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

#define FileName "data.dat" 
#define DataString "Now is the winter of our discontent\n"


int main() {
  struct flock lock;
  lock.l_type = F_WRLCK;    /* read/write (exclusive versus shared) 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_RDWR | O_CREAT, 0666)) < 0)  {
       perror("open failed..."); exit(-1); }
  if (fcntl(fd, F_SETLK, &lock) < 0) {
       perror("fcntl failed to get lock..."); exit(-2); }
  else {
    write(fd, DataString, strlen(DataString)); /* populate data file */
    fprintf(stderr, "Process %d has written to data file...\n", lock.l_pid); }

  /* Now 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; }