소켓 네트워크 프로그래밍) 데이터 전송 방식 -> 고정 + 가변 길이 데이터 전송

책) TCP/IP 윈도우 소켓 프로그래밍 5장 

TCP 서버 - 클라이언트 모델에서 데이터를 전송하는 방식은 다양한 방식이 있다. 

 

1. 고정 길이 데이터 전송

2. 가변 길이 데이터 전송

3. 고정 + 가변 길이 데이터 전송

4. 송신자 데이터 전송후 접속 종료, 수신자는 recv()함수의 리턴값이 0이 될때까지 읽는다.

 

클라이언트 측에서 먼저 길이를 send 후 데이터를 전송하고, 서버측은 길이를 recv로 받은 후 받은 길이 만큼 recv한다.

 

//DummyClient

#pragma warning(disable:4996)
#pragma comment(lib, "ws2_32.lib")
#include <winsock2.h>
#include <stdio.h>
#include <stdlib.h>

#define MAXLINE 50
#define PORTNUM 5000

void err_quit(const char* msg) {
    LPVOID lpMsgBuf;
    FormatMessage(
        FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
        NULL, WSAGetLastError(),
        MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
        (LPTSTR)&lpMsgBuf, 0, NULL);
    printf("[%s] %s", msg, (char*)lpMsgBuf);
    LocalFree(lpMsgBuf);
}


int main()
{
    WSADATA wsaData;
    SOCKET sockfd;
    int addrlen;
    char buf[MAXLINE];

    WSAEVENT sEvent;
    struct sockaddr_in sockaddr;

    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0)
    {
        printf("dlload error\n");
        return 1;
    }

    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd == INVALID_SOCKET)
    {
        printf("socket create error\n");
        return 1;
    }

    ZeroMemory(&sockaddr, sizeof(sockaddr));
    sockaddr.sin_family = AF_INET;
    sockaddr.sin_port = htons(PORTNUM);
    sockaddr.sin_addr.s_addr = inet_addr("127.0.0.1");

    if (::connect(sockfd, (SOCKADDR*)&sockaddr, sizeof(sockaddr))) err_quit("Connet");

    const char* testdata[] = {
        "안녕하세요",
        "반가워요",
        "오늘따라 할 이야기가 많을 것 같네요.",
        "저도 그렇네요" 
    };

    for (int i = 0; i < 4; i++) {
        //데이터 입력
        int len = strlen(testdata[i]);
        strncpy(buf, testdata[i], len);

        //데이터 보내기(고정길이)
        int retVal = send(sockfd, (char*)&len, sizeof(int), 0);
        if (retVal == SOCKET_ERROR) {
            err_quit("Send");
            break;
        }

        //데이터 보내기(가변길이)
        retVal = send(sockfd, buf, len, 0);
        if (retVal == SOCKET_ERROR) {
            err_quit("Send");
            break;
        }
        printf("[TCP 클라이언트] %d바이트를 보냈습니다.\n", retVal);
    }

    printf("소켓 통신 종료");
    closesocket(sockfd);
    WSACleanup();
    return 0;
}

---

//DummyServer

#pragma warning(disable:4996)
#pragma comment(lib, "ws2_32.lib")
#include <winsock2.h>
#include <stdio.h>
#include <stdlib.h>

#define MAXLINE 512
#define PORTNUM 5000

void err_quit(const char* msg) {
    LPVOID lpMsgBuf;
    FormatMessage(
        FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
        NULL, WSAGetLastError(),
        MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
        (LPTSTR)&lpMsgBuf, 0, NULL);
    printf("[%s] %s", msg, (char*)lpMsgBuf);
    LocalFree(lpMsgBuf);
}

int recvn(SOCKET s, char* buf, int len, int flag) {
    int received;
    char* ptr = buf;
    int left = len;

    while (left > 0) {
        received = recv(s, ptr, left, flag);
        if (received == SOCKET_ERROR)
            return SOCKET_ERROR;
        else if (received == 0)
            break;
        left -= received;
        ptr += received;
    }
    return (len - left);
}
int main()
{
    WSADATA wsaData;
    SOCKET sockfd, clientfd;
    int addrlen;
    char buf[MAXLINE+1];
    int len = 0;
    WSAEVENT sEvent;
    struct sockaddr_in sockaddr,clientaddr;

    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0)
    {
        printf("dlload error\n");
        return 1;
    }

    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd == INVALID_SOCKET)
    {
        printf("socket create error\n");
        return 1;
    }

    sockaddr.sin_family = AF_INET;
    sockaddr.sin_port = htons(PORTNUM);
    sockaddr.sin_addr.s_addr = htonl(INADDR_ANY);

    if (::bind(sockfd, (SOCKADDR*)&sockaddr, sizeof(sockaddr)) == SOCKET_ERROR) err_quit("Bind");
    if (::listen(sockfd, 5) == SOCKET_ERROR) err_quit("Listen");

    while (1)
    {
        addrlen = sizeof(clientaddr);
        clientfd = ::accept(sockfd, (SOCKADDR*)&clientaddr, &addrlen);
        if (clientfd == INVALID_SOCKET) {
            err_quit("Accept");
            break;
        }
        printf("[TCP서버] 클라이언트 접속 IP : %s %d\n", 
            inet_ntoa(clientaddr.sin_addr), ntohs(clientaddr.sin_port));
        
        while (1) {
            //데이터받기(고정길이)
            int retVal = recvn(clientfd, (char*)&len, sizeof(int), 0);
            if (retVal == SOCKET_ERROR) {
                err_quit("Recv");
                break;
            }
            else if (retVal == 0) break;

            //데이터받기(가변길이)
            retVal = recvn(clientfd, buf, len, 0);
            if (retVal == SOCKET_ERROR) {
                err_quit("Recv");
                break;
            }
            else if (retVal == 0) break;

            //받은 데이터 출력
            buf[retVal] = '\0';
            printf("[TCP/%s : %d] %s\n", inet_ntoa(clientaddr.sin_addr),
                ntohs(clientaddr.sin_port), buf);


        }
        closesocket(clientfd);
        printf("소켓 통신 종료");
    }

    closesocket(sockfd);
    WSACleanup();
    return 0;
}