What is sync.Cond and when do you use it instead of channels?

sync.Cond is a condition variable — a synchronisation primitive for goroutines to wait for or announce a state change on a shared resource. Use it when: many goroutines wait for a complex shared-state predicate; waking all waiters at once is needed; or channels would add unnecessary data-passing overhead.

// Bounded buffer using sync.Cond
type BoundedQueue struct {
    mu       sync.Mutex
    notFull  *sync.Cond
    notEmpty *sync.Cond
    items    []any
    capacity int
}

func NewBoundedQueue(cap int) *BoundedQueue {
    q := &BoundedQueue{capacity: cap}
    q.notFull  = sync.NewCond(&q.mu)
    q.notEmpty = sync.NewCond(&q.mu)
    return q
}

func (q *BoundedQueue) Put(item any) {
    q.mu.Lock()
    defer q.mu.Unlock()
    for len(q.items) == q.capacity { // LOOP — never if! (spurious wakeups)
        q.notFull.Wait()             // atomically releases lock and sleeps
    }
    q.items = append(q.items, item)
    q.notEmpty.Signal() // wake one waiting consumer
}

func (q *BoundedQueue) Get() any {
    q.mu.Lock()
    defer q.mu.Unlock()
    for len(q.items) == 0 {
        q.notEmpty.Wait()
    }
    item := q.items[0]
    q.items = q.items[1:]
    q.notFull.Signal()
    return item
}

// cond.Signal()    — wake ONE goroutine (when only one should act)
// cond.Broadcast() — wake ALL goroutines (when state change affects all)

Key rule: always check the condition in a for loop, never an if. cond.Wait() can return spuriously (OS wakeup without Signal/Broadcast), so the condition must be re-verified after each wakeup. Wait() atomically releases the associated mutex and sleeps; the mutex is re-acquired before returning.

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