Question

我正在使用C ++在Windows上编写服务器，我使用recv()面临一种奇怪的行为。

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;
}

我称之为：

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()返回正数，缓冲区仍为空。我错过了什么？

Answer 1

我在你的代码中看到了一些问题。

每次致电readset时，您都没有重置select()变量。 select()修改变量。对于单插槽情况，这不是太糟糕，但你应该养成每次重置变量的习惯。
您没有检查recv()返回的错误。您认为任何非优雅断开都是成功的，但并非总是如此。

readN()

true

，您正在将buffer参数输出到std::cout，但是buffer将指向数据的 END ，而不是 BEGINNING ，因为它是由读取循环推进的。这可能是你对空白缓冲区＆＃34;是来自。 readN()本身甚至不应该输出数据，因为您在readN()退出后执行此操作，否则最终会出现冗余输出消息。
如果readN()返回true，则使用期望空终止buffer字符串的std::cout将最终operator<<传递给char，但是你的缓冲区不能保证以空值终止。

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，具体取决于您使用的是阻塞还是非阻塞套接字。

如果屏蔽，请使用setsockopt(SO_RCVTIMEO)代替select()。如果recv()因超时而失败，WSAGetLastError()将报告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;
}

如果非阻止，请不要致电select()，除非recv()要求您致电：{/ p>

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;
}

Answer 2

在readN()的末尾有

std::cout << "\t" << buffer << std::endl;

问题是缓冲区现在指向buffer + size的{{1}}的原始值。该值已被

修改

buffer

这应输出缓冲区

buffer += res;

使用以下std::cout << "\t" << (buffer - size) << std::endl;对readN()进行试验后，如果套接字不是无效句柄（ncat发送的文本/二进制数据），则main()似乎有效。如果套接字是无效句柄，则函数会快速返回。

readN()

成功recv后清空缓冲区

2 个答案: