flock(): removing locked file without race condition?

Question

I\'m using flock() for inter-process named mutexes (i.e. some process can decide to hold a lock on \"some_name\", which is implemented by locking a file named \"some_name\"

Answer 1

1. In Unix it is possible to delete a file while it is opened - the inode will be kept until all processes have ended that have it in their file descriptor list
2. In Unix it is possible to check that a file has been removed from all directories by checking the link count as it becomes zero

So instead of comparing the ino-value of the old/new file paths you can simply check the nlink count on the file that is already open. It assumes that it is just an ephemeral lock file and not a real mutex resource or device.

lockfile = "/tmp/some_name.lock";

for(int attempt; attempt < timeout; ++attempt) {
    int fd = open(lockfile, O_CREAT, 0444);
    int done = flock(fd, LOCK_EX | LOCK_NB);
    if (done != 0) { 
        close(fd);
        sleep(1);     // lock held by another proc
        continue;
    }
    struct stat st0;
    fstat(fd, &st0);
    if(st0.st_nlink == 0) {
       close(fd);     // lockfile deleted, create a new one
       continue;
    }
    do_something();
    unlink(lockfile); // nlink :=0 before releasing the lock
    flock(fd, LOCK_UN);
    close(fd);        // release the ino if no other proc 
    return true;
}
return false;