Q: How can I write a function that takes a format string and a variable number of arguments, like printf, and passes them to printf to do most of the work?
A: Use vprintf, vfprintf, or vsprintf. These routines are like their counterparts printf, fprintf, and sprintf, except that instead of a variable-length argument list, they accept a single va_list pointer.
As an example, here is an error function which prints an error message, preceded by the string ``error: '' and terminated with a newline:
#include <stdio.h>
#include <stdarg.h>
void error(const char *fmt, ...)
{
va_list argp;
fprintf(stderr, "error: ");
va_start(argp, fmt);
vfprintf(stderr, fmt, argp);
va_end(argp);
fprintf(stderr, "\n");
}
See also question 15.7.
References:
K&R2 Sec. 8.3 p. 174, Sec. B1.2 p. 245
ISO Secs. 7.9.6.7,7.9.6.8,7.9.6.9
H&S Sec. 15.12 pp. 379-80
PCS Sec. 11 pp. 186-7
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