How to use std::mutex in different threads?

Question

How to properly write multithreaded code with mutex:

std::mutex m, m2;    

... thread
m2.lock(); 
if ((++reference) == 1) m.lock(); 
m2.unlock();

... differenet thread
m2.lock();
if ((reference--) == 0) m.unlock(); // error here
m2.unlock ();

When I call m.unlock() visual studio 2012 raises error R6010. Mutex m2 works fine, because it locks and unlocks in one thread.

I tried to replace code with std::contidional_variable, but its not notified at start and first enter to cond_var.wait_one waits infinitely.

UPD: Replace with conditional_variable and all works fine now. Article: C++0x has no semaphores? How to synchronize threads?

In third thread this mutex is locks, do some work and unlocks. Inside this thread everything looks fine. I commented it and nothing changed (error R6010 raised). — Alexey, Sep 24 '16 at 09:51
Note that the parentheses around the increment and decrement of `reference` are not needed (and distracting). — Pete Becker, Sep 24 '16 at 13:33

score 1 · Answer 1 · edited May 23 '17 at 12:06

Mutex needs to be unlocked by the owning thread (the thread that locked it):

If the mutex is not currently locked by the calling thread, it causes undefined behavior. (http://www.cplusplus.com/reference/mutex/mutex/unlock/)

You'll need to use conditional variable - I think building a semaphore implementation around it will be nice in your case. See the accepted answer here: C++0x has no semaphores? How to synchronize threads?

How to use std::mutex in different threads?

1 Answers1