网络消息接收长度的问题

#define WIN32_LEAN_AND_MEAN
#define _WINSOCK_DEPRECATED_NO_WARNINGS

#include <windows.h>
#include <WinSock2.h>
#include <cstdio>

#pragma comment(lib , "ws2_32.lib")

enum CMD {
    CMD_LOGIN,
    CMD_LOGIN_RESULT,
    CMD_LOGOUT,
    CMD_LOGOUT_RESULT,
    CMD_ERROR
};

struct DataHeader {
    short dataLength;  // 包体长度
    short cmd;         
};

// DataPackage
struct Login : public DataHeader{  
    Login() {
        dataLength = sizeof(Login);
        cmd = CMD_LOGIN;
    }
    char userName[32];
    char PassWord[32];
};

struct LoginResult : public DataHeader{
    LoginResult() {
        dataLength = sizeof(LoginResult);
        cmd = CMD_LOGIN_RESULT;
        result = 0;
    }
    int result;
};

struct LogOut : public DataHeader {
    LogOut() {
        dataLength = sizeof(LogOut);
        cmd = CMD_LOGOUT;
    }
    char userName[32];
};

struct LoginOutResult : public DataHeader {
    LoginOutResult() {
        dataLength = sizeof(LoginOutResult);
        cmd = CMD_LOGIN_RESULT;
        result = 0;
    }
    int result;
};

int main()
{
    // 启动Windows socket 2.x环境
    WORD ver = MAKEWORD(2, 2);
    WSADATA dat;
    WSAStartup(ver, &dat);
    //-----------------
    
    // 用Socket API建立简易TCP服务器端
    // 1建立一个Socket
    SOCKET sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    // 2 bind 绑定用于接收客户端连接的网络端口
    sockaddr_in _sin = {};
    _sin.sin_family = AF_INET;
    _sin.sin_port = htons(4567);
    _sin.sin_addr.S_un.S_addr = INADDR_ANY;//inet_addr("127.0.0.1");
    if (bind(sock, (sockaddr*)&_sin, sizeof(_sin)) == SOCKET_ERROR) {
        printf("Bind Port Error\n");
    }
    else {
        printf("Bind Port Success\n");
    }
    // 3 listen 监听网络端口
    if (listen(sock, 5) == SOCKET_ERROR) {
        printf("listen failed\n");
    }
    else {
        printf("listen success\n");
    }
    // 4 accept 等待客户端连接
    sockaddr_in clientAddr = {};
    int cAddrlen = sizeof(sockaddr_in);
    SOCKET csock = INVALID_SOCKET;
    csock = accept(sock, (sockaddr*)&clientAddr, &cAddrlen);
    if (csock == INVALID_SOCKET) {
        printf("failed , recieve a invalid client socket\n");
    }
    printf("new client join:socket = %d , IP = %s \n", csock , inet_ntoa(clientAddr.sin_addr));

while (true) {
        // 缓冲区
        char szRecv[1024] = {};
        // 接收客户端的请求数据
        int len = recv(csock, szRecv , sizeof(DataHeader), 0);
        DataHeader* header = (DataHeader*)szRecv;
        if (len <= 0) {
            printf("client exit\n");
            break;
        }
        // 处理请求
        switch (header->cmd) {
            case CMD_LOGIN:
            {
                recv(csock, szRecv + sizeof(DataHeader), header->dataLength - sizeof(DataHeader), 0);
                Login* login = (Login*)szRecv;
                printf("收到命令:CMD_LOGIN , 数据长度: %d , 用户名:%s"
                "密码: %s \n", login->dataLength , login->userName, login->PassWord);
                // 忽略判断用户名密码是否正确的过程
                LoginResult ret;
                send(csock, (char*)&ret, sizeof(LoginResult), 0);
            }
            break;
            case CMD_LOGOUT:
            {
                recv(csock, szRecv + sizeof(DataHeader), header->dataLength - sizeof(DataHeader), 0);
                LogOut* logout = (LogOut*)szRecv;
                printf("收到命令:CMD_LOGIN , 数据长度: %d , 用户名:%s"
                    "\n", logout->dataLength, logout->userName);
                // 忽略判断用户名密码是否正确的过程
                LoginResult ret;
                send(csock, (char*)&ret, sizeof(LoginResult), 0);
            }
            break;
            default:
           {
            DataHeader header = { 0 , CMD_ERROR};
            send(csock, (char*)&header, sizeof(DataHeader), 0);
           }
            break;
         }
    }
    printf("任务完成\n");
    getchar();
    // 6 关闭套接字
    closesocket(sock);
    WSACleanup();
    return 0;
}

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