CODEWITHSUNDEEP
multithreadingc++11
Kasata Ata
Kasata Ata

Reputation: 125

Locking mutex before using std::condition_variable

I have a question related to std::condition_variable. I read a lot about it and all examples shown locked mutex before using std::condition_variable like that:

std::unique_lock<std::mutex> lock(mutex);
condition_variable.wait(lock);
//...

or like that:

std::unique_lock<std::mutex> lock(mutex);
condition_variable.notify_one();
//...

Is it necessery to lock mutex before using condition variable or is it thread-safe?

Upvotes: 0

Views: 58

Answers (1)

cantordust
cantordust

Reputation: 1612

As mentioned in the comments, notify_one() doesn't need a locked mutex in order to be called, but wait(lock) does. condition_variable is somewhat unfortunately named because it gives the option to wait unconditionally for something to happen. A more useful version is wait(lock, condition), where the thread waits only if the condition is not true, otherwise it doesn't wait at all.

An analogy can be made with a hotel reception desk, where the condition_variable in a worker thread is a receptionist, the mutex represents a critical shared resource (say, a computer for making reservations), and the condition is the presence of patrons. Checking into a hotel involves a lot of steps, most of which can be done in parallel by different receptionists, but that one step of entering your details into the computer can't - receptionists have to use the computer one at a time.

Assume that the first thing that receptionists do when they get to work is check if the computer is free (no other receptionists are using it). In the case of wait(lock), receptionists will do that and then immediately fall asleep regardless of whether there are people waiting at the reception to be checked in. In the case of wait(lock, condition) they will fall asleep only if the condition is not true (there are no people waiting).

Now, if a receptionist is asleep, they are not going to notice when there are people at the desk waiting to be checked in. That's what notify_one() does - wakes up a receptionist. The action of waking up a receptionist does not depend on the state of the computer (the mutex) - you can keep ringing the bell at the reception desk even if all the receptionists are awake and checking people in like crazy (think a 4-year-old left without supervision who finds the bell...). notify_one() wakes up one receptionist, whereas notify_all() wakes up all of them.

The analogy is not perfect, but it illustrates the dependence between a condition_variable and a locked mutex.

Upvotes: 1

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