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<h5 class="subsubsection">A.2.2.5 Argument Access Macros</h5>

<p>Here are descriptions of the macros used to retrieve variable arguments. 
These macros are defined in the header file <samp><span class="file">stdarg.h</span></samp>. 
<a name="index-stdarg_002eh-3726"></a>
<!-- stdarg.h -->
<!-- ISO -->

<div class="defun">
&mdash; Data Type: <b>va_list</b><var><a name="index-va_005flist-3727"></a></var><br>
<blockquote><p>The type <code>va_list</code> is used for argument pointer variables. 
</p></blockquote></div>

<!-- stdarg.h -->
<!-- ISO -->
<div class="defun">
&mdash; Macro: void <b>va_start</b> (<var>va_list ap, last-required</var>)<var><a name="index-va_005fstart-3728"></a></var><br>
<blockquote><p>This macro initializes the argument pointer variable <var>ap</var> to point
to the first of the optional arguments of the current function;
<var>last-required</var> must be the last required argument to the function.

        <p>See <a href="Old-Varargs.html#Old-Varargs">Old Varargs</a>, for an alternate definition of <code>va_start</code>
found in the header file <samp><span class="file">varargs.h</span></samp>. 
</p></blockquote></div>

<!-- stdarg.h -->
<!-- ISO -->
<div class="defun">
&mdash; Macro: <var>type</var> <b>va_arg</b> (<var>va_list ap, type</var>)<var><a name="index-va_005farg-3729"></a></var><br>
<blockquote><p>The <code>va_arg</code> macro returns the value of the next optional argument,
and modifies the value of <var>ap</var> to point to the subsequent argument. 
Thus, successive uses of <code>va_arg</code> return successive optional
arguments.

        <p>The type of the value returned by <code>va_arg</code> is <var>type</var> as
specified in the call.  <var>type</var> must be a self-promoting type (not
<code>char</code> or <code>short int</code> or <code>float</code>) that matches the type
of the actual argument. 
</p></blockquote></div>

<!-- stdarg.h -->
<!-- ISO -->
<div class="defun">
&mdash; Macro: void <b>va_end</b> (<var>va_list ap</var>)<var><a name="index-va_005fend-3730"></a></var><br>
<blockquote><p>This ends the use of <var>ap</var>.  After a <code>va_end</code> call, further
<code>va_arg</code> calls with the same <var>ap</var> may not work.  You should invoke
<code>va_end</code> before returning from the function in which <code>va_start</code>
was invoked with the same <var>ap</var> argument.

        <p>In the GNU C library, <code>va_end</code> does nothing, and you need not ever
use it except for reasons of portability.

        </blockquote></div>

   <p>Sometimes it is necessary to parse the list of parameters more than once
or one wants to remember a certain position in the parameter list.  To
do this, one will have to make a copy of the current value of the
argument.  But <code>va_list</code> is an opaque type and one cannot necessarily
assign the value of one variable of type <code>va_list</code> to another variable
of the same type.

<!-- stdarg.h -->
<!-- GNU -->
<div class="defun">
&mdash; Macro: void <b>__va_copy</b> (<var>va_list dest, va_list src</var>)<var><a name="index-g_t_005f_005fva_005fcopy-3731"></a></var><br>
<blockquote><p>The <code>__va_copy</code> macro allows copying of objects of type
<code>va_list</code> even if this is not an integral type.  The argument pointer
in <var>dest</var> is initialized to point to the same argument as the
pointer in <var>src</var>.

        <p>This macro is a GNU extension but it will hopefully also be available in
the next update of the ISO C standard. 
</p></blockquote></div>

   <p>If you want to use <code>__va_copy</code> you should always be prepared for the
possibility that this macro will not be available.  On architectures where a
simple assignment is invalid, hopefully <code>__va_copy</code> <em>will</em> be available,
so one should always write something like this:

<pre class="smallexample">     {
       va_list ap, save;
       ...
     #ifdef __va_copy
       __va_copy (save, ap);
     #else
       save = ap;
     #endif
       ...
     }
</pre>
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