#include <stdio.h>

void die(void)
{
  fprintf(stderr,"This is an uninformative message.\n");
  exit(111);
}

int main(int argc,char **argv)
{
  int p;
  int b;
  FILE *f;

  if (argc < 4) die();
  if (sscanf(argv[2],"%d",&p) < 1) die();
  if (sscanf(argv[3],"%d",&b) < 1) die();

  f = fopen(argv[1],"r+");
  if (!f) die();
  if (fseek(f,p,0) == -1) die();
  if (fprintf(f,"%c",b) == -1) die();
  if (fflush(f) == -1) die();

  exit(0);
}