Sunday, April 30, 2017

6 Pthread condition variable in Linux

Posted by beni April 30, 2017

6 Pthread condition variable in Linux


Condition variable one type of thread synchronization mechanism.

By using condition variable we can tell one thread that when to execute.

using pthread_cond_signal pass the signal to another thread, this signal can be catch by another thread using pthread_cond_wait.


Below program
  • main thread will create two thread
  • second thread will wait on pthread_cond_wait to get signal.
  • first thread will increment the globalCount. When globalCount become 5 thread1 will send signal to thread2
  • now thread2 will get signal and add 20 to globalCount and exited from thread handler
  • then thread 1 increment the gloablCount until 15, and exited from thread handler function
PROGRAM : 
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
 
int globalCount = 0;
pthread_mutex_t mutex;
pthread_cond_t condvar;
 
void *t1_fun(void *arg) //thread 1 handler function
{
    int i = 0;
    int t_num = (int)arg;
 
    for (i=0; i<15; i++) {
        pthread_mutex_lock(&mutex);
        globalCount++;
        if (globalCount == 5) {
        pthread_cond_signal(&condvar);
            printf("t1_fun(): thread %d, globalCount = %d Signaled ", t_num, globalCount);
        }
        printf("t1_fun(): thread %d, globalCount = %d ", t_num, globalCount);
        pthread_mutex_unlock(&mutex);
        sleep(1);
    }
    pthread_exit(NULL);
}
 
void *condVar_fun(void *arg) //thread 2 handler function
{
 int t_num = (int)arg;
        
    printf("Thread %d created and running  ", t_num);
    pthread_mutex_lock(&mutex);
        
 pthread_cond_wait(&condvar, &mutex);
    printf("condVar_fun(): Thread %d Condition signal received  ", t_num);
    globalCount += 20;
 printf("condVar_fun(): Thread %d globalCount = %d  ", t_num, globalCount); 
        
    pthread_mutex_unlock(&mutex);
    pthread_exit(NULL);
}
 
int main(int argc, char *argv[])
{
    pthread_t mythread1;
    pthread_t mythread2;
    pthread_attr_t myattr;
    void *joinResult;
    int x = 0;
    int t_arg = 1;
        
    pthread_attr_init(&myattr);
    pthread_attr_setdetachstate(&myattr, PTHREAD_CREATE_JOINABLE);
        
    pthread_mutex_init(&mutex, NULL);
    pthread_cond_init (&condvar, NULL);
        
    if((pthread_create(&mythread1, &myattr, t1_fun,  (void*)t_arg) != 0)){
        printf("Error, thread not created properly ");
        return 1;
    }
    t_arg = 2;
    if((pthread_create(&mythread2, &myattr, condVar_fun,  (void*)t_arg) != 0)){
        printf("Error, thread not created properly ");
        return 1;
    }
   
    pthread_attr_destroy(&myattr);
    if(pthread_join(mythread1, &joinResult) != 0 ){
        printf("Error pthread join  ");
        return 1;
    }
    printf("Main : Thread1 joined with result of %d ", (int)joinResult);
        if

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