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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:46 2005 by faqproc version 2.7 --><!-- from source file varargs.sgml dated Sun Nov 25 08:09:42 2001 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/varargs/varargs1.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:00 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 15.4</title><link href="proto2.html" rev=precedes><link href="vprintf.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="proto2.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="vprintf.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a>&nbsp;<a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>&middot;</font><!-- qtag -->Question 15.4</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How can I write a function that takes a variable number ofarguments?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Usethe facilities ofthe <TT>&lt;stdarg.h&gt;</TT>header.</p><p>Here is a function which concatenates an arbitrary number ofstrings into <TT>malloc</TT>'ed memory:</p><pre>#include &lt;stdlib.h&gt;		/* for malloc, NULL, size_t */#include &lt;stdarg.h&gt;		/* for va_ stuff */#include &lt;string.h&gt;		/* for strcat et al. */char *vstrcat(const char *first, ...){	size_t len;	char *retbuf;	va_list argp;	char *p;	if(first == NULL)		return NULL;	len = strlen(first);	va_start(argp, first);	while((p = va_arg(argp, char *)) != NULL)		len += strlen(p);	va_end(argp);	retbuf = malloc(len + 1);	/* +1 for trailing \0 */	if(retbuf == NULL)		return NULL;		/* error */	(void)strcpy(retbuf, first);	va_start(argp, first);		/* restart; 2nd scan */	while((p = va_arg(argp, char *)) != NULL)		(void)strcat(retbuf, p);	va_end(argp);	return retbuf;}</pre>(Note thata second call to <TT>va_start</TT> is neededto re-start the scanwhen the argument list is processed a second time.Note the calls to <TT>va_end</TT>:they're important for portability,even if they don't seem to do anything.)<p>A call to <TT>vstrcat</TT> looks something like<pre>	char *str = vstrcat("Hello, ", "world!", (char *)NULL);</pre>Note the cast on the last argument;see questions <a href="../null/null2.html">5.2</a> and <a href="proto2.html">15.3</a>.(Also note that the caller must free the returned,<TT>malloc</TT>'ed storage.)</p><p><TT>vstrcat</TT> acceptsa variable number of arguments,all of type <TT>char&nbsp;*</TT>.Here is an example which accepts a variable number of arguments of different types;it is a stripped-down versionof the familiar <TT>printf</TT> function.Note that each invocation of <TT>va_arg()</TT> specifies the type of the argumentbeingretrievedfrom the argument list.</p><p>(The <TT>miniprintf</TT> function here uses<TT>baseconv</TT>from question <a href="../misc/hexio.html">20.10</a> to formatnumbers.Itissignificantlyimperfectin that it will not usually be able to printthe smallest integer, <TT>INT_MIN</TT>, properly.)<pre>#include &lt;stdio.h&gt;#include &lt;stdarg.h&gt;#ifdef MAINvoid miniprintf(const char *, ...);main(){	miniprintf("Hello, world!\n");	miniprintf("%c %d %s\n", '1', 2, "three");	miniprintf("%o %d %x\n", 10, 10, 10);	miniprintf("%u\n", 0xffff);	return 0;}#endifextern char *baseconv(unsigned int, int);voidminiprintf(const char *fmt, ...){	const char *p;	int i;	unsigned u;	char *s;	va_list argp;	va_start(argp, fmt);	for(p = fmt; *p != '\0'; p++) {		if(*p != '%') {			putchar(*p);			continue;		}		switch(*++p) {		case 'c':			i = va_arg(argp, int);			/* *not* va_arg(argp, char); see Q <a href="float.html">15.10</a> */			putchar(i);			break;		case 'd':			i = va_arg(argp, int);			if(i &lt; 0) {				/* XXX won't handle INT_MIN */				i = -i;				putchar('-');			}			fputs(baseconv(i, 10), stdout);			break;		case 'o':			u = va_arg(argp, unsigned int);			fputs(baseconv(u, 8), stdout);			break;		case 's':			s = va_arg(argp, char *);			fputs(s, stdout);			break;		case 'u':			u = va_arg(argp, unsigned int);			fputs(baseconv(u, 10), stdout);			break;		case 'x':			u = va_arg(argp, unsigned int);			fputs(baseconv(u, 16), stdout);			break;		case '%':			putchar('%');			break;		}	}	va_end(argp);}</pre></p><p>See also question <a href="oldvarargs.html">15.7</a>.</p><p>References:K&amp;R2 Sec. 7.3 p. 155, Sec. B7 p. 254<br>ISO Sec. 7.8<br>Rationale Sec. 4.8<br>H&amp;S Sec. 11.4 pp. 296-9<br>CT&amp;P Sec. A.3 pp. 139-141<br>PCS Sec. 11 pp. 184-5, Sec. 13 p. 242<br></p><!-- aend --><p><hr><a href="proto2.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="vprintf.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a>&nbsp;<a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a>&nbsp;<a href="../eskimo.html">about eskimo</a>&nbsp;<a href="../search.html">search</a>&nbsp;<a href="../feedback.html">feedback</a>&nbsp;<a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/varargs/varargs1.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:00 GMT --></html>
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