How to allow certain threads to have priority in locking a mutex use PTHREADS

Question

Assume that the following code is being executed by 10 threads.

pthread_mutex_lock(&lock)
Some trivial code
pthread_mutex_unlock(&lock)

Answer 1

To implement that with pthreads you would need N lists, one per thread priority. The lists would contain pointers to the thread's pthread_cond_t variables.

Schematic untested meta-code:

/* the main lock */
pthread_mutex_t TheLock = PTHREAD_MUTEX_INITIALIZER;

/* service structures: prio lists and the lock for them */
pthread_mutex_t prio_list_guard = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t *prio_lists[MY_MAX_PRIO][MY_MAX_THREAD]; /* 0 == highest prio */

/* lock */
void
prio_lock(int myprio)
{
    pthread_cond_t x;

    pthread_mutex_lock( &prio_list_guard );

    if (0 == pthread_mutex_trylock( &TheLock )) {
        pthread_mutex_unlock( &prio_list_guard );
        return 0;
    }

    pthread_cond_init( &x, 0 );
    LIST_ADD( prio_lists[myprio], &x )

    while(1)    /* handle spurious wake-ups */
    {
        pthread_cond_wait( &prio_list_guard, &x );
        if (0 == pthread_mutex_trylock( &TheLock )) 
        {
            LIST_REMOVE( prio_lists[myprio], &x );
            pthread_mutex_unlock( &prio_list_guard );
            return 0;
        }
    }
}

/* unlock */
void
prio_unlock()
{
    int i;
    pthread_cond_t *p;

    pthread_mutex_lock( &prio_list_guard );

    for (i=0; i

The code also handles spurious wake-ups from pthread_cond_wait(), but frankly I have never seen that happening.

Edit1. Note that prio_lists above is a primitive form of a priority queue.