ndiag | level architecture diagramming/documentation tool | Architecture library

If you want to wait for a specific condition to happen use the class of functions pthread_cond_*.

If you just want to wait for the thread to do their stuff remove while(!wakeup){}. This works because

int pthread_join(pthread_t thread, void **value_ptr);

The pthread_join() function suspends execution of the calling thread until the target thread terminates, unless the target thread has already terminated.

So in the following snippet you first create limit threads and they start running right away. Then you wait for every thread to end, that is you suspend the calling thread until the termination of limit threads.

Your second code is wrong and has an undefined behavior. The reason for that is that by declaring nEle, si and sj private, you create some local (per-thread) versions of these variables, without giving them any value. Therefore, nEle notably, which is the upper bound of you for loop, can have whatever value, likely increasing quite dramatically the length of your computation.

In order to fix your code, the snippet you gave should look like this (with a few simplifications, not tested obviously):