c++ - 从 recvfrom 获取未记录的错误代码

Question

当尝试使用 recvfrom 读取 UDP 数据包时，函数返回 -1 表示错误。我当然会调用 WSAGetLastError 来找出问题所在。报告的错误编号是 183。我似乎找不到关于该编号含义的任何引用。

编辑:

while (bytesRecv != SOCKET_ERROR)
    {
        //  get data from the server
        bytesRecv = recvfrom(m_socket, (char*)&receiveData, sizeof(ReceiveData), 0, (struct sockaddr *) &server_addr, &server_addr_len);
        logError("Bytes recieved: ", bytesRecv);
        // if data was recieved from the server
        if (bytesRecv > 0)
        {
            //Data packet processing code
        }
        else
        {
            if (bytesRecv == SOCKET_ERROR)
            {
                logError("Error: Reading data: ", WSAGetLastError());
            }
        }
    }

编辑:

void logError(const std::string &text, int errorCode)
{
    std::ofstream log_file("error_log_file.txt", std::ios_base::out | std::ios_base::app);

    log_file << text << errorCode << "\n";
}

Answer 1

问题不在于 WSAGetLastError() 本身。真正的问题是您在调用 WSAGetLastError() 之前调用了 logError()，并且 logError() 最终将最后一个错误代码重置为183.

logError() 使用 std::ofstream 打开文件进行追加。在 Windows 上，该操作最终将调用 CreateFile()使用 OPEN_ALWAYS 标志，其文档说明:

Opens a file, always.

If the specified file exists, the function succeeds and the last-error code is set to ERROR_ALREADY_EXISTS (183).

If the specified file does not exist and is a valid path to a writable location, the function creates a file and the last-error code is set to zero.

...

If the function fails, the return value is INVALID_HANDLE_VALUE. To get extended error information, call GetLastError.

在内部，WSAGetLastError() 只是映射到 GetLastError()(一个众所周知但未记录的实现细节)。因此，无论CreateFile()打开文件成功与否，WSAGetLastError()报告的错误码都会重置为打开操作的结果。

您对 logError() 的调用在错误的位置。它需要在你的 if (bytesRecv > 0) block 中移动(顺便说一句，UDP 支持 0 长度数据报，所以你应该使用 >= 而不是 >):

while (true)
{
    //  get data from the server
    bytesRecv = recvfrom(m_socket, (char*)&receiveData, sizeof(ReceiveData), 0, (struct sockaddr *) &server_addr, &server_addr_len);
    // if data was received from the server
    if (bytesRecv >= 0)
    {
        logError("Bytes received: ", bytesRecv); // <-- moved here!!!
        //Data packet processing code
    }
    else // if (bytesRecv == SOCKET_ERROR)
    {
        logError("Error: Reading data: ", WSAGetLastError());
        break;
    }
}

或者:

while (true)
{
    //  get data from the server
    bytesRecv = recvfrom(m_socket, (char*)&receiveData, sizeof(ReceiveData), 0, (struct sockaddr *) &server_addr, &server_addr_len);
    // if data was received from the server
    if (bytesRecv == SOCKET_ERROR)
    {
        logError("Error: Reading data: ", WSAGetLastError());
        break;
    }

    logError("Bytes received: ", bytesRecv); // <-- moved here!!!
    //Data packet processing code
}