How to stop and continue a pthread?

Question

I'm coding in C (actually in OOC which is then compiled to C).

How do I instruct a thread to wait at a particular checkpoint until some other thread tells it to continue?

I'm actually using a tight loop in the thread and polling a variable I'm changing from the main thread, but I think that is not well performing, right? Besides that, when I do a tight loop in a thread, should I include a sleep in the loop to avoid consuming much cpu power just looping?

Answer 1

It seems that you are looking for pthread_cond_wait and its related functions.

Here is a quote from the manpage with an example:

Consider two shared variables x and y, protected by the mutex mut, and a condition variable cond that is to be signaled whenever x becomes greater than y.

          int x,y;
          pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
          pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

Waiting until x is greater than y is performed as follows:

          pthread_mutex_lock(&mut);
          while (x <= y) {
                  pthread_cond_wait(&cond, &mut);
          }
          /* operate on x and y */
          pthread_mutex_unlock(&mut);

Modifications on x and y that may cause x to become greater than y should signal the condition if needed:

          pthread_mutex_lock(&mut);
          /* modify x and y */
          if (x > y) pthread_cond_broadcast(&cond);
          pthread_mutex_unlock(&mut);

In the end it does what you are asking for: the thread calling pthread_cond_wait(&cond) is blocked until an other thread (your main thread for example) calls pthread_cond_broadcast(&cond). During this time it is just in a blocked state and doesn't consume CPU cycles.