提供互斥锁以更正线程

Question

If two threads are being blocked on a lock, how can you choose which get's it?

For example thread1 calls mutex_lock(aMut) but aMut is already locked so thread1 blocks. Along comes thread2 and it calls mutex lock on aMut and it blocks. How can it be ensured aMut is given to thread2 ?

here is my attempt that doesn't work

/*global variables*/
pthread_mutex_t aMut = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t  cond_var = PTHREAD_COND_INITIIALIZER;
pthread_cond_t  changeNumPrior = PTHREAD_COND_INITIIALIZER;
int highPriorityThreads = 0;

/*function that is passed to pthread_create()*/
void* functionThatIsPassedToPthreadCreate(void* arg)
{
   bool highPriority = false;
   if(arg->someConditionIsMet)
   {
     highPriorityThreads++;
     highPriority = true;
   }
   pthread_mutex_lock(&changeNumPrior);
    while(!highPriority && highPriorities > 0)//this doesn't work
    {
        pthread_cond_wait(&cond_var, &changeNumPrior);
    }
    pthread_mutex_unlock(&changeNumPrior);

    pthread_mutex_lock(&aMut);//if this blocks, by the time this mutex is aquired, there now may be high priority threads
    //code that requires exclusive access to shared resource protected by aMut
    pthread_mutex_unlock(&aMut);
    pthread_cond_signal(&cond_var);
}

Answer 1

Just saying "Now I want this task to be scheduled and now this one" is not possible, but some thread libraries allow to set the priority of tasks, so it will be favored when scheduling.

For pthreads, you can do this with pthread_getschedparam .