What does mutex and semaphore actually do?

Question

I want some clarification regarding mutex and semaphore.
My question is,

1. What mutex actually do when a thread tries to enter a region locked by a mutex, a. it waits for the lock to be released? or b. it goes to sleep until the lock is released. In that case how it is wake up again when the lock is released?
2. Same question as 1, but in this case it is semaphore.
3. Can you give me some code regarding busy waiting in pthread in C, and also a case where thread goes to sleep instead of waiting? does sleep mean it is blocked or sleeping is another kind of busy waiting?
4. i want to know some programs where this situations are covered, for example some c source codes where busy waiting, blocking etc are implemented.

Answer 1

When a thread tries to acquire a lock on a mutex, if that mutex is already held then typically it will use a call to the OS kernel to indicate that it is waiting, and then when the thread that currently holds the lock unlocks the mutex then it will make a call to the OS kernel to wake one of the waiting threads.

The same applies to a semaphore, except it only blocks if the count is decremented below zero, and threads are only woken when the count is increased back above zero.

A busy wait is where you don't block or sleep when waiting for something, but repeatedly poll in a loop, so the processor is always busy, but not doing anything useful.

To truly achieve a busy wait, you need an atomic variable, but POSIX threads does not provide such a thing, so you cannot truly write a busy wait in pthreads. The closest you can get is to lock a mutex, read a flag, unlock the mutex, loop if the flag was not set. This repeatedly locks and unlocks the mutex, but does not wait for the data to be ready. In this situation you should use a condition variable instead.

Typically, you say a thread is sleeping if it has called something like usleep to suspend its own execution for a specified period of time. This is as opposed to to blocking, where it is waiting for a specific signal which will be provided by another thread.

Answer 2

Please take a look at: https://stackoverflow.com/a/24582076/3163691

Yes, both mutex and semaphore are synchronizing kernel objects which when a thread tries to acquire one of them, this thread is put to sleep if that object is already owned by other thread.

As you already guessed, this sleeping is a crucial feature because it allows other threads to do more useful work than just 'looping/polling'.

The sleeping of one of these threads ends when the thread who owns the object release it.

[OS Scheduler doesn't give any execution slice to sleeping threads].

Contrast it with a lock & spinlock where a thread is in a 'looping/busy-waiting' state wasting precious CPU time doing almost nothing. Therefore, spinlocks should be avoided in User code. Use a critical section, mutex or semaphore instead!.

As you can see from the above link, both objects are different and should be used in the right context which fits best.

Think of a mutex as a lock which allows just one thread to own it. And that it has many safe attributes (ownership, termination notification, recursion, etc.).

And, think of a semaphore as a lock which allows just a specified number of threads to own it. However, it doesn't have the many useful attributes of a mutex.

Hope this helps.