c++ - 成功接收后清空缓冲区

Question

我正在用 C++ 在 Windows 上编写一个服务器，我在使用 recv() 时遇到了一个奇怪的行为。

我写了这个函数:

bool readN(SOCKET s, int size, char* buffer){
    fd_set readset;
    struct timeval tv;
    int left, res;
    FD_ZERO(&readset);
    FD_SET(s, &readset);
    left = size;
    std::cout << "-----called readN to read " << size << " byte" << std::endl;
    while (left > 0) {
        tv.tv_sec = MAXWAIT;
        tv.tv_usec = 0;
        res = select(0, &readset, NULL, NULL, &tv);
        if (res > 0) {
            res = recv(s, buffer, left, 0);
            if (res == 0) {//connection closed by client
                return false;
            }

            left -= res;
            std::cout << "\treceived " << res << " left " << left << std::endl;
            if (left != 0) {
                buffer += res;
            }

        }
        else if (res == 0) { //timer expired
            return false;
        }
        else { //socket error
            return false;
        }
    }
    std::cout << "\t" << buffer << std::endl;
    return true;
}

我这样调用它:

std::unique_ptr<char[]> buffer = std::make_unique<char[]>(size_);
if (readN(sck, size_, buffer.get())) {
    std::cout << "----read message----" << std::endl;
    std::cout <<"\t"<< buffer.get()<< std::endl;
}

问题是即使recv() 返回一个正数，缓冲区仍然是空的。我错过了什么？

Answer 1

我发现您的代码中存在一些问题。

1. 您没有重置 readset每次调用 select() 时都会发生变化. select()修改变量。对于单插槽情况，这还算不错，但您应该养成每次都重置变量的习惯。

2. 您没有检查 recv() 返回的错误.您假设任何非正常断开都是成功的，但这并不总是正确的。

3. 在 readN() 的末尾返回前true , 你正在输出 buffer参数 std::cout , 然而 buffer将指向数据的END，而不是BEGINNING，因为它是由读取循环推进的。这可能是您对“空缓冲区”感到困惑的来源。 readN()本身甚至根本不应该输出数据，因为你在 readN() 之后这样做退出，否则你最终会得到冗余的输出消息。

4. 如果 readN()返回 true，您将传递最后一个 buffer至 std::cout使用 operator<<期望空终止 char字符串，但不能保证您的缓冲区以 null 结尾。

尝试更像这样的东西:

bool readN(SOCKET s, int size, char* buffer){
    fd_set readset;
    struct timeval tv;
    int res;
    std::cout << "-----called readN to read " << size << " byte(s)" << std::endl;
    while (size > 0) {
        FD_ZERO(&readset);
        FD_SET(s, &readset);
        tv.tv_sec = MAXWAIT;
        tv.tv_usec = 0;

        res = select(0, &readset, NULL, NULL, &tv);
        if (res > 0) {
            res = recv(s, buffer, size, 0);
            if (res == SOCKET_ERROR) {
                res = WSAGetLastError();
                if (res == WSAEWOULDBLOCK) {
                    continue; //call select() again
                }
                return false; //socket error
            }

            if (res == 0) {
                return false;  //connection closed by client
            }

            buffer += res;
            size -= res;

            std::cout << "\treceived " << res << " byte(s), " << size << " left" << std::endl;
        }

        /*
        else if (res == 0) {
            return false; //timer expired
        }
        else {
            return false; //socket error
        }
        */

        else {
            return false; //timer expired or socket error
        }
    }

    return true;
}

std::unique_ptr<char[]> buffer = std::make_unique<char[]>(size_);
if (readN(sck, size_, buffer.get())) {
    std::cout << "----read message----" << std::endl;
    std::cout << "\t";
    std::cout.write(buffer.get(), size_);
    std::cout << std::endl;
}

话虽如此，我建议使用 readN() 的替代实现方式，取决于您使用的是阻塞套接字还是非阻塞套接字。

如果阻塞，使用setsockopt(SO_RCVTIMEO)而不是 select() .如果recv()因超时而失败，WSAGetLastError()会报WSAETIMEDOUT :

sck = socket(...);

DWORD timeout = MAXWAIT * 1000;
setsockopt(sck, SOL_SOCKET, SO_RCVTIMEO, (char*)&timeout, sizeof(timeout));

bool readN(SOCKET s, int size, char* buffer){
    int res;
    std::cout << "-----called readN to read " << size << " byte(s)" << std::endl;
    while (size > 0) {
        res = recv(s, buffer, size, 0);
        if (res == SOCKET_ERROR) {
            /*
            res = WSAGetLastError();
            if (res == WSAETIMEDOUT) {
                return false; //timer expired
            }
            else {
                return false; //socket error
            }
            */
            return false; //timer expired or socket error
        }

        if (res == 0) {
            return false; //connection closed by client
        }

        buffer += res;
        size -= res;

        std::cout << "\treceived " << res << " byte(s), " << size << " left" << std::endl;
    }

    return true;
}

如果是非阻塞的，不要调用select()除非recv()要求你调用它:

bool readN(SOCKET s, int size, char* buffer){
    fd_set readset;
    struct timeval tv;
    int res;
    std::cout << "-----called readN to read " << size << " byte(s)" << std::endl;
    while (size > 0) {
        res = recv(s, buffer, size, 0);
        if (res == SOCKET_ERROR) {
            res = WSAGetLastError();
            if (res != WSAEWOULDBLOCK) {
                return false; //socket error
            }

            FD_ZERO(&readset);
            FD_SET(s, &readset);
            tv.tv_sec = MAXWAIT;
            tv.tv_usec = 0;

            res = select(0, &readset, NULL, NULL, &tv);
            if (res > 0) {
                continue; //call recv() again
            }

            /*
            else if (res == 0) {
                return false; //timer expired
            }
            else {
                return false; //socket error
            }
            */

            return false; //timer expired or socket error
        }

        if (res == 0) {
            return false; //connection closed by client
        }

        buffer += res;
        size -= res;

        std::cout << "\treceived " << res << " byte(s), " << size << " left" << std::endl;
    }

    return true;
}