pthread_cond_wait for 2 threads

c, linux, pthreads

Solution

You have two problems. The first is that you aren't using `while()` loops correctly - for example, here:

//wait for turn
while( pthread_cond_wait(&condA, &mutex) != 0 )

i++;

The body of the `while` loop is the statement `i++` - this will execute `pthread_cond_wait()` and `i++` until the `pthread_cond_wait()` returns an error, so this is essentially an endless loop.

The second is that you can't use a pthreads condition variable on its own - it needs to be paired with some actual shared state (at its simplest, this shared state might just be a flag variable protected by a mutex). The `pthread_cond_wait()` function is used to wait for the shared state to reach a certain value, and the `pthread_cond_signal()` function is used when a thread has altered the shared state. Reworking your example to use such a variable:

//global variables
/* STATE_A = THREAD A runs next, STATE_B = THREAD B runs next */
enum { STATE_A, STATE_B } state = STATE_A;
pthread_cond_t      condA  = PTHREAD_COND_INITIALIZER;
pthread_cond_t      condB  = PTHREAD_COND_INITIALIZER;
pthread_mutex_t     mutex = PTHREAD_MUTEX_INITIALIZER;

void *threadA()
{
    int i = 0, rValue, loopNum;

    while(i<5)
    {
        /* Wait for state A */
        pthread_mutex_lock(&mutex);
        while (state != STATE_A)
            pthread_cond_wait(&condA, &mutex);
        pthread_mutex_unlock(&mutex);

        //do stuff
        for(loopNum = 1; loopNum <= 5; loopNum++)
            printf("Hello %d\n", loopNum);

        /* Set state to B and wake up thread B */
        pthread_mutex_lock(&mutex);
        state = STATE_B;
        pthread_cond_signal(&condB);
        pthread_mutex_unlock(&mutex);

        i++;
    }

    return 0;
}

void *threadB()
{
    int n = 0, rValue;

    while(n<5)
    {
        /* Wait for state B */
        pthread_mutex_lock(&mutex);
        while (state != STATE_B)
            pthread_cond_wait(&condB, &mutex);
        pthread_mutex_unlock(&mutex);

        //do stuff
        printf("Goodbye\n");

        /* Set state to A and wake up thread A */
        pthread_mutex_lock(&mutex);
        state = STATE_A;
        pthread_cond_signal(&condA);
        pthread_mutex_unlock(&mutex);

        n++;
    }

    return 0;
}

Note that the use of two condition variables `condA` and `condB` is unnecessary here - the code would be just as correct if only one condition variable was used instead.

Problem

Original source