~/Go sync.Cond Explained for Concurrent Goroutines

Jul 11, 2022

A Go sync.Cond allows goroutines to wait for or signal the occurrence of an event. It is built on top of a Locker, typically a Mutex.

Basic usage:

Create a Cond with NewCond(Locker).
Use Wait to block until a signal.
Use Signal to wake one waiter.
Use Broadcast to wake all waiters.

Common pattern for usage:

// Example using sync.Cond
var mu sync.Mutex
cond := sync.NewCond(&mu)
ready := false

go func() {
    mu.Lock()
    for !ready {
        cond.Wait()
    }
    mu.Unlock()
}()

mu.Lock()
ready = true
cond.Signal() // or cond.Broadcast()
mu.Unlock()

Always check conditions in a loop. The condition may not be true when unblocked. Learn why.

Alternatives: For simple cases, channels are easier. Signal mirrors sending on a channel, and Broadcast mirrors closing.

Notes:

Do not copy a Cond once used.
Wait releases the lock and suspends your goroutine until another goroutine calls Signal or Broadcast.
Locker must be held when observing or changing the condition.