Cesar Forcing

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

# define IS_PRINTABLE(c) ((c >= ' ' && c <= '~'))

int     main(int argc, char **argv)
{
  int       fd;
  int       i;
  int       j;
  char      buffer[1024];
  int       n;

  j = 0;
  i = 1;
  if (argc != 2)
    {
      printf("Usage : %s <filetobrute>\n", argv[0]);
      return (1);
    }
  if ((fd = open(argv[1], O_RDONLY)) == -1)
    {
      printf("Can't open file : '%s'\n", argv[1]);
      perror("open");
      return (1);
    }
  while ((n = read(fd, buffer, 1023)) > 0)
    buffer[n] = 0;
  /* premier test */
  while (i != 20)
    {
      printf("[+%d] ", i);
      while (buffer[j])
    {
      if (IS_PRINTABLE(buffer[j] + i))
        printf("%c", buffer[j] + i);
      else
        printf(" ");
      j++;
    }
      printf("\n");
      j = 0;
      i++;
    }
  i = 0;
  /* second test */
  while (i != -20)
    {
      printf("[-%d] ", i);
      while (buffer[j])
    {
      if (IS_PRINTABLE(buffer[j] - i))
        printf("%c", buffer[j] - i);
      else
        printf(" ");
      j++;
    }
      printf("\n");
      j = 0;
      i--;
    }
}