Conditional wait and signal in multi-threading

Last Updated : 27 Jan, 2023

What are conditional wait and signal in multi-threading?
Explanation: When you want to sleep a thread, condition variable can be used. In C under Linux, there is a function pthread_cond_wait() to wait or sleep.
On the other hand, there is a function pthread_cond_signal() to wake up sleeping or waiting thread.
Threads can wait on a condition variable.

Prerequisite : Multithreading

Syntax of pthread_cond_wait() :

int pthread_cond_wait(pthread_cond_t *restrict cond, 
                   pthread_mutex_t *restrict mutex);

Parameter :

cond : condition variable
mutex : is mutex lock

Return Value :

On success, 0 is returned ; otherwise, an error 
number shall be returned to indicate the error.

The pthread_cond_wait() release a lock specified by mutex and wait on condition cond variable.

Syntax of pthread_cond_signal() :

int pthread_cond_signal(pthread_cond_t *cond);

Parameter :

cond : condition variable

Return Value :

On success, 0 is returned ; otherwise, an error number
shall be returned to indicate the error.

The pthread_cond_signal() wake up threads waiting for the condition variable.

Note : The above two functions works together.

Below is the implementation of condition, wait and signal functions.

// C program to implement cond(), signal()
// and wait() functions
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>

// Declaration of thread condition variable
pthread_cond_t cond1 = PTHREAD_COND_INITIALIZER;

// declaring mutex
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

int done = 1;

// Thread function
void* foo()
{

    // acquire a lock
    pthread_mutex_lock(&lock);
    if (done == 1) {

        // let's wait on condition variable cond1
        done = 2;
        printf("Waiting on condition variable cond1\n");
        pthread_cond_wait(&cond1, &lock);
    }
    else {

        // Let's signal condition variable cond1
        printf("Signaling condition variable cond1\n");
        pthread_cond_signal(&cond1);
    }

    // release lock
    pthread_mutex_unlock(&lock);

    printf("Returning thread\n");

    return NULL;
}

// Driver code
int main()
{
    pthread_t tid1, tid2;

    // Create thread 1
    pthread_create(&tid1, NULL, foo, NULL);

    // sleep for 1 sec so that thread 1
    // would get a chance to run first
    sleep(1);

    // Create thread 2
    pthread_create(&tid2, NULL, foo, NULL);

    // wait for the completion of thread 2
    pthread_join(tid2, NULL);

    return 0;
}