Question

I am trying to write a customized threadpool suited to my purpose using pthreads, and I am new to pthreads. I read these (POSIX threads programming and Linux Tutorial Posix Threads) tutorials online and they were quite helpful, but i still have some (maybe silly) doubts regarding mutexes and condition variables:

1. What is the scope of a mutex? Will a global mutex lock all the global variables so that only one thread can access them at a time? If i have two global mutexes, would they lock the same set of variables? What about a mutex that is declared inside a class or a function, what will happen when i lock/unlock it?
2. If i just plan to just read a global variable, and not modify it at all, should i still use a mutex lock?
3. If i am correct, a condition variable is used to wake up other threads which are sleeping (or blocked using pthread_cond_wait()) on some condition. The wake up call to sleeping threads is given by pthread_cond_signal() or pthread_cond_broadcast() from some other thread. How is the flow of control supposed to occur so that some all or one thread wake(s) up to do a work and wait until next work is available? I am particularly interested in a scenario with 4 threads.
4. Is there a way to set the affinity of a thread to a particular processor core before it is created (so that it starts execution on the desired core and no shifting of cores occur after creation)?

I am sorry if the questions look silly, but as i said, i am new to this. Any help, comments, code or pointer to good resources is appreciated. thanks in advance for your help.

Answer 1

That’s a lot of questions. A few answers.

(1a) The scope of a mutex is whatever you program it to be. In that sense it is no different from any other kind of variable.

(1b) A global mutex will protect whatever variables you program it to protect. I think from your other questions you might have a fundamental misunderstanding here. There is nothing magical about mutexes. You can’t just declare one and say “Ok, protect these variables”, you have to incorporate the mutex in your code. So if you have two functions that use variable X and one does a mutex lock/unlock around any changes to the variable and the other function completely ignores that a mutex even exists you really aren’t protecting anything. The best example I can think of is advisory file locks – one program can use them but if another doesn’t then that file isn’t locked.

(1c) As a rule, don’t have multiple mutexes locking the same data. It is an invitation to problems. Again the use of mutexes depends on programmed cooperation. If function A is protecting data B with mutex C while function D is protecting data B with mutex E then data B isn’t protected at all. Function A can hold the lock on mutex C but since function D pays no attention to it it will just overwrite data B anyway.

(1d) Basic scoping rules apply.

(2) No. If the variable isn’t going to change in any way that would make it inconsistent among threads then you don’t need to lock it.

(3) There are a number of detailed answers on this on SO that go into considerable detail on this. Search around a bit.

(4) Not that I am aware.