offseter.c

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

int main (int argc, char* argv[]) {
  if(argc < 3) {
    printf("Usage: offseter [FILE] [OFFSET]\n");
    return 1;
  }

  unsigned char offset = atoi(argv[2]);
  FILE *fi, *fo;
  unsigned char c;

  fi = fopen(argv[1], "r");
  if(!fi) {
    printf("%s: No such file or directory\n", argv[1]);
    return 1;
  }
  fo = fopen("out.txt", "w");

  while(!feof(fi)) {
    c = fgetc(fi);
    fputc(c + offset, fo);
  }

  fclose(fi);
  fclose(fo);

  return 0;
}