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<h1 id="firstHeading" class="firstHeading">va_start</h1> <table class="t-dcl-begin"> <tr class="t-dsc-header"> <th> Defined in header <code><stdarg.h></code> </th> <th> </th> <th> </th> </tr> <tr class="t-dcl t-until-c23"> <td><pre data-language="c">void va_start( va_list ap, parmN );</pre></td> <td class="t-dcl-nopad"> </td> <td><span class="t-mark-rev t-until-c23">(until C23)</span></td> </tr> <tr class="t-dcl t-since-c23"> <td> <pre data-language="c">void va_start( va_list ap, ... );</pre>
</td> <td class="t-dcl-nopad"> </td> <td> <span class="t-mark-rev t-since-c23">(since C23)</span> </td> </tr> </table> <p>The <code>va_start</code> macro enables access to the variable arguments<span class="t-rev-inl t-until-c23"><span> following the named argument <code>parmN</code></span><span><span class="t-mark-rev t-until-c23">(until C23)</span></span></span>.</p>
<p><code>va_start</code> shall be invoked with an instance to a valid <code>va_list</code> object <code>ap</code> before any calls to <code>va_arg</code>.</p>
<table class="t-rev-begin"> <tr class="t-rev t-until-c23">
<td> <p>If <code>parmN</code> is declared with <code>register</code> storage class specifier, with an array type, with a function type, or with a type not compatible with the type that results from default argument promotions, the behavior is undefined.</p>
</td> <td><span class="t-mark-rev t-until-c23">(until C23)</span></td>
</tr> <tr class="t-rev t-since-c23">
<td> <p>Only the first argument passed to <code>va_start</code> is evaluated. Any additional arguments are neither expanded nor used in any way.</p>
</td> <td><span class="t-mark-rev t-since-c23">(since C23)</span></td>
</tr> </table> <h3 id="Parameters"> Parameters</h3> <table class="t-par-begin"> <tr class="t-par"> <td> ap </td> <td> - </td> <td> an instance of the <code>va_list</code> type </td>
</tr> <tr class="t-par"> <td> parmN </td> <td> - </td> <td> the named parameter preceding the first variable parameter </td>
</tr>
</table> <h3 id="Expanded_value"> Expanded value</h3> <p>(none)</p>
<h3 id="Example"> Example</h3> <div class="t-example"> <div class="c source-c"><pre data-language="c">#include <stdio.h>
#include <stdarg.h>
int add_nums_C99(int count, ...)
{
int result = 0;
va_list args;
va_start(args, count); // count can be omitted since C23
for (int i = 0; i < count; ++i) {
result += va_arg(args, int);
}
va_end(args);
return result;
}
#if __STDC_VERSION__ > 201710L
// Same as above, valid since C23
int add_nums_C23(...)
{
int result = 0;
va_list args;
va_start(args);
int count = va_arg(args, int);
for (int i = 0; i < count; ++i) {
result += va_arg(args, int);
}
va_end(args);
return result;
}
#endif
int main(void)
{
printf("%d\n", add_nums_C99(4, 25, 25, 50, 50));
#if __STDC_VERSION__ > 201710L
printf("%d\n", add_nums_C23(4, 25, 25, 50, 50));
#endif
}</pre></div> <p>Possible output:</p>
<div class="text source-text"><pre data-language="c">150
150</pre></div> </div> <h3 id="References"> References</h3> <ul>
<li> C17 standard (ISO/IEC 9899:2018): </li>
<ul><li> 7.16.1.4 The va_start macro (p: 198-199) </li></ul>
<li> C11 standard (ISO/IEC 9899:2011): </li>
<ul><li> 7.16.1.4 The va_start macro (p: 271-272) </li></ul>
<li> C99 standard (ISO/IEC 9899:1999): </li>
<ul><li> 7.15.1.4 The va_start macro (p: 251-252) </li></ul>
<li> C89/C90 standard (ISO/IEC 9899:1990): </li>
<ul><li> 4.8.1.1 The va_start macro </li></ul>
</ul> <h3 id="See_also"> See also</h3> <table class="t-dsc-begin"> <tr class="t-dsc"> <td> <div><a href="va_arg" title="c/variadic/va arg"> <span class="t-lines"><span>va_arg</span></span></a></div> </td> <td> accesses the next variadic function argument <br> <span class="t-mark">(function macro)</span> </td>
</tr> <tr class="t-dsc"> <td> <div><a href="va_copy" title="c/variadic/va copy"> <span class="t-lines"><span>va_copy</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-c99">(C99)</span></span></span></div> </td> <td> makes a copy of the variadic function arguments <br> <span class="t-mark">(function macro)</span> </td>
</tr> <tr class="t-dsc"> <td> <div><a href="va_end" title="c/variadic/va end"> <span class="t-lines"><span>va_end</span></span></a></div> </td> <td> ends traversal of the variadic function arguments <br> <span class="t-mark">(function macro)</span> </td>
</tr> <tr class="t-dsc"> <td> <div><a href="va_list" title="c/variadic/va list"> <span class="t-lines"><span>va_list</span></span></a></div> </td> <td> holds the information needed by va_start, va_arg, va_end, and va_copy <br> <span class="t-mark">(typedef)</span> </td>
</tr> <tr class="t-dsc"> <td colspan="2"> <span><a href="https://en.cppreference.com/w/cpp/utility/variadic/va_start" title="cpp/utility/variadic/va start">C++ documentation</a></span> for <code>va_start</code> </td>
</tr> </table> <div class="_attribution">
<p class="_attribution-p">
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<a href="https://en.cppreference.com/w/c/variadic/va_start" class="_attribution-link">https://en.cppreference.com/w/c/variadic/va_start</a>
</p>
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