#include <limits.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv) {
  off_t pos;
  FILE *f;
  unsigned long blkgroesse;
  unsigned long blknummer;

  /* Argumente pruefen */
  if (argc < 5)
    return 1; /* zu wenig Argumente */
  blkgroesse = strtoul(argv[2], 0, 0);
  if (blkgroesse == ULONG_MAX)
    return 1; /* ungueltiger Wert */
  blknummer = strtoul(argv[3], 0, 0);
  if (blknummer == ULONG_MAX)
    return 1; /* ungueltiger Wert */
  /* oeffnen */
  f = fopen(argv[1], "r+");
  if (f == NULL)
    return 1;
  /* An Schreibposition spulen */
  pos = (off_t)(blkgroesse * blknummer);
  if (pos < blkgroesse)
    return 1; /* Integerueberlauf */
  pos = fseek(f, pos, SEEK_SET);
  if (pos == (off_t)-1)
    return 1;
  fprintf(f, "%s", argv[4]); /* schreiben */

  return 0;
}