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/* Open File Description Locks Usage Example
Copyright (C) 1991-2020 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 <https://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;
}
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