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1 /* | |
2 * Copyright 2015 Google Inc. | |
3 * | |
4 * Use of this source code is governed by a BSD-style license that can be | |
5 * found in the LICENSE file. | |
6 */ | |
7 | |
8 #ifndef SkSemaphore_DEFINED | |
9 #define SkSemaphore_DEFINED | |
10 | |
11 #include "SkTypes.h" | |
12 #include "SkAtomics.h" | |
13 | |
14 /** | |
15 * SkSemaphore is a fast mostly-user-space semaphore. | |
16 * | |
17 * A semaphore is logically an atomic integer with a few special properties: | |
18 * - The integer always starts at 0. | |
19 * - You can only increment or decrement it, never read or write it. | |
20 * - Increment is spelled 'signal()'; decrement is spelled 'wait()'. | |
21 * - If a call to wait() decrements the counter to <= 0, | |
22 * the calling thread sleeps until another thread signal()s it back above 0. | |
23 */ | |
24 class SkSemaphore : SkNoncopyable { | |
25 public: | |
26 // Initializes the counter to 0. | |
27 // (Though all current implementations could start from an arbitrary value.) | |
28 SkSemaphore(); | |
29 ~SkSemaphore(); | |
30 | |
31 // Increment the counter N times. | |
32 // Generally it's better to call signal(N) instead of signal() N times. | |
33 void signal(int N = 1); | |
34 | |
35 // Decrement the counter by 1, | |
36 // then if the counter is <= 0, sleep this thread until the counter is > 0. | |
37 void wait(); | |
38 | |
39 private: | |
40 // This implementation follows the general strategy of | |
41 // 'A Lightweight Semaphore with Partial Spinning' | |
42 // found here | |
43 // http://preshing.com/20150316/semaphores-are-surprisingly-versatile/ | |
44 // That article (and entire blog) are very much worth reading. | |
45 // | |
46 // We wrap an OS-provided semaphore with a user-space atomic counter that | |
47 // lets us avoid interacting with the OS semaphore unless strictly required: | |
48 // moving the count from >0 to <=0 or vice-versa, i.e. sleeping or waking th
reads. | |
49 struct OSSemaphore; | |
50 | |
51 SkAtomic<int> fCount; | |
52 OSSemaphore* fOSSemaphore; | |
53 }; | |
54 | |
55 #endif//SkSemaphore_DEFINED | |
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