Programming

Hi all,

I use F_SETLKW to lock a file or keep waiting. How can I interrupt the blocking lock after waiting for a period? I want to add an argument to let user set waiting time. Maybe I can use non-blocking lock(F_SETLK) and a loop to check it. But it may consume too many resources. Any idea?

p.s: C/C++, LINUX platform

Thanks,
Marco Chen

alarm/SIGALRM sounds like the right thing here.

That what I'm trying now. But it's weird. I wrote a signal handler function to judge if set lock successfully or wait timeout. There are A, B ,C three processes trying to get file lock. A starts to lock file first, then B starts waiting, then C starts waitng. (alarm after ten seconds). Now A releases its lock but C gets file lock instead of B. I don't know how could it happen. Since I use "F_SETLKW", it sholud wait sequentially.

//timeout handler function
static void timeout_handler (int signum) {
struct flock fl = { F_WRLCK, SEEK_SET, 0, 0, 0 };
int fd;
fl.l_type = F_RDLCK;

printf("enter locking timeout check\n");

if ((fd = open("lock.c", O_RDWR)) == -1) {
perror("open");
exit(1);
}

if (fcntl(fd, F_SETLK, &fl) == -1){
printf("timeout pid: %d\n",fl.l_pid);
printf("Timeout!\n");
exit(1);
}
}

//main function as below
......

printf("Press <RETURN> to try to get lock: ");
getchar();
printf("Trying to get lock...");

struct sigaction action, old_action;
unsigned int old_timeout;

/* Cancel any existing alarm. */
old_timeout = alarm (0);

/* Establish signal handler. */
action.sa_handler = timeout_handler;
sigemptyset (&action.sa_mask);
action.sa_flags = SA_RESTART;
sigaction (SIGALRM, &action, &old_action);

alarm(TIMEOUT);

fcntl(fd, F_SETLKW, &fl);

/* Reset the signal handler and alarm. */
sigaction (SIGALRM, &old_action, NULL);
alarm(old_timeout);

........

A, B and C are all independent processes. There is no rule that says that he who starts waiting for a lock first is the first to get it, but there is a rule that says only one can have the lock at any time.

It may not seem fair, but it is democratic.

thank you, porter.