code>struct</code> and <code>union</code> types when you define
such types.  This keyword is followed by an attribute specification
inside double parentheses.  Seven attributes are currently defined for
types: <code>aligned</code>, <code>packed</code>, <code>transparent_union</code>,
<code>unused</code>, <code>deprecated</code>, <code>visibility</code>, and
<code>may_alias</code>.  Other attributes are defined for functions
(see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>) and for variables (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).

 <p>You may also specify any one of these attributes with &lsquo;<samp><span class="samp">__</span></samp>&rsquo;
preceding and following its keyword.  This allows you to use these
attributes in header files without being concerned about a possible
macro of the same name.  For example, you may use <code>__aligned__</code>
instead of <code>aligned</code>.

 <p>You may specify type attributes in an enum, struct or union type
declaration or definition, or for other types in a <code>typedef</code>
declaration.

 <p>For an enum, struct or union type, you may specify attributes either
between the enum, struct or union tag and the name of the type, or
just past the closing curly brace of the <em>definition</em>.  The
former syntax is preferred.

 <p>See <a href="Attribute-Syntax.html#Attribute-Syntax">Attribute Syntax</a>, for details of the exact syntax for using
attributes.


<a name="index-g_t_0040code_007baligned_007d-attribute-2799"></a>
<dl><dt><code>aligned (</code><var>alignment</var><code>)</code><dd>This attribute specifies a minimum alignment (in bytes) for variables
of the specified type.  For example, the declarations:

     <pre class="smallexample">          struct S { short f[3]; } __attribute__ ((aligned (8)));
          typedef int more_aligned_int __attribute__ ((aligned (8)));
</pre>
     <p class="noindent">force the compiler to ensure (as far as it can) that each variable whose
type is <code>struct S</code> or <code>more_aligned_int</code> is allocated and
aligned <em>at least</em> on a 8-byte boundary.  On a SPARC, having all
variables of type <code>struct S</code> aligned to 8-byte boundaries allows
the compiler to use the <code>ldd</code> and <code>std</code> (doubleword load and
store) instructions when copying one variable of type <code>struct S</code> to
another, thus improving run-time efficiency.

     <p>Note that the alignment of any given <code>struct</code> or <code>union</code> type
is required by the ISO C standard to be at least a perfect multiple of
the lowest common multiple of the alignments of all of the members of
the <code>struct</code> or <code>union</code> in question.  This means that you <em>can</em>
effectively adjust the alignment of a <code>struct</code> or <code>union</code>
type by attaching an <code>aligned</code> attribute to any one of the members
of such a type, but the notation illustrated in the example above is a
more obvious, intuitive, and readable way to request the compiler to
adjust the alignment of an entire <code>struct</code> or <code>union</code> type.

     <p>As in the preceding example, you can explicitly specify the alignment
(in bytes) that you wish the compiler to use for a given <code>struct</code>
or <code>union</code> type.  Alternatively, you can leave out the alignment factor
and just ask the compiler to align a type to the maximum
useful alignment for the target machine you are compiling for.  For
example, you could write:

     <pre class="smallexample">          struct S { short f[3]; } __attribute__ ((aligned));
</pre>
     <p>Whenever you leave out the alignment factor in an <code>aligned</code>
attribute specification, the compiler automatically sets the alignment
for the type to the largest alignment that is ever used for any data
type on the target machine you are compiling for.  Doing this can often
make copy operations more efficient, because the compiler can use
whatever instructions copy the biggest chunks of memory when performing
copies to or from the variables that have types that you have aligned
this way.

     <p>In the example above, if the size of each <code>short</code> is 2 bytes, then
the size of the entire <code>struct S</code> type is 6 bytes.  The smallest
power of two that is greater than or equal to that is 8, so the
compiler sets the alignment for the entire <code>struct S</code> type to 8
bytes.

