Check to see if a pthread mutex is locked or unlocked (After a thread has locked itself)

Question

I need to see if a mutex is locked or unlocked in an if statement so I check it like this...

if(mutex[id] != 2){
    /* do stuff */
}

but w

Answer 1

You can use pthread_mutex_trylock. If that succeeds, the mutex was unclaimed and you now own it (so you should release it and return "unheld", in your case). Otherwise, someone is holding it.

I have to stress though that "check to see if a mutex is unclaimed" is a very bad idea. There are inherent race conditions in this kind of thinking. If such a function tells you at time t that the lock is unheld, that says absolutely nothing about whether or not some other thread acquired the lock at t+1.

In case this is better illustrated with a code example, consider:

bool held = is_lock_held();

if (!held)
{
  // What exactly can you conclude here?  Pretty much nothing.
  // It was unheld at some point in the past but it might be held
  // by the time you got to this point, or by the time you do your
  // next instruction...
}