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package semaphore

import ""

Package semaphore provides a weighted semaphore implementation.

Example_workerPool demonstrates how to use a semaphore to limit the number of goroutines working on parallel tasks.

This use of a semaphore mimics a typical “worker pool” pattern, but without the need to explicitly shut down idle workers when the work is done.


ctx := context.TODO()

var (
    maxWorkers = runtime.GOMAXPROCS(0)
    sem        = semaphore.NewWeighted(int64(maxWorkers))
    out        = make([]int, 32)

// Compute the output using up to maxWorkers goroutines at a time.
for i := range out {
    // When maxWorkers goroutines are in flight, Acquire blocks until one of the
    // workers finishes.
    if err := sem.Acquire(ctx, 1); err != nil {
        log.Printf("Failed to acquire semaphore: %v", err)

    go func(i int) {
        defer sem.Release(1)
        out[i] = collatzSteps(i + 1)

// Acquire all of the tokens to wait for any remaining workers to finish.
// If you are already waiting for the workers by some other means (such as an
// errgroup.Group), you can omit this final Acquire call.
if err := sem.Acquire(ctx, int64(maxWorkers)); err != nil {
    log.Printf("Failed to acquire semaphore: %v", err)



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Package Files


type Weighted Uses

type Weighted struct {
    // contains filtered or unexported fields

Weighted provides a way to bound concurrent access to a resource. The callers can request access with a given weight.

func NewWeighted Uses

func NewWeighted(n int64) *Weighted

NewWeighted creates a new weighted semaphore with the given maximum combined weight for concurrent access.

func (*Weighted) Acquire Uses

func (s *Weighted) Acquire(ctx context.Context, n int64) error

Acquire acquires the semaphore with a weight of n, blocking until resources are available or ctx is done. On success, returns nil. On failure, returns ctx.Err() and leaves the semaphore unchanged.

If ctx is already done, Acquire may still succeed without blocking.

func (*Weighted) Release Uses

func (s *Weighted) Release(n int64)

Release releases the semaphore with a weight of n.

func (*Weighted) TryAcquire Uses

func (s *Weighted) TryAcquire(n int64) bool

TryAcquire acquires the semaphore with a weight of n without blocking. On success, returns true. On failure, returns false and leaves the semaphore unchanged.

Package semaphore imports 3 packages (graph) and is imported by 228 packages. Updated 2020-02-27. Refresh now. Tools for package owners.