     <p>Note that although you can ask the compiler to select a time-efficient
alignment for a given type and then declare only individual stand-alone
objects of that type, the compiler's ability to select a time-efficient
alignment is primarily useful only when you plan to create arrays of
variables having the relevant (efficiently aligned) type.  If you
declare or use arrays of variables of an efficiently-aligned type, then
it is likely that your program also does pointer arithmetic (or
subscripting, which amounts to the same thing) on pointers to the
relevant type, and the code that the compiler generates for these
pointer arithmetic operations is often more efficient for
efficiently-aligned types than for other types.

     <p>The <code>aligned</code> attribute can only increase the alignment; but you
can decrease it by specifying <code>packed</code> as well.  See below.

     <p>Note that the effectiveness of <code>aligned</code> attributes may be limited
by inherent limitations in your linker.  On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment.  (For some linkers, the maximum supported alignment may
be very very small.)  If your linker is only able to align variables
up to a maximum of 8-byte alignment, then specifying <code>aligned(16)</code>
in an <code>__attribute__</code> still only provides you with 8-byte
alignment.  See your linker documentation for further information.

     <br><dt><code>packed</code><dd>This attribute, attached to <code>struct</code> or <code>union</code> type
definition, specifies that each member (other than zero-width bit-fields)
of the structure or union is placed to minimize the memory required.  When
attached to an <code>enum</code> definition, it indicates that the smallest
integral type should be used.

     <p><a name="index-fshort_002denums-2800"></a>Specifying this attribute for <code>struct</code> and <code>union</code> types is
equivalent to specifying the <code>packed</code> attribute on each of the
structure or union members.  Specifying the <samp><span class="option">-fshort-enums</span></samp>
flag on the line is equivalent to specifying the <code>packed</code>
attribute on all <code>enum</code> definitions.

     <p>In the following example <code>struct my_packed_struct</code>'s members are
packed closely together, but the internal layout of its <code>s</code> member
is not packed&mdash;to do that, <code>struct my_unpacked_struct</code> needs to
be packed too.

     <pre class="smallexample">          struct my_unpacked_struct
           {
              char c;
              int i;
           };

          struct __attribute__ ((__packed__)) my_packed_struct
            {
               char c;
               int  i;
               struct my_unpacked_struct s;
            };
</pre>
     <p>You may only specify this attribute on the definition of an <code>enum</code>,
<code>struct</code> or <code>union</code>, not on a <code>typedef</code> that does not
also define the enumerated type, structure or union.

     <br><dt><code>transparent_union</code><dd>This attribute, attached to a <code>union</code> type definition, indicates
that any function parameter having that union type causes calls to that
function to be treated in a special way.

     <p>First, the argument corresponding to a transparent union type can be of
any type in the union; no cast is required.  Also, if the union contains
a pointer type, the corresponding argument can be a null pointer
constant or a void pointer expression; and if the union contains a void
pointer type, the corresponding argument can be any pointer expression.
If the union member type is a pointer, qualifiers like <code>const</code> on
the referenced type must be respected, just as with normal pointer
conversions.

     <p>Second, the argument is passed to the function using the calling
conventions of the first member of the transparent union, not the calling
conventions of the union itself.  All members of the union must have the
same machine representation; this is necessary for this argument passing
to work properly.

     <p>Transparent unions are designed for library functions that have multiple
interfaces for compatibility reasons.  For example, suppose the
<code>wait</code> function must accept either a value of type <code>int *</code> to
comply with POSIX, or a value of type <code>union wait *</code> to comply with
the 4.1BSD interface.  If <code>wait</code>'s parameter were <code>void *</code>,
<code>wait</code> would accept both kinds of arguments, but it would also
accept any other pointer type and this would make argument type checking
less useful.  Instead, <code>&lt;sys/wait.h&gt;</code> might define the interface
as follows:

     <pre class="smallexample">          typedef union __attribute__ ((__transparent_union__))
            {
              int *__ip;
              union wait *__up;
            } wait_status_ptr_t;

          pid_t wait (wait_status_ptr_t);
</pre>
     <p class="noindent">This interface allows either <code>int *</code> or <code>union wait *</code>
arguments to be passed, using the <code>int *</code> calling convention.
The program can call <code>wait</code> with arguments of either type:

