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Conditional wait and signal in multi-threading
  • Difficulty Level : Medium
  • Last Updated : 22 Jul, 2019

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 conition 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;
}

Output :


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