Conditional Variable vs Semaphore

Question

When should one use a semaphore and when should one use a conditional variable (CondVar) ?

Answer 1

Let's reveal what's under the hood.

Conditional variable is essentially a wait-queue, that supports blocking-wait and wakeup operations, i.e. you can put a thread into the wait-queue and set its state to BLOCK, and get a thread out from it and set its state to READY.

Note that to use a conditional variable, two other elements are needed:

• a condition (typically implemented by checking a flag or a counter)
• a mutex that protects the condition

The protocol then becomes,

1. acquire mutex
2. check condition
3. block and release mutex if condition is true, else release mutex

Semaphore is essentially a counter + a mutex + a wait queue. And it can be used as it is without external dependencies. You can use it either as a mutex or as a conditional variable.

Therefore, semaphore can be treated as a more sophisticated structure than conditional variable, while the latter is more lightweight and flexible.