Question

Is it possible to use fcntl() inside a function other than main() ? Does the file get unlocked after calling it ? I say this because in this case fcntl() and most everything else inside lockfile() are out-of-scope when the function returns.

int lockfile(void){
    int fd;
    const char *path = "path-to-lockfile";

    struct flock fl;
    fl.l_type = F_WRLCK;
    fl.l_whence = SEEK_SET;
    fl.l_start = 0;
    fl.l_len = 0
    fl.l_pid = getpid();

    fd = open(path, O_RDWR|O_CREAT);

    fcntl(fd, F_SETLKW, &fl);

    return fd;
}

Answer 1

The fcntl call places a lock on the file. It remains until the file is closed or the lock is released. The structures are only needed to tell fcntl what to do.