作者： chexlong (1 篇文章) 日期： 二月 14, 2012 在 5:34 下午
读写锁实际是一种特殊的自旋锁，它把对共享资源的访问者划分成读者和写者，读者只对共享资源进行读访问，写者则需要对共享资源进行写操作。这种锁相对于自旋锁而言，能提高并发性，因为在多处理器系统中，它允许同时有多个读者来访问共享资源，最大可能的读者数为实际的逻辑CPU数。写者是排他性的，一个读写锁同时只能有一个写者或多个读者（与CPU数相关），但不能同时既有读者又有写者。
现在Win32的API，用C++实现自己的读写锁。这组API包括：CreateMutex，CreateEvent，WaitForSingleObject，WaitForMultipleObjects，ResetEvent，ReleaseMutex，SetEvent，CloseHandle。以下代码在VS2005下，已经编译通过。
RWLockImpl.h
01.#ifndef _RWLockImpl_Header
02.#define _RWLockImpl_Header
03.
04.#include
05.#include
06.#include
07.#include
08.
09.using namespace std;
10.
11./*
12. 读写锁允许当前的多个读用户访问保护资源，但只允许一个写读者访问保护资源
13.*/
14.
15.//-----------------------------------------------------------------
16.class CRWLockImpl
17.{
18.protected:
19. CRWLockImpl();
20. ~CRWLockImpl();
21. void ReadLockImpl();
22. bool TryReadLockImpl();
23. void WriteLockImpl();
24. bool TryWriteLockImpl();
25. void UnlockImpl();
26.
27.private:
28. void AddWriter();
29. void RemoveWriter();
30. DWORD TryReadLockOnce();
31.
32. HANDLE m_mutex;
33. HANDLE m_readEvent;
34. HANDLE m_writeEvent;
35. unsigned m_readers;
36. unsigned m_writersWaiting;
37. unsigned m_writers;
38.};
39.
40.//-----------------------------------------------------------------
41.
42.class CMyRWLock: private CRWLockImpl
43.{
44.public:
45.
46. //创建读/写锁
47. CMyRWLock(){};
48.
49. //销毁读/写锁
50. ~CMyRWLock(){};
51.
52. //获取读锁
53. //如果其它一个线程占有写锁，则当前线程必须等待写锁被释放，才能对保护资源进行访问
54. void ReadLock();
55.
56. //尝试获取一个读锁
57. //如果获取成功，则立即返回true，否则当另一个线程占有写锁，则返回false
58. bool TryReadLock();
59.
60. //获取写锁
61. //如果一个或更多线程占有读锁，则必须等待所有锁被释放
62. //如果相同的一个线程已经占有一个读锁或写锁，则返回结果不确定
63. void WriteLock();
64.
65. //尝试获取一个写锁
66. //如果获取成功，则立即返回true，否则当一个或更多其它线程占有读锁，返回false
67. //如果相同的一个线程已经占有一个读锁或写锁，则返回结果不确定
68. bool TryWriteLock();
69.
70. //释放一个读锁或写锁
71. void Unlock();
72.
73.private:
74. CMyRWLock(const CMyRWLock&);
75. CMyRWLock& operator = (const CMyRWLock&);
76.};
77.
78.inline void CMyRWLock::ReadLock()
79.{
80. ReadLockImpl();
81.}
82.
83.inline bool CMyRWLock::TryReadLock()
84.{
85. return TryReadLockImpl();
86.}
87.
88.inline void CMyRWLock::WriteLock()
89.{
90. WriteLockImpl();
91.}
92.
93.inline bool CMyRWLock::TryWriteLock()
94.{
95. return TryWriteLockImpl();
96.}
97.
98.inline void CMyRWLock::Unlock()
99.{
100. UnlockImpl();
101.}
102.
103.
104.#endif
RWLockImpl.cpp
01.#include "RWLockImpl.h"
02.
03.CRWLockImpl::CRWLockImpl(): m_readers(0), m_writersWaiting(0), m_writers(0)
04.{
05. m_mutex = CreateMutex(NULL, FALSE, NULL);
06. if (m_mutex == NULL)
07. cout<<"cannot create reader/writer lock"<<endl;
08.
