Reader Writer lock implementation from Isilon interview.

rwlock.c

typedef struct RWLock {
	int readers;
	bool writer;
	bool writeRequest;
}

void LockForRead(RWLock *l) {
	m.Lock();
	while(l->writer || l->writeRequest) {
		m.Unlock();
		Sleep(10);
		m.Lock();
	}
	om.Lock();
	l->readers++;
	om.Unlock();
	m.Unlock();
}

void LockForWrite(RWLock *l) {
	m.Lock();
	l->writeRequest = true;
	while(l->readers > 0 && l->writer) {
		m.Unlock();
		Sleep(10);
		m.Lock();
	}
	l->writer = true;
	l->writeRequest = false;
	m.Unlock();
}

void UnlockForRead(RWLock *l) {
	om.Lock();
	l->readers--;
	om.Unlock();
}

void UnlockForWrite(RWLock *l) {
	m.Lock();
	l->writer = false;
	m.Unlock();
}