Compute the length printf would print.

printflen.c

#include <stdio.h>
#include <stdarg.h>

/* returns the length printf would print. */
int
printflen(const char* format, ...) {
  va_list ap;
  char buf[1];
  int len;

  va_start(ap, format);
  len = vsnprintf(buf, sizeof (buf), format, ap);
  va_end(ap);
  return len;
}