5 Condition variables and semaphores

In the previous chapter, we saw how we can use mutexes to protect critical sections of our code and prevent multiple goroutines from executing at the same time. Mutexes are not the only synchronization tool that we have available: condition variables give us extra controls that complement exclusive locking. They give us the ability to wait for a certain condition to occur before unblocking the execution. Semaphores go one step further than mutexes in that they allow us to control how many concurrent goroutines can execute a certain section at the same time. In addition, semaphores can be used to store a signal (of an occurring event) for later access by an execution.

Apart from being useful in our concurrent applications, condition variables and semaphores are additional primitive building blocks that we can use to build more complex tools and abstractions. In this chapter, we will also re-examine our readers-writer lock, developed in the previous chapter, and improve it using condition variables.

5.1 Condition variables

Condition variables give us extra functionality on top of mutexes. We can use them in situations where a goroutine needs to block and wait for a particular condition to occur. Let’s look at an example to understand how they’re used.

5.1.1 Combining mutexes with condition variables

5.1.2 Missing the signal

5.1.3 Synchronizing multiple goroutines with waits and broadcasts

5.1.4 Revisiting readers–writer locks using condition variables