gistfile1.c

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

char * number2filename(char *start, int digits, int num, char *end) {
  int size = strlen(start) + strlen(end) + 1;
	
  int formatSize = size + 3 + log10(digits) + 1;
  char *format = (char *)malloc(formatSize);
  snprintf(format, formatSize, "%s%%0%dd%s", start, digits, end);
  
  size += digits;
  char *s = (char *)malloc(size);
  snprintf(s, size, format, num);
	
  free(format);
  return s;
}

int main(int argc, char *argv[]) {
  printf("%s", number2filename("capture_", 5, 15, ".jpeg"));
}