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244376
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0
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0
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10
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2008-10-28T18:46:10.487
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2008-10-28T19:28:26.033
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2008-10-28T19:28:26.033
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20713
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20713
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244316
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2
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33
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text
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<p>Using standard pre-tested, pre-built stuff is always good (for example, Boost as another answer suggested), but this is something that's not too hard to build yourself. Here's a dumb little implementation pulled out from a project of mine:</p> <pre><code>#include <pthread.h> struct rwlock { pthread_mutex_t lock; pthread_cond_t read, write; unsigned readers, writers, read_waiters, write_waiters; }; void reader_lock(struct rwlock *self) { pthread_mutex_lock(&self->lock); if (self->writers || self->write_waiters) { self->read_waiters++; do pthread_cond_wait(&self->read, &self->lock); while (self->writers || self->write_waiters); self->read_waiters--; } self->readers++; pthread_mutex_unlock(&self->lock); } void reader_unlock(struct rwlock *self) { pthread_mutex_lock(&self->lock); self->readers--; if (self->write_waiters) pthread_cond_signal(&self->write); pthread_mutex_unlock(&self->lock); } void writer_lock(struct rwlock *self) { pthread_mutex_lock(&self->lock); if (self->readers || self->writers) { self->write_waiters++; do pthread_cond_wait(&self->write, &self->lock); while (self->readers || self->writers); self->write_waiters--; } self->writers = 1; pthread_mutex_unlock(&self->lock); } void writer_unlock(struct rwlock *self) { pthread_mutex_lock(&self->lock); self->writers = 0; if (self->write_waiters) pthread_cond_signal(&self->write); else if (self->read_waiters) pthread_cond_broadcast(&self->read); pthread_mutex_unlock(&self->lock); } void rwlock_init(struct rwlock *self) { self->readers = self->writers = self->read_waiters = self->write_waiters = 0; pthread_mutex_init(&self->lock, NULL); pthread_cond_init(&self->read, NULL); pthread_cond_init(&self->write, NULL); } </code></pre> <p><code>pthreads</code> not really being Windows-native, but the general idea is here. This implementation is slightly biased towards writers (a horde of writers can starve readers indefinitely); just modify <code>writer_unlock</code> if you'd rather the balance be the other way around.</p> <p>Yes, this is C and not C++. Translation is an exercise left to the reader.</p> <h2>Edit</h2> <p>Greg Rogers pointed out that the POSIX standard does specify <code>pthread_rwlock_*</code>. This doesn't help if you don't have <code>pthreads</code>, but it stirred my mind into remembering: <a href="http://sourceware.org/pthreads-win32/" rel="noreferrer">Pthreads-w32</a> should work! Instead of porting this code to non-<code>pthreads</code> for your own use, just use Pthreads-w32 on Windows, and native <code>pthreads</code> everywhere else.</p>
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