read/write lock implementation using mutex only?

Question

I was trying to implement read/write lock using mutex only (just for learning). Just when i thought i have covered all corner cases (as the program worked with variety of combin

Answer 1

class ReadWriteLock {
    mutex writeLock;
    mutex readLock;
    int readCount;
public:
    ReadWriteLock() {
        readCount = 0;
    }
    void LockWrite() {
        writeLock.lock();
    }
    void UnlockWrite() {
        writeLock.unlock();
    }
    void LockRead() {
        lock_guard lock(readLock);
        ++readCount;
        if (1 == readCount) {
            LockWrite();
        }
    }
    void UnlockRead() {
        lock_guard lock(readLock);
        --readCount;
        if (0 == readCount) {
            UnlockWrite();
        }
    }
};

As Alexey pointed out, if the last read thread to UnlockWrite isn't the first read thread to LockWrite, the behavior is undefined. See std::mutex::unlock http://www.cplusplus.com/reference/mutex/mutex/unlock/ Windows ReleaseMutex: http://msdn.microsoft.com/en-us/library/windows/desktop/ms685066(v=vs.85).aspx