Check to see if a pthread mutex is locked or unloc

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 when I check it gcc gives me the following error:

error: invalid operands to binary != (have 'ptherad_mutex_t' and 'int')

So how can I check to see if the mutex is locked or not?

EDIT:

A key component to my problem is that my threads (by design) lock themselves right AFTER passing control to another thread. So when thread A passes control to thread B thread A is locked, thread B does some stuff, then when thread B is done it will unlock thread A.

The problem with this is that if thread B attempts to unlock thread A and thread A has not yet completed locking itself then the call to unlock is lost and thread A remains locked which causes a dead lock.

UPDATE:

I remade my program taking caf's suggestion but I am still running into problems. I have molded my program into the structure caf provided the best I can but I cannot even tell what is causing the dead lock now... I have created a new question here seeking help with my code.

Below is a runnable version of caf's suggestion. I made a small reordering in the function for thread a, without which both thread a and thread b would have been locked upon their creation, waiting for a condition that could never change.

#include <pthread.h>

int run_a = 0;
pthread_mutex_t lock_a = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond_a = PTHREAD_COND_INITIALIZER;

int run_b = 0;
pthread_mutex_t lock_b = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond_b = PTHREAD_COND_INITIALIZER;

void *a(void *);
void *b(void *);

int main(){
    int status;
    pthread_t thread_a;
    pthread_t thread_b;

    pthread_create(&thread_a, NULL, a, (void *)0);
    pthread_create(&thread_b, NULL, b, (void *)0);

    pthread_join(thread_a, (void **)&status);
    pthread_join(thread_b, (void **)&status);

}

/* thread A */
void *a(void *i){
    while (1) {
        printf("thread A is running\n");
        sleep(1);

        /* unlock thread B */
        pthread_mutex_lock(&lock_b);
            run_b = 1;
            pthread_cond_signal(&cond_b);
        pthread_mutex_unlock(&lock_b);

        /* wait for thread A to be runnable */
        pthread_mutex_lock(&lock_a);
            while (!run_a)
                pthread_cond_wait(&cond_a, &lock_a);
            run_a = 0;
        pthread_mutex_unlock(&lock_a);      
    }
}

/* thread B */
void *b(void *i){
    while (1) {
        /* wait for thread B to be runnable */
        pthread_mutex_lock(&lock_b);
            while (!run_b)
                pthread_cond_wait(&cond_b, &lock_b);
            run_b = 0;
        pthread_mutex_unlock(&lock_b);

        printf("thread B is running\n");
        sleep(1);

        /* unlock thread A */
        pthread_mutex_lock(&lock_a);
            run_a = 1;
            pthread_cond_signal(&cond_a);
        pthread_mutex_unlock(&lock_a);
    }
}

Answer 1

Mutexes are not the right primitive for the scheme that you want to implement. You should be using condition variables:

int run_thread_a = 0;
pthread_mutex_t run_lock_a = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t run_cond_a = PTHREAD_COND_INITIALIZER;

int run_thread_b = 0;
pthread_mutex_t run_lock_b = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t run_cond_b = PTHREAD_COND_INITIALIZER;

/* thread A */
while (1) {
    /* Wait for Thread A to be runnable */
    pthread_mutex_lock(&run_lock_a);
    while (!run_thread_a)
        pthread_cond_wait(&run_cond_a, &run_lock_a);
    run_thread_a = 0;
    pthread_mutex_unlock(&run_lock_a);

    /* Do some work */

    /* Now wake thread B */
    pthread_mutex_lock(&run_lock_b);
    run_thread_b = 1;
    pthread_cond_signal(&run_cond_b);
    pthread_mutex_unlock(&run_lock_b);
}

/* thread B */
while (1) {
    /* Wait for Thread B to be runnable */
    pthread_mutex_lock(&run_lock_b);
    while (!run_thread_b)
        pthread_cond_wait(&run_cond_b, &run_lock_b);
    run_thread_b = 0;
    pthread_mutex_unlock(&run_lock_b);

    /* Do some work */

    /* Now wake thread A */
    pthread_mutex_lock(&run_lock_a);
    run_thread_a = 1;
    pthread_cond_signal(&run_cond_a);
    pthread_mutex_unlock(&run_lock_a);
}

Each thread will block in pthread_cond_wait() until the other thread signals it to wake up. This will not deadlock.

It can easily be extended to many threads, by allocating one int, pthread_cond_t and pthread_mutex_t per thread.

Answer 2

You can't compare a pthread_mutex_t with a int.

You can use

int pthread_mutex_trylock(pthread_mutex_t *mutex);

to check that.

Answer 3

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...
}