使用信号量机制实现经典的读者-写者问题，每个线程代表一个读者或一个写者。 每个线程按照相应测试数据文件的要求进行读写操作。请用信号量机制实现读 者优先或写者优先的读者-写者问题。(C++)

在C++中，我们可以使用互斥锁（mutex）和信号量（semaphore）来解决经典读者-写者问题，使得同时只有一个写者可以访问共享资源，并确保读者优先。这里是一个简单的例子：

#include <iostream>
#include <thread>
#include <queue>
#include <mutex> // std::mutex for mutual exclusion
#include <condition_variable> // std::condition_variable for waiting and signaling

std::mutex mtx; // shared mutex
std::condition_variable cv_reader, cv_writer; // condition variables
int readers_wanted = 0, writers_wanted = 1; // semaphore counts

void reader() {
    std::unique_lock<std::mutex> lock(mtx);
    while (writers_wanted > 0 && readers_wanted == 0) {
        cv_reader.wait(lock); // wait until a writer finishes or there's space for a reader
    }
    ++readers_wanted;
    std::cout << "Reader enters" << std::endl;
    // perform reading operations
    // ...

    // release resources
    --readers_wanted;
    if (readers_wanted == 0) {
        cv_writer.notify_one(); // signal a writer to enter
    }
    lock.unlock();
}

void writer() {
    std::unique_lock<std::mutex> lock(mtx);
    while (readers_wanted > 0 || writers_wanted == 0) {
        cv_writer.wait(lock); // wait until no readers are present and room for a writer
    }
    ++writers_wanted;
    std::cout << "Writer enters" << std::endl;
    // perform writing operation
    // ...

    // release resources
    --writers_wanted;
    if (writers_wanted == 0) {
        cv_reader.notify_all(); // notify all waiting readers
    }
    lock.unlock();
}

int main() {
    std::thread t1(reader), t2(writer);
    t1.join();
    t2.join();
    return 0;
}

在这个例子中，`cv_reader`用于控制读者何时进入，`cv_writer`用于控制写者何时进入。当有写者正在写入时，只有当所有读者都已经完成并释放了资源（即`readers_wanted == 0`），才会允许新的读者进入；反之亦然。