#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>

/* Your basic Stevens cut-and-paste */
static int
lock_reg (int fd, int cmd, int type, off_t offset, int whence, off_t
len)
{
  struct flock lock;

  lock.l_type = type; /* F_RDLCK, F_WRLCK, F_UNLCK */
  lock.l_start = offset; /* byte offset relative to whence */
  lock.l_whence = whence; /* SEEK_SET, SEEK_CUR, SEEK_END */
  lock.l_len = len; /* #bytes, 0 for eof */

  return fcntl (fd, cmd, &lock);
}

#define lock_entire_file(fd) \
  lock_reg ((fd), F_SETLK, F_WRLCK, 0, SEEK_SET, 0)
#define unlock_entire_file(fd) \
  lock_reg ((fd), F_SETLK, F_UNLCK, 0, SEEK_SET, 0)

int 
main (int argc, char **argv)
{
  int result;
  int fd;

  if (argc != 2)
    {
      fprintf (stderr, "Must pass in a single file to lock\n");
      return 1;
    }

  fd = open (argv[1], O_RDWR);
  if (fd < 0)
    {
      fprintf (stderr, "Failed to open '%s': %s\n", 
               argv[1], strerror (errno));
      return 1;
    }

  result = lock_entire_file (fd);

  if (result < 0)
    {
      fprintf (stderr, "Failed to lock '%s': %s\n",
               argv[1], strerror (errno));
      
      return 1;
    }

  printf ("Successfully locked '%s', unlocking and exiting\n",
  argv[1]);

  close (fd);
  
  return 0;
}