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

int main(int argc, char **argv) {
  off_t pos;
  FILE *f;
  /* Eingaben */
  char *filename = argv[1];
  int blkgroesse = atoi(argv[2]);
  unsigned int blknummer = atoi(argv[3]);
  char *string = argv[4];

  f = fopen(filename, "r+");
  if (f == NULL)
    return 1;
  /* An Schreibposition spulen */
  pos = blkgroesse * blknummer;
  pos = fseek(f, pos, SEEK_SET);
  if (pos == (off_t)-1)
    return 1;
  fprintf(f, string); /* schreiben */

  return 0;
}