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Node:<a name="Type%20Attributes">Type Attributes</a>,
Next:<a rel="next" accesskey="n" href="Alignment.html#Alignment">Alignment</a>,
Previous:<a rel="previous" accesskey="p" href="Variable-Attributes.html#Variable%20Attributes">Variable Attributes</a>,
Up:<a rel="up" accesskey="u" href="C-Extensions.html#C%20Extensions">C Extensions</a>
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<h3 class="section">Specifying Attributes of Types</h3>
<p>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. Six attributes are currently defined for types:
<code>aligned</code>, <code>packed</code>, <code>transparent_union</code>, <code>unused</code>,
<code>deprecated</code> and <code>may_alias</code>. Other attributes are defined for
functions (see <a href="Function-Attributes.html#Function%20Attributes">Function Attributes</a>) and for variables
(see <a href="Variable-Attributes.html#Variable%20Attributes">Variable Attributes</a>).
<p>You may also specify any one of these attributes with <code>__</code>
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 the <code>aligned</code> and <code>transparent_union</code>
attributes either in a <code>typedef</code> declaration or just past the
closing curly brace of a complete enum, struct or union type
<em>definition</em> and the <code>packed</code> attribute only past the closing
brace of a definition.
<p>You may also specify attributes between the enum, struct or union
tag and the name of the type rather than after the closing brace.
<p>See <a href="Attribute-Syntax.html#Attribute%20Syntax">Attribute Syntax</a>, for details of the exact syntax for using
attributes.
<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>force the compiler to insure (as far as it can) that each variable whose
type is <code>struct S</code> or <code>more_aligned_int</code> will be 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 which 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 which 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 which 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 will also be doing 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 will often be 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> will still only provide you with 8 byte
alignment. See your linker documentation for further information.
<br><dt><code>packed</code>
<dd>This attribute, attached to an <code>enum</code>, <code>struct</code>, or
<code>union</code> type definition, specified that the minimum required memory
be used to represent the type.
<p>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 <code>-fshort-enums</code>
flag on the line is equivalent to specifying the <code>packed</code>
attribute on all <code>enum</code> definitions.
<p>You may only specify this attribute after a closing curly brace on an
<code>enum</code> definition, not in a <code>typedef</code> declaration, unless that
declaration also contains the definition of the <code>enum</code>.
<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 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><sys/wait.h></code> might define the interface
as follows:
<pre class="smallexample"> typedef union
{
int *__ip;
union wait *__up;
} wait_status_ptr_t __attribute__ ((__transparent_union__));
pid_t wait (wait_status_ptr_t);
</pre>
<p>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="example"> int w1 () { int w; return wait (&w); }
int w2 () { union wait w; return wait (&w); }
</pre>
<p>With this interface, <code>wait</code>'s implementation might look like this:
<pre class="example"> 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 will 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>
<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>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.
<p>The <code>deprecated</code> attribute can also be used for functions and
variables (see <a href="Function-Attributes.html#Function%20Attributes">Function Attributes</a>, see <a href="Variable-Attributes.html#Variable%20Attributes">Variable Attributes</a>.)
<br><dt><code>may_alias</code>
<dd>Accesses to objects with types with this attribute are not subjected to
type-based alias analysis, but are instead assumed to be able to alias
any other type of objects, just like the <code>char</code> type. See
<code>-fstrict-aliasing</code> for more information on aliasing issues.
<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 *) &a;
b[1] = 0;
if (a == 0x12345678)
abort();
exit(0);
}
</pre>
<p>If you replaced <code>short_a</code> with <code>short</code> in the variable
declaration, the above program would abort when compiled with
<code>-fstrict-aliasing</code>, which is on by default at <code>-O2</code> or
above in recent GCC versions.
</dl>
<p>To specify multiple attributes, separate them by commas within the
double parentheses: for example, <code>__attribute__ ((aligned (16),
packed))</code>.
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