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