Condition variables

These are functions which are prefixed with either pthread_cond_ or pthread_condattr_. They apply to condition variables and their attribute objects.

Condition variables in Pthreads follow the same pattern of having an initialization and a destroy function in addition to having the same for managing a pthread_condattr_t attribution structure.

This example covers basic usage of Pthreads condition variables:

#include <pthread.h> 
#include <stdlib.h>
#include <unistd.h>
 
   #define COUNT_TRIGGER 10 
   #define COUNT_LIMIT 12 
 
   int count = 0; 
   int thread_ids[3] = {0,1,2}; 
   pthread_mutex_t count_mutex; 
   pthread_cond_t count_cv; 

In the preceding code, we get the standard headers, and define a count trigger and limit, whose purpose will become clear in a moment. We also define a few global variables: a count variable, the IDs for the threads we wish to create, as well as a mutex and condition variable:

void* add_count(void* t)  { 
    int tid = (long) t; 
    for (int i = 0; i < COUNT_TRIGGER; ++i) { 
        pthread_mutex_lock(&count_mutex); 
        count++; 
        if (count == COUNT_LIMIT) { 
            pthread_cond_signal(&count_cv); 
        } 
 
        pthread_mutex_unlock(&count_mutex); 
        sleep(1); 
    } 
 
    pthread_exit(0); 
} 

This preceding function, essentially, just adds to the global counter variable after obtaining exclusive access to it with the count_mutex. It also checks whether the count trigger value has been reached. If it has, it will signal the condition variable.

To give the second thread, which also runs this function, a chance to get the mutex, we sleep for 1 second in each cycle of the loop:

void* watch_count(void* t) { 
    int tid = (int) t; 
 
    pthread_mutex_lock(&count_mutex); 
    if (count < COUNT_LIMIT) { 
        pthread_cond_wait(&count_cv, &count_mutex); 
    } 
 
    pthread_mutex_unlock(&count_mutex); 
    pthread_exit(0); 
} 

In this second function, we lock the global mutex before checking whether we have reached the count limit yet. This is our insurance in case the thread running this function does not get called before the count reaches the limit.

Otherwise, we wait on the condition variable providing the condition variable and locked mutex. Once signaled, we unlock the global mutex, and exit the thread.

A point to note here is that this example does not account for spurious wake-ups. Pthreads condition variables are susceptible to such wake-ups which necessitate one to use a loop and check whether some kind of condition has been met:

int main (int argc, char* argv[]) { 
    int tid1 = 1, tid2 = 2, tid3 = 3; 
    pthread_t threads[3]; 
    pthread_attr_t attr; 
 
    pthread_mutex_init(&count_mutex, 0); 
    pthread_cond_init (&count_cv, 0); 
 
    pthread_attr_init(&attr); 
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE); 
    pthread_create(&threads[0], &attr, watch_count, (void *) tid1); 
    pthread_create(&threads[1], &attr, add_count, (void *) tid2); 
    pthread_create(&threads[2], &attr, add_count, (void *) tid3); 
 
    for (int i = 0; i < 3; ++i) { 
        pthread_join(threads[i], 0); 
    } 
 
    pthread_attr_destroy(&attr); 
    pthread_mutex_destroy(&count_mutex); 
    pthread_cond_destroy(&count_cv); 
    return 0; 
}  

Finally, in the main function, we create the three threads, with two running the function which adds to the counter, and the third running the function which waits to have its condition variable signaled.

In this method, we also initialize the global mutex and condition variable. The threads we create further have the "joinable" attribute explicitly set.

Finally, we wait for each thread to finish, after which we clean up, destroying the attribute structure instance, mutex, and condition variable before exiting.

Using the pthread_cond_broadcast() function, it's further possible to signal all threads which are waiting for a condition variable instead of merely the first one in the queue. This enables one to use condition variables more elegantly with some applications, such as where one has a lot of worker threads waiting for new dataset to arrive without having to notify every thread individually.