Format C string with length pre-computation.

snprintf.c

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

static char *mysprintf(const char *format, ...);

int main(void) {
  char *str = mysprintf("%s fmt with various values: PI ~= %.2f", "Test", 3.14);
  printf("%s (len = %zd)\n", str, strlen(str));
  free(str);
  return 0;
}

static char *mysprintf(const char *format, ...) {
  va_list ap;
  char *fstr = NULL;
  va_start(ap, format);
  int len = vsnprintf(NULL, 0, format, ap);
  va_end(ap);
  fstr = malloc(len + 1);
  va_start(ap, format);
  if (fstr) vsnprintf(fstr, len + 1, format, ap);
  va_end(ap);
  return fstr;
}