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/*
* Copyright 2015 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#ifndef SkSemaphore_DEFINED
#define SkSemaphore_DEFINED
#include "SkTypes.h"
#include "SkAtomics.h"
/**
* SkSemaphore is a fast mostly-user-space semaphore.
*
* A semaphore is logically an atomic integer with a few special properties:
* - The integer always starts at 0.
* - You can only increment or decrement it, never read or write it.
* - Increment is spelled 'signal()'; decrement is spelled 'wait()'.
* - If a call to wait() decrements the counter to <= 0,
* the calling thread sleeps until another thread signal()s it back above 0.
*/
class SkSemaphore : SkNoncopyable {
public:
// Initializes the counter to 0.
// (Though all current implementations could start from an arbitrary value.)
SkSemaphore();
~SkSemaphore();
// Increment the counter N times.
// Generally it's better to call signal(N) instead of signal() N times.
void signal(int N = 1);
// Decrement the counter by 1,
// then if the counter is <= 0, sleep this thread until the counter is > 0.
void wait();
private:
// This implementation follows the general strategy of
// 'A Lightweight Semaphore with Partial Spinning'
// found here
// http://preshing.com/20150316/semaphores-are-surprisingly-versatile/
// That article (and entire blog) are very much worth reading.
//
// We wrap an OS-provided semaphore with a user-space atomic counter that
// lets us avoid interacting with the OS semaphore unless strictly required:
// moving the count from >0 to <=0 or vice-versa, i.e. sleeping or waking threads.
struct OSSemaphore;
SkAtomic<int> fCount;
OSSemaphore* fOSSemaphore;
};
#endif//SkSemaphore_DEFINED
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