1 /* Open File Description Locks Usage Example
2 Copyright (C) 1991-2021 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 *
thread_start(void * arg)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
main(int argc,char ** argv)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 }
78