     <pre class="smallexample">          int w1 () { int w; return wait (&amp;w); }
          int w2 () { union wait w; return wait (&amp;w); }
</pre>
     <p class="noindent">With this interface, <code>wait</code>'s implementation might look like this:

     <pre class="smallexample">          pid_t wait (wait_status_ptr_t p)
          {
            return waitpid (-1, p.__ip, 0);
          }
</pre>
     <br><dt><code>unused</code><dd>When attached to a type (including a <code>union</code> or a <code>struct</code>),
this attribute means that variables of that type are meant to appear
possibly unused.  GCC does not produce a warning for any variables of
that type, even if the variable appears to do nothing.  This is often
the case with lock or thread classes, which are usually defined and then
not referenced, but contain constructors and destructors that have
nontrivial bookkeeping functions.

     <br><dt><code>deprecated</code><dt><code>deprecated (</code><var>msg</var><code>)</code><dd>The <code>deprecated</code> attribute results in a warning if the type
is used anywhere in the source file.  This is useful when identifying
types that are expected to be removed in a future version of a program.
If possible, the warning also includes the location of the declaration
of the deprecated type, to enable users to easily find further
information about why the type is deprecated, or what they should do
instead.  Note that the warnings only occur for uses and then only
if the type is being applied to an identifier that itself is not being
declared as deprecated.

     <pre class="smallexample">          typedef int T1 __attribute__ ((deprecated));
          T1 x;
          typedef T1 T2;
          T2 y;
          typedef T1 T3 __attribute__ ((deprecated));
          T3 z __attribute__ ((deprecated));
</pre>
     <p class="noindent">results in a warning on line 2 and 3 but not lines 4, 5, or 6.  No
warning is issued for line 4 because T2 is not explicitly
deprecated.  Line 5 has no warning because T3 is explicitly
deprecated.  Similarly for line 6.  The optional <var>msg</var>
argument, which must be a string, is printed in the warning if
present.

     <p>The <code>deprecated</code> attribute can also be used for functions and
variables (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>, see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>.)

     <br><dt><code>may_alias</code><dd>Accesses through pointers to types with this attribute are not subject
to type-based alias analysis, but are instead assumed to be able to alias
any other type of objects.
In the context of section 6.5 paragraph 7 of the C99 standard,
an lvalue expression
dereferencing such a pointer is treated like having a character type.
See <samp><span class="option">-fstrict-aliasing</span></samp> for more information on aliasing issues.
This extension exists to support some vector APIs, in which pointers to
one vector type are permitted to alias pointers to a different vector type.

     <p>Note that an object of a type with this attribute does not have any
special semantics.

     <p>Example of use:

     <pre class="smallexample">          typedef short __attribute__((__may_alias__)) short_a;

          int
          main (void)
          {
            int a = 0x12345678;
            short_a *b = (short_a *) &amp;a;

            b[1] = 0;

            if (a == 0x12345678)
              abort();

            exit(0);
          }
</pre>
     <p class="noindent">If you replaced <code>short_a</code> with <code>short</code> in the variable
declaration, the above program would abort when compiled with
<samp><span class="option">-fstrict-aliasing</span></samp>, which is on by default at <samp><span class="option">-O2</span></samp> or
above in recent GCC versions.

     <br><dt><code>visibility</code><dd>In C++, attribute visibility (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>) can also be
applied to class, struct, union and enum types.  Unlike other type
attributes, the attribute must appear between the initial keyword and
the name of the type; it cannot appear after the body of the type.

     <p>Note that the type visibility is applied to vague linkage entities
associated with the class (vtable, typeinfo node, etc.).  In
particular, if a class is thrown as an exception in one shared object
and caught in another, the class must have default visibility.
Otherwise the two shared objects are unable to use the same
typeinfo node and exception handling will break.

