/* Open File Description Locks Usage Example

   Copyright (C) 1991-2018 Free Software Foundation, Inc.

   This program is free software; you can redistribute it and/or

   modify it under the terms of the GNU General Public License

   as published by the Free Software Foundation; either version 2

   of the License, or (at your option) any later version.

   This program is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License

   along with this program; if not, see <http://www.gnu.org/licenses/>.

*/

#define _GNU_SOURCE

#include <stdio.h>

#include <sys/types.h>

#include <sys/stat.h>

#include <unistd.h>

#include <fcntl.h>

#include <pthread.h>

#define FILENAME	"/tmp/foo"

#define NUM_THREADS	3

#define ITERATIONS	5

void *

thread_start (void *arg)

{

  int i, fd, len;

  long tid = (long) arg;

  char buf[256];

  struct flock lck = {

    .l_whence = SEEK_SET,

    .l_start = 0,

    .l_len = 1,

  };

  fd = open ("/tmp/foo", O_RDWR | O_CREAT, 0666);

  for (i = 0; i < ITERATIONS; i++)

    {

      lck.l_type = F_WRLCK;

      fcntl (fd, F_OFD_SETLKW, &lck;;

      len = sprintf (buf, "%d: tid=%ld fd=%d\n", i, tid, fd);

      lseek (fd, 0, SEEK_END);

      write (fd, buf, len);

      fsync (fd);

      lck.l_type = F_UNLCK;

      fcntl (fd, F_OFD_SETLK, &lck;;

      /* sleep to ensure lock is yielded to another thread */

      usleep (1);

    }

  pthread_exit (NULL);

}

int

main (int argc, char **argv)

{

  long i;

  pthread_t threads[NUM_THREADS];

  truncate (FILENAME, 0);

  for (i = 0; i < NUM_THREADS; i++)

    pthread_create (&threads[i], NULL, thread_start, (void *) i);

  pthread_exit (NULL);

  return 0;

}