How do I recover a semaphore when the process that decremented it to zero crashes?

Question

I have multiple apps compiled with g++, running in Ubuntu. I\'m using named semaphores to co-ordinate between different processes.

All works fine except in

Answer 1

Simply do a sem_unlink() immediately after the sem_open(). Linux will remove after all processes have closed the resource, which includes internal closes.

Answer 2

Use a lock file instead of a semaphore, much like @Stéphane's solution but without the flock() calls. You can simply open the file using an exclusive lock:

//call to open() will block until it can obtain an exclusive lock on the file.
errno = 0;
int fd = open("/tmp/.lockfile", 
    O_CREAT | //create the file if it's not present.
    O_WRONLY | //only need write access for the internal locking semantics.
    O_EXLOCK, //use an exclusive lock when opening the file.
    S_IRUSR | S_IWUSR); //permissions on the file, 600 here.

if (fd == -1) {
    perror("open() failed");
    exit(EXIT_FAILURE);
}

printf("Entered critical section.\n);
//Do "critical" stuff here.

//exit the critical section
errno = 0;
if (close(fd) == -1) {
    perror("close() failed");
    exit(EXIT_FAILURE);
}

printf("Exited critical section.\n");