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Id
3960316
data
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0
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0
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3
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CreationDate
2010-10-18T14:47:43.390
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2015-08-10T12:39:28.117
LastEditDate
2015-08-10T12:39:28.117
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111698
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1008726
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2
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6
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text
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Old question, but this is something that should work. It doesn't spin on contention. Readers incur limited extra cost if they have little or no contention, because <code>SetEvent</code> is called lazily (look at the edit history for a more heavyweight version that doesn't have this optimization). <pre><code>#include <windows.h> typedef struct _RW_LOCK { CRITICAL_SECTION countsLock; CRITICAL_SECTION writerLock; HANDLE noReaders; int readerCount; BOOL waitingWriter; } RW_LOCK, *PRW_LOCK; void rwlock_init(PRW_LOCK rwlock) { InitializeCriticalSection(&rwlock->writerLock); InitializeCriticalSection(&rwlock->countsLock); /* * Could use a semaphore as well. There can only be one waiter ever, * so I'm showing an auto-reset event here. */ rwlock->noReaders = CreateEvent (NULL, FALSE, FALSE, NULL); } void rwlock_rdlock(PRW_LOCK rwlock) { /* * We need to lock the writerLock too, otherwise a writer could * do the whole of rwlock_wrlock after the readerCount changed * from 0 to 1, but before the event was reset. */ EnterCriticalSection(&rwlock->writerLock); EnterCriticalSection(&rwlock->countsLock); ++rwlock->readerCount; LeaveCriticalSection(&rwlock->countsLock); LeaveCriticalSection(&rwlock->writerLock); } int rwlock_wrlock(PRW_LOCK rwlock) { EnterCriticalSection(&rwlock->writerLock); /* * readerCount cannot become non-zero within the writerLock CS, * but it can become zero... */ if (rwlock->readerCount > 0) { EnterCriticalSection(&rwlock->countsLock); /* ... so test it again. */ if (rwlock->readerCount > 0) { rwlock->waitingWriter = TRUE; LeaveCriticalSection(&rwlock->countsLock); WaitForSingleObject(rwlock->noReaders, INFINITE); } else { /* How lucky, no need to wait. */ LeaveCriticalSection(&rwlock->countsLock); } } /* writerLock remains locked. */ } void rwlock_rdunlock(PRW_LOCK rwlock) { EnterCriticalSection(&rwlock->countsLock); assert (rwlock->readerCount > 0); if (--rwlock->readerCount == 0) { if (rwlock->waitingWriter) { /* * Clear waitingWriter here to avoid taking countsLock * again in wrlock. */ rwlock->waitingWriter = FALSE; SetEvent(rwlock->noReaders); } } LeaveCriticalSection(&rwlock->countsLock); } void rwlock_wrunlock(PRW_LOCK rwlock) { LeaveCriticalSection(&rwlock->writerLock); } </code></pre> You could decrease the cost for readers by using a single <code>CRITICAL_SECTION</code>: <ul> <li><code>countsLock</code> is replaced with <code>writerLock</code> in rdlock and rdunlock</li> <li><code>rwlock->waitingWriter = FALSE</code> is removed in wrunlock</li> <li>wrlock's body is changed to <pre><code>EnterCriticalSection(&rwlock->writerLock); rwlock->waitingWriter = TRUE; while (rwlock->readerCount > 0) { LeaveCriticalSection(&rwlock->writerLock); WaitForSingleObject(rwlock->noReaders, INFINITE); EnterCriticalSection(&rwlock->writerLock); } rwlock->waitingWriter = FALSE; /* writerLock remains locked. */ </code></pre></li> </ul> However this loses in fairness, so I prefer the above solution.
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1. This table or related slice is empty.
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1. POWin32 Read/Write Lock Using Only Critical Sections
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1. PTAnswer
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1. USPaolo Bonzini
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1. USPaolo Bonzini
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2. VO
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1. COC++ realization http://www.glennslayden.com/code/win32/reader-writer-lock
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 USKindDragon
2. COFunny that we wrote almost exactly the same code.
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 USPaolo Bonzini

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