Resume a program if variable's value changes

Question

Is there a way in c/c++ to pause a program (might be with SIGSTOP), and then to continue (SIGCONT ?) it when another thread changes a value of some variable?

For example:

int main(void) {

    int a = 0;
    createThread(&a); //creates a thread which will change a's value

    pauseMyself(); //pauses main process till a's value is changed

    /* main process gets here only when a's value has been changed */

    fprintf(stderr, "a's value has been changed\n");

    return 0;
}

I want to see if it is possible so I won't need to sample a's value each interval - Like a while() loop which checks a's value each 200ms (sleeps for 200ms)

One possible solution for this problem is using semaphores:

I should initialize semaphores value to 0 (!), Then in the main(), instead of pauseMyself() I'll do sem_wait(), means that the main process will stop there.

In the thread function I'll do sem_post() after it changes a's value, so the main process will continue.

Is there another, nicer way?

Thanks.

Answer 1

I believe what you need is pthread condition variable, a synchronization device:

A condition (short for ‘‘condition variable’’) is a synchronization device that allows threads to suspend execution and relinquish the processors until some predicate on shared data is satisfied. The basic operations on conditions are: signal the condition (when the predicate becomes true), and wait for the condition, suspending the thread execution until another thread signals the condition.