c - running 2 threads in parallel with a shared variable

Question

Just a beginner to threads, I'm just doing a task which involves these 2 threads.

#include <stdio.h>
#include <pthread.h>

int count = 0;

void waitFor(unsigned int secs)
{
    unsigned int retTime = time(0) + secs;
    while(time(0) < retTime);
}

void func1(void * args)
{
    printf("In func1 ...\n");
    long i = 0;
    while(1){
        i++;
        if(count == 1)
            break;
    }
    printf("The total number counted is: %ld \n", i);
    count = 0;
    i = 0;
}

void func2(void * args)
{
    printf("In func2 ...\n");
    waitFor(3);
    count = 1;
}


int main()
{
    pthread_t th1, th2;

    int j = 0;
    while(j++ < 4){
        printf("\nRound:\t%d\n", j);

        pthread_create(&th1, NULL, (void*)func1,NULL);
        pthread_create(&th2, NULL, (void*)func2, NULL);

        pthread_join(th1,NULL);
        pthread_join(th2,NULL);
        waitFor(3);
    }

    return 0;
}

I've read various references and to my understanding pthread_join() means that if there are 2 or more threads, then they will wait for one thread to finish its execution and then next one will start executing and so on.

But when i run this program, the moment pthread_join(th1) is executed, both threads are created and executed 'concurrently'. How is this happening? Output:

Round:  1
In func2 ...
In func1 ...
The total number counted is: 897651254 

Round:  2
In func1 ...
In func2 ...
The total number counted is: 1051386065

........

My goal is to run these 2 threads in parallel. For now, join seems to do this; or am I going wrong somewhere?

And I've read that using volatile is not preferred for threads in C. So is there any way I could use count as a signal from thread 2 to 1?

Answer 1

Quote:

my understanding pthread_join() means that if there are 2 or more threads, then they will wait for one thread to finish its execution and then next one will start executing and so on

That is incorrect. Join simply means that the process waits until the thread has terminated.

Quote:

the moment pthread_join(th1) is executed, both threads are created and executed 'concurrently'.

That is incorrect. The threads are created and start when calling pthread_create Note: By start I mean that they are ready to execute. However, it is the OS that decides when they actually get to execute so it may take some time before they execute.

To share count between two threads you can use a mutex.

int count = 0;
pthread_mutex_t lock;

When accessing count you must first lock the mutex, read/write the variable and unlock the mutex.

Example:

pthread_mutex_lock(&lock);
count = 1;
pthread_mutex_unlock(&lock);

Example:

long i = 0;
while(1)
{
    ... code not using count ....

    pthread_mutex_lock(&lock);
    if(count == 1) 
    {
        pthread_mutex_unlock(&lock);
        break;
    }
    pthread_mutex_unlock(&lock);

    ... code not using count ....
}

And in main you'll need to initialize the mutex like:

pthread_mutex_init(&lock,NULL);