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author | Russ Cox <rsc@golang.org> | 2014-09-08 00:08:51 -0400 |
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committer | Russ Cox <rsc@golang.org> | 2014-09-08 00:08:51 -0400 |
commit | 8528da672cc093d4dd06732819abc1f7b6b5a46e (patch) | |
tree | 334be80d4a4c85b77db6f6fdb67cbf0528cba5f5 /src/sync/once.go | |
parent | 73bcb69f272cbf34ddcc9daa56427a8683b5a95d (diff) | |
download | go-8528da672cc093d4dd06732819abc1f7b6b5a46e.tar.gz |
build: move package sources from src/pkg to src
Preparation was in CL 134570043.
This CL contains only the effect of 'hg mv src/pkg/* src'.
For more about the move, see golang.org/s/go14nopkg.
Diffstat (limited to 'src/sync/once.go')
-rw-r--r-- | src/sync/once.go | 43 |
1 files changed, 43 insertions, 0 deletions
diff --git a/src/sync/once.go b/src/sync/once.go new file mode 100644 index 000000000..161ae3b3e --- /dev/null +++ b/src/sync/once.go @@ -0,0 +1,43 @@ +// Copyright 2009 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package sync + +import ( + "sync/atomic" +) + +// Once is an object that will perform exactly one action. +type Once struct { + m Mutex + done uint32 +} + +// Do calls the function f if and only if Do is being called for the +// first time for this instance of Once. In other words, given +// var once Once +// if once.Do(f) is called multiple times, only the first call will invoke f, +// even if f has a different value in each invocation. A new instance of +// Once is required for each function to execute. +// +// Do is intended for initialization that must be run exactly once. Since f +// is niladic, it may be necessary to use a function literal to capture the +// arguments to a function to be invoked by Do: +// config.once.Do(func() { config.init(filename) }) +// +// Because no call to Do returns until the one call to f returns, if f causes +// Do to be called, it will deadlock. +// +func (o *Once) Do(f func()) { + if atomic.LoadUint32(&o.done) == 1 { + return + } + // Slow-path. + o.m.Lock() + defer o.m.Unlock() + if o.done == 0 { + f() + atomic.StoreUint32(&o.done, 1) + } +} |