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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html><head><!-- Copyright 1997 The Open Group, All Rights Reserved --><title><stdarg.h></title></head><body bgcolor=white><center><font size=2>The Single UNIX ® Specification, Version 2<br>Copyright © 1997 The Open Group</font></center><hr size=2 noshade><h4><a name = "tag_000_008_849"> </a>NAME</h4><blockquote>stdarg.h - handle variable argument list</blockquote><h4><a name = "tag_000_008_850"> </a>SYNOPSIS</h4><blockquote><pre><code>#include <stdarg.h>void va_start(va_list <i>ap</i>, <i>argN</i>);<i>type</i> va_arg(va_list <i>ap</i>, <i>type</i>);void va_end(va_list <i>ap</i>);</code></pre></blockquote><h4><a name = "tag_000_008_851"> </a>DESCRIPTION</h4><blockquote>The<i><stdarg.h></i>header contains a set of macros which allows portable functions that acceptvariable argument lists to be written.Functions that have variable argument lists (suchas <i>printf</i>()) but do not use these macros areinherently non-portable, as different systems use differentargument-passing conventions.<p>The type <b>va_list</b> is defined for variables used to traversethe list.<p>The <i>va_start</i>() macro is invoked to initialise <i>ap</i>to the beginning of the list before any calls to<i><a href="va_arg.html">va_arg()</a></i>.<p>The object<i>ap</i>may be passed as an argument to another function; if that functioninvokes the<i>va_arg</i>()macro with parameter<i>ap</i>,the value of<i>ap</i>in the calling function is indeterminate and must be passed tothe<i>va_end</i>()macro prior to any further reference to<i>ap</i>.The parameter<i>argN</i>is the identifier of the rightmost parameter in the variableparameter list in the function definition (the one just beforethe , ...).If the parameter<i>argN</i>is declared with the<b>register</b>storage class, with a function type or array type, or with a typethat is not compatible with the type that results afterapplication of the default argument promotions, the behaviour isundefined.<p>The<i><a href="va_arg.html">va_arg()</a></i>macro will return the next argument in the listpointed to by <i>ap</i>. Each invocation of<i><a href="va_arg.html">va_arg()</a></i>modifies <i>ap</i> so that the values of successive arguments arereturned in turn.The <i>type</i> parameter is the type the argument is expected to be.This is the type name specified such that the type of a pointer to an objectthat has the specified type can be obtained simply by suffixing a * totype.Different types can be mixed, but it is up to the routine to knowwhat type of argument is expected.<p>The<i><a href="va_end.html">va_end()</a></i>macro is used to clean up; it invalidates <i>ap</i> for use(unless<i><a href="va_start.html">va_start()</a></i>is invoked again).<p>Multiple traversals, each bracketed by<i><a href="va_start.html">va_start()</a></i><i><a href="va_end.html">va_end()</a></i>,are possible.</blockquote><h4><a name = "tag_000_008_852"> </a>EXAMPLES</h4><blockquote>This example is a possible implementation of<i><a href="exec.html">execl()</a></i>.<pre><code>#include <stdarg.h>#define MAXARGS 31/* * execl is called by * execl(file, arg1, arg2, ..., (char *)(0)); */int execl (const char *file, const char *args, ...){ va_list ap; char *array[MAXARGS]; int argno = 0; va_start(ap, args); while (args != 0) { array[argno++] = args; args = va_arg(ap, const char *);}va_end(ap);return execv(file, array);}</code></pre></blockquote><h4><a name = "tag_000_008_853"> </a>APPLICATION USAGE</h4><blockquote>It is up to the calling routine to communicate to the calledroutine how many argumentsthere are, since it is not always possible for the called routineto determine this in any other way.For example,<i><a href="exec.html">execl()</a></i>is passed a null pointer to signal the end of the list.The<i><a href="printf.html">printf()</a></i>function can tell how many arguments are there by the<i>format</i>argument.</blockquote><h4><a name = "tag_000_008_854"> </a>FUTURE DIRECTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_000_008_855"> </a>SEE ALSO</h4><blockquote><i><a href="exec.html">exec</a></i>,<i><a href="printf.html">printf</i>()</a>.</blockquote><hr size=2 noshade><center><font size=2>UNIX ® is a registered Trademark of The Open Group.<br>Copyright © 1997 The Open Group<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]</font></center><hr size=2 noshade></body></html>⌨️ 快捷键说明
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