Question 15.4

How can I write a function that takes a variable number of arguments?

Use the facilities of the <stdarg.h> header.

Here is a function which concatenates an arbitrary number of strings into malloc'ed memory:

#include <stdlib.h>		/* for malloc, NULL, size_t */
#include <stdarg.h>		/* for va_ stuff */
#include <string.h>		/* for strcat et al. */

char *vstrcat(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 for second scan */

	while((p = va_arg(argp, char *)) != NULL)
		(void)strcat(retbuf, p);

	va_end(argp);

	return retbuf;
}

Usage is something like

	char *str = vstrcat("Hello, ", "world!", (char *)NULL);

Note the cast on the last argument; see questions 5.2 and 15.3. (Also note that the caller must free the returned, malloc'ed storage.)