09. m_readEvent = CreateEvent(NULL, TRUE, TRUE, NULL);
10. if (m_readEvent == NULL)
11. cout<<"cannot create reader/writer lock"<<endl;
12.
13. m_writeEvent = CreateEvent(NULL, TRUE, TRUE, NULL);
14. if (m_writeEvent == NULL)
15. cout<<"cannot create reader/writer lock"<<endl;
16.}
17.
18.CRWLockImpl::~CRWLockImpl()
19.{
20. CloseHandle(m_mutex);
21. CloseHandle(m_readEvent);
22. CloseHandle(m_writeEvent);
23.}
24.
25.inline void CRWLockImpl::AddWriter()
26.{
27. switch (WaitForSingleObject(m_mutex, INFINITE))
28. {
29. case WAIT_OBJECT_0:
30. if (++m_writersWaiting == 1)
31. ResetEvent(m_readEvent);
32. ReleaseMutex(m_mutex);
33. break;
34. default:
35. cout<<"cannot lock reader/writer lock"<<endl;
36. }
37.}
38.
39.inline void CRWLockImpl::RemoveWriter()
40.{
41. switch (WaitForSingleObject(m_mutex, INFINITE))
42. {
43. case WAIT_OBJECT_0:
44. if (--m_writersWaiting == 0 && m_writers == 0)
45. SetEvent(m_readEvent);
46. ReleaseMutex(m_mutex);
47. break;
48. default:
49. cout<<"cannot lock reader/writer lock"<<endl;
50. }
51.}
52.
53.void CRWLockImpl::ReadLockImpl()
54.{
55. HANDLE h[2];
56. h[0] = m_mutex;
57. h[1] = m_readEvent;
58. switch (WaitForMultipleObjects(2, h, TRUE, INFINITE))
59. {
60. case WAIT_OBJECT_0:
61. case WAIT_OBJECT_0 + 1:
62. ++m_readers;
63. ResetEvent(m_writeEvent);
64. ReleaseMutex(m_mutex);
65. assert(m_writers == 0);
66. break;
67. default:
68. cout<<"cannot lock reader/writer lock"<<endl;
69. }
70.}
71.
72.bool CRWLockImpl::TryReadLockImpl()
73.{
74. for (;;)
75. {
76. if (m_writers != 0 || m_writersWaiting != 0)
77. return false;
78.
79. DWORD result = TryReadLockOnce();
80. switch (result)
81. {
82. case WAIT_OBJECT_0:
83. case WAIT_OBJECT_0 + 1:
84. return true;
85. case WAIT_TIMEOUT:
86. continue;
87. default:
88. cout<<"cannot lock reader/writer lock"<<endl;
89. }
90. }
91.}
92.
93.void CRWLockImpl::WriteLockImpl()
94.{
95. AddWriter();
96. HANDLE h[2];
97. h[0] = m_mutex;
98. h[1] = m_writeEvent;
99. switch (WaitForMultipleObjects(2, h, TRUE, INFINITE))
100. {
101. case WAIT_OBJECT_0:
102. case WAIT_OBJECT_0 + 1:
103. --m_writersWaiting;
104. ++m_readers;
105. ++m_writers;
106. ResetEvent(m_readEvent);
107. ResetEvent(m_writeEvent);
108. ReleaseMutex(m_mutex);
109. assert(m_writers == 1);
110. break;
111. default:
112. RemoveWriter();
113. cout<<"cannot lock reader/writer lock"<<endl;
114. }
115.}
116.
117.bool CRWLockImpl::TryWriteLockImpl()
118.{
119. AddWriter();
120. HANDLE h[2];
121. h[0] = m_mutex;
122. h[1] = m_writeEvent;
123. switch (WaitForMultipleObjects(2, h, TRUE, 1))
124. {
125. case WAIT_OBJECT_0:
126. case WAIT_OBJECT_0 + 1:
127. --m_writersWaiting;
128. ++m_readers;
129. ++m_writers;
130. ResetEvent(m_readEvent);
131. ResetEvent(m_writeEvent);
132. ReleaseMutex(m_mutex);
133. assert(m_writers == 1);
134. return true;
135. case WAIT_TIMEOUT:
136. RemoveWriter();
137. default:
138. RemoveWriter();
139. cout<<"cannot lock reader/writer lock"<<endl;
140. }
141. return false;
142.}
143.
