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

#define FileName "data.dat" 

int main() {
  struct flock lock;
  lock.l_type = F_WRLCK;    /* read/write (exclusive) lock */
  lock.l_whence = SEEK_SET; /* base for seek offsets */
  lock.l_start = 0;         /* 1st byte in file */
  lock.l_len = 0;           /* 0 here means 'until EOF' */
  lock.l_pid = getpid();    /* process id */
  
  int fd; /* file descriptor to identify a file within a process */
  if ((fd = open(FileName, O_RDONLY)) < 0) {
       perror("open to read failed..."); exit(-1); }
  
  /* If the file is write-locked, we can't continue. */
  fcntl(fd, F_GETLK, &lock); /* sets lock.l_type to F_UNLCK if no write lock */
  if (lock.l_type != F_UNLCK) {
       perror("file is still write locked..."); exit(-1); }
  
  lock.l_type = F_RDLCK; /* prevents any writing during the reading */
  if (fcntl(fd, F_SETLK, &lock) < 0) {
       perror("can't get a read-only lock..."); exit(-1); }

  /* Read the bytes (they happen to be ASCII codes) one at a time. */
  int c; /* buffer for read bytes */
  while (read(fd, &c, 1) > 0)    /* 0 signals EOF */
    write(STDOUT_FILENO, &c, 1); /* write one byte to the standard output */

  /* Release the lock explicitly. */
  lock.l_type = F_UNLCK;
  if (fcntl(fd, F_SETLK, &lock) < 0) {
       perror("explicit unlocking failed..."); exit(-1); }
  
  close(fd); 
  return 0; }