manual/examples/ofdlocks.c

   1 /* Open File Description Locks Usage Example
   2    Copyright (C) 1991-2020 Free Software Foundation, Inc.
   3
   4    This program is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU General Public License
   6    as published by the Free Software Foundation; either version 2
   7    of the License, or (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program; if not, see <https://www.gnu.org/licenses/>.
  16 */
  17
  18 #define _GNU_SOURCE
  19 #include <stdio.h>
  20 #include <sys/types.h>
  21 #include <sys/stat.h>
  22 #include <unistd.h>
  23 #include <fcntl.h>
  24 #include <pthread.h>
  25
  26 #define FILENAME        "/tmp/foo"
  27 #define NUM_THREADS     3
  28 #define ITERATIONS      5
  29
  30 void *
  31 thread_start (void *arg)
  32 {
  33   int i, fd, len;
  34   long tid = (long) arg;
  35   char buf[256];
  36   struct flock lck = {
  37     .l_whence = SEEK_SET,
  38     .l_start = 0,
  39     .l_len = 1,
  40   };
  41
  42   fd = open ("/tmp/foo", O_RDWR | O_CREAT, 0666);
  43
  44   for (i = 0; i < ITERATIONS; i++)
  45     {
  46       lck.l_type = F_WRLCK;
  47       fcntl (fd, F_OFD_SETLKW, &lck);
  48
  49       len = sprintf (buf, "%d: tid=%ld fd=%d\n", i, tid, fd);
  50
  51       lseek (fd, 0, SEEK_END);
  52       write (fd, buf, len);
  53       fsync (fd);
  54
  55       lck.l_type = F_UNLCK;
  56       fcntl (fd, F_OFD_SETLK, &lck);
  57
  58       /* sleep to ensure lock is yielded to another thread */
  59       usleep (1);
  60     }
  61   pthread_exit (NULL);
  62 }
  63
  64 int
  65 main (int argc, char **argv)
  66 {
  67   long i;
  68   pthread_t threads[NUM_THREADS];
  69
  70   truncate (FILENAME, 0);
  71
  72   for (i = 0; i < NUM_THREADS; i++)
  73     pthread_create (&threads[i], NULL, thread_start, (void *) i);
  74
  75   pthread_exit (NULL);
  76   return 0;
  77 }