读写多线程中的读者首选和作家首选 [英] Reader Preferred and Writer Preferred in Read-Write Multithreading

问题描述

我被分配使用POSIX线程编写读写实现.我想知道两种情况下我的实现是否正确:

I was given the assignment to write a Read-Write implementation using POSIX threads. I would like to know if my implementation is correct for both cases:

已删除可怕的尝试

尝试2号

首选读者:

变量:

int readersActive_;

sem_t lock_;
sem_t writeLock_;

实施:

void PalindromeDatabase::lockReaders()
{
    sem_wait(&lock_);
    {
        ++readersActive_;

        if (readersActive_ == 1)
            sem_wait(&writeLock_);
    }
    sem_post(&lock_);
}

void PalindromeDatabase::unlockReaders()
{
    sem_wait(&lock_);
    {
        --readersActive_;

        if (readersActive_ == 0)
            sem_post(&writeLock_);
    }
    sem_post(&lock_);
}

void PalindromeDatabase::lockWriters()
{
    sem_wait(&writeLock_);
}

void PalindromeDatabase::unlockWriters()
{
    sem_post(&writeLock_);
}

作家首选:

变量:

int readersActive_;
int readersWaiting_;

int writersActive_;
int writersWaiting_;

pthread_mutex_t lock_;
pthread_cond_t read_;
pthread_cond_t write_;

实施:

void PalindromeDatabase::lockReaders()
{
    pthread_mutex_lock(&lock_);
    {
        if (writersActive_ || writersWaiting_)
        {
            ++readersWaiting_;

            do
            {
                pthread_cond_wait(&read_, &lock_);
            } while(writersActive_ || writersWaiting_);

            --readersWaiting_;
        }

        ++readersActive_;
    }
    pthread_mutex_unlock(&lock_);
}

void PalindromeDatabase::unlockReaders()
{
    pthread_mutex_lock(&lock_);
    {
        --readersActive_;

        if (writersWaiting_)
            pthread_cond_signal(&write_);
    }
    pthread_mutex_unlock(&lock_);
}

void PalindromeDatabase::lockWriters()
{
    pthread_mutex_lock(&lock_);
    {
        if (readersActive_ || writersActive_)
        {
            ++writersWaiting_;

            do
            {
                pthread_cond_wait(&write_, &lock_);
            } while(readersActive_ || writersActive_);

            --writersWaiting_;
        }

        writersActive_ = 1;
    }
    pthread_mutex_unlock(&lock_);
}

void PalindromeDatabase::unlockWriters()
{
    pthread_mutex_lock(&lock_);
    {
        writersActive_ = 0;

        if (writersWaiting_)
            pthread_cond_signal(&write_);
        else if (readersWaiting_)
            pthread_cond_broadcast(&read_);
    }
    pthread_mutex_unlock(&lock_);
}

线程很有趣-它们也使我的大脑受伤.如果我错了，请不要给出直接的答案，但要以正确的方向指导我，因为这是一项家庭作业，而且我以及其他与我一样的人一定会从我们自己的解决方案中受益.

Threads are fun- they also make my brain hurt. If I'm wrong, please refrain from giving a direct answer, but guide me in the right direction as this is a homework assignment and I, and other people like me, would surely benefit from getting the solution on our own.

推荐答案

通过提示的方式，我不认为

By way of hint, I don't think

    // If I'm the last reader, we can let the writers work.
if (--numberOfReadersActive_)
    pthread_cond_signal(&readersActive_);

正在按照评论中的内容进行操作.这也可能就是您陷入僵局的原因.

is doing what it says in the comment. This might also be why you are getting a deadlock.

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