144.void CRWLockImpl::UnlockImpl()
145.{
146. switch (WaitForSingleObject(m_mutex, INFINITE))
147. {
148. case WAIT_OBJECT_0:
149. m_writers = 0;
150. if (m_writersWaiting == 0) SetEvent(m_readEvent);
151. if (--m_readers == 0) SetEvent(m_writeEvent);
152. ReleaseMutex(m_mutex);
153. break;
154. default:
155. cout<<"cannot unlock reader/writer lock"<<endl;
156. }
157.}
158.
159.DWORD CRWLockImpl::TryReadLockOnce()
160.{
161. HANDLE h[2];
162. h[0] = m_mutex;
163. h[1] = m_readEvent;
164. DWORD result = WaitForMultipleObjects(2, h, TRUE, 1);
165. switch (result)
166. {
167. case WAIT_OBJECT_0:
168. case WAIT_OBJECT_0 + 1:
169. ++m_readers;
170. ResetEvent(m_writeEvent);
171. ReleaseMutex(m_mutex);
172. assert(m_writers == 0);
173. return result;
174. case WAIT_TIMEOUT:
175. default:
176. cout<<"cannot lock reader/writer lock"<<endl;
177. }
178. return result;
179.}
180.
下边是测试代码
01.// MyRWLockWin32.cpp : 定义控制台应用程序的入口点。
02.//
03.
04.#include "RWLockImpl.h"
05.
06.//创建一个读写锁对象
07.CMyRWLock g_myRWLock;
08.volatile int g_counter = 0;
09.
10.//线程函数
11.unsigned int __stdcall StartThread(void *pParam)
12.{
13. int lastCount = 0;
14. for (int i = 0; i < 10000; ++i)
15. {
16. g_myRWLock.ReadLock();
17. lastCount = g_counter;
18. //在读锁域，两个线程不断循环交替访问全局变量g_counter
19. for (int k = 0; k < 100; ++k)
20. {
21. if (g_counter != lastCount)
22. cout<<"the value of g_counter has been updated."<<endl;
23. Sleep(0);
24. }
25. g_myRWLock.Unlock();
26.
27.
28. g_myRWLock.WriteLock();
29. //在写锁域，只有一个线程可以修改全局变量g_counter的值
30. for (int k = 0; k < 100; ++k)
31. {
32. --g_counter;
33. Sleep(0);
34. }
35. for (int k = 0; k < 100; ++k)
36. {
37. ++g_counter;
38. Sleep(0);
39. }
40. ++g_counter;
41. if (g_counter <= lastCount)
42. cout<<"the value of g_counter is error."<<endl;
43. g_myRWLock.Unlock();
44. }
45.
46. return (unsigned int)0;
47.}
48.
49.int main(int argc, char* argv[])
50.{
51. HANDLE hThread1, hThread2;
52. unsigned int uiThreadId1, uiThreadId2;
53.
54. //创建两个工作线程
55. hThread1 = (HANDLE)_beginthreadex(NULL, 0, &StartThread, (void *)NULL, 0, &uiThreadId1);
56. hThread2 = (HANDLE)_beginthreadex(NULL, 0, &StartThread, (void *)NULL, 0, &uiThreadId2);
57.
58. //等待线程结束
59. DWORD dwRet = WaitForSingleObject(hThread1,INFINITE);
60. if ( dwRet == WAIT_TIMEOUT )
61. {
62. TerminateThread(hThread1,0);
63. }
64. dwRet = WaitForSingleObject(hThread2,INFINITE);
65. if ( dwRet == WAIT_TIMEOUT )
66. {
67. TerminateThread(hThread2,0);
68. }
69.
70. //关闭线程句柄，释放资源
71. CloseHandle(hThread1);
72. CloseHandle(hThread2);
73.
74. assert (g_counter == 20000);
75.
76. system("pause");
77. return 0;
78.}
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