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\n", t_num, globalCount);
        }
        printf("t1_fun(): thread %d, globalCount = %d\n", 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 \n", t_num);
    pthread_mutex_lock(&mutex);
        
    pthread_cond_wait(&condvar, &mutex);
    printf("condVar_fun(): Thread %d Condition signal received \n", t_num);
    globalCount += 20;
    printf("condVar_fun(): Thread %d globalCount = %d \n", 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\n");
        return 1;
    }
    t_arg = 2;
    if((pthread_create(&mythread2, &myattr, condVar_fun,  (void*)t_arg) != 0)){
        printf("Error, thread not created properly\n");
        return 1;
    }
   
    pthread_attr_destroy(&myattr);
    if(pthread_join(mythread1, &joinResult) != 0 ){
        printf("Error pthread join \n");
        return 1;
    }
    printf("Main : Thread1 joined with result of %d\n", (int)joinResult);
        if(pthread_join(mythread2, &joinResult) != 0 ){
        printf("Error pthread join \n");
        return 1;
    }
    printf("Main : Thread2 joined with result of %d\n", (int)joinResult);
    printf("main finishes the work\n");
    printf("\nCount at end : %d\n", globalCount);
        
    pthread_attr_destroy(&myattr);
    pthread_mutex_destroy(&mutex);
    pthread_cond_destroy(&condvar);
    pthread_exit(NULL);
}
 

OUTPUT : 

t1_fun(): thread 1, globalCount = 1
Thread 2 created and running 
t1_fun(): thread 1, globalCount = 2
t1_fun(): thread 1, globalCount = 3
t1_fun(): thread 1, globalCount = 4
t1_fun(): thread 1, globalCount = 5 Signaled
t1_fun(): thread 1, globalCount = 5
condVar_fun(): Thread 2 Condition signal received 
condVar_fun(): Thread 2 globalCount = 25 
t1_fun(): thread 1, globalCount = 26
t1_fun(): thread 1, globalCount = 27
t1_fun(): thread 1, globalCount = 28
t1_fun(): thread 1, globalCount = 29
t1_fun(): thread 1, globalCount = 30
t1_fun(): thread 1, globalCount = 31
t1_fun(): thread 1, globalCount = 32
t1_fun(): thread 1, globalCount = 33
t1_fun(): thread 1, globalCount = 34
t1_fun(): thread 1, globalCount = 35
Main : Thread1 joined with result of 0
Main : Thread2 joined with result of 0
main finishes the work

Count at end : 35