Pthread - What is the difference between time.h::sleep() and pthread.h::pthread_yield()?

Question

I spent a good long while looking for info on the differences between time.h::sleep() and pthread.h::pthread_yield() but was unable to find any solid reference material and so I am posting this question.

What is the difference between time.h::sleep() and pthread.h::pthread_yield()?

Update:

The reason I ask is because I was using sleep() to sleep() each individual thread... and my application started having issues when there was 8 threads vs 4 threads. When I went online to see if sleep() only affects each thread, I couldn't find any good reference stating whether Sleep() affects the entire process OR sleep() only affects the individual thread.

Answer 1

sleep() causes your program to stop executing for a certain length of time. No matter what else happens on the system, your thread will not start again until at least the length of time passed to sleep() has elapsed. pthread_yield() notifies the operating system that your thread is done working, and that it can switch execution to another thread. However, if there is no higher-priority thread that needs to do work at that time, your thread may start again immediately.

IOWs, after sleep() your thread is guaranteed to stop running even if no one else needs to run, while pthread_yield() is just a polite way to give other threads a chance to run if they need to.

Update in response to question update: both sleep() and pthread_yield() affect only the calling thread.

Answer 2

sleep(s) takes the current thread of execution and suspends it until s seconds have passed (or it is woken up by a signal.)

In more practical terms, when you call sleep(), that thread will cease execution and just... wait until the specified time has passed. Once it passes, that thread is placed into the ready queue.

pthread_yield() says "take this thread, and put it into the ready queue." Your thread will stop execution and be in the 'waiting' state to be selected/run by the scheduler. This does not gaurantee that your thread will not immediately resume running. But it gives another thread a chance to run at a given point in its execution.

I am going to go out on a limb and say that sleep(0) would accomplish the same thing as a pthread_yield() - both stopping execution and placing the thread in the ready queue.

Answer 3

From pthread_yield:

The pthread_yield subroutine forces the calling thread to relinquish use of its processor, and to wait in the run queue before it is scheduled again. If the run queue is empty when the pthread_yield subroutine is called, the calling thread is immediately rescheduled.

From the sleep manpage:

sleep() makes the calling process sleep until seconds seconds have elapsed or a signal arrives which is not ignored.

If you don't want to have a real time delay in your threads and just want to allow other threads to do their work, then pthread_yield is better suited than sleep.