 </dl>

 <p>To specify multiple attributes, separate them by commas within the
double parentheses: for example, &lsquo;<samp><span class="samp">__attribute__ ((aligned (16),
packed))</span></samp>&rsquo;.

<h4 class="subsection">6.37.1 ARM Type Attributes</h4>

<p>On those ARM targets that support <code>dllimport</code> (such as Symbian
OS), you can use the <code>notshared</code> attribute to indicate that the
virtual table and other similar data for a class should not be
exported from a DLL.  For example:

<pre class="smallexample">     class __declspec(notshared) C {
     public:
       __declspec(dllimport) C();
       virtual void f();
     }

     __declspec(dllexport)
     C::C() {}
</pre>
 <p class="noindent">In this code, <code>C::C</code> is exported from the current DLL, but the
virtual table for <code>C</code> is not exported.  (You can use
<code>__attribute__</code> instead of <code>__declspec</code> if you prefer, but
most Symbian OS code uses <code>__declspec</code>.)

 <p><a name="MeP-Type-Attributes"></a>

<h4 class="subsection">6.37.2 MeP Type Attributes</h4>

<p>Many of the MeP variable attributes may be applied to types as well.
Specifically, the <code>based</code>, <code>tiny</code>, <code>near</code>, and
<code>far</code> attributes may be applied to either.  The <code>io</code> and
<code>cb</code> attributes may not be applied to types.

 <p><a name="i386-Type-Attributes"></a>

<h4 class="subsection">6.37.3 i386 Type Attributes</h4>

<p>Two attributes are currently defined for i386 configurations:
<code>ms_struct</code> and <code>gcc_struct</code>.

     <dl>
<dt><code>ms_struct</code><dt><code>gcc_struct</code><dd><a name="index-g_t_0040code_007bms_005fstruct_007d-2801"></a><a name="index-g_t_0040code_007bgcc_005fstruct_007d-2802"></a>
If <code>packed</code> is used on a structure, or if bit-fields are used
it may be that the Microsoft ABI packs them differently
than GCC normally packs them.  Particularly when moving packed
data between functions compiled with GCC and the native Microsoft compiler
(either via function call or as data in a file), it may be necessary to access
either format.

     <p>Currently <samp><span class="option">-m[no-]ms-bitfields</span></samp> is provided for the Microsoft Windows X86
compilers to match the native Microsoft compiler.
</dl>

 <p><a name="PowerPC-Type-Attributes"></a>

<h4 class="subsection">6.37.4 PowerPC Type Attributes</h4>

<p>Three attributes currently are defined for PowerPC configurations:
<code>altivec</code>, <code>ms_struct</code> and <code>gcc_struct</code>.

 <p>For full documentation of the <code>ms_struct</code> and <code>gcc_struct</code>
attributes please see the documentation in <a href="i386-Type-Attributes.html#i386-Type-Attributes">i386 Type Attributes</a>.

 <p>The <code>altivec</code> attribute allows one to declare AltiVec vector data
types supported by the AltiVec Programming Interface Manual.  The
attribute requires an argument to specify one of three vector types:
<code>vector__</code>, <code>pixel__</code> (always followed by unsigned short),
and <code>bool__</code> (always followed by unsigned).

<pre class="smallexample">     __attribute__((altivec(vector__)))
     __attribute__((altivec(pixel__))) unsigned short
     __attribute__((altivec(bool__))) unsigned
</pre>
 <p>These attributes mainly are intended to support the <code>__vector</code>,
<code>__pixel</code>, and <code>__bool</code> AltiVec keywords.

 <p><a name="SPU-Type-Attributes"></a>

<h4 class="subsection">6.37.5 SPU Type Attributes</h4>

<p>The SPU supports the <code>spu_vector</code> attribute for types.  This attribute
allows one to declare vector data types supported by the Sony/Toshiba/IBM SPU
Language Extensions Specification.  It is intended to support the
<code>__vector</code> keyword.

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