客户端/服务器之间的 C++/Winsock TCP 发送/接收问题

Question

当我连续发送然后在客户端接收时，我遇到了通过套接字进行客户端/服务器通信的问题。示例:

案例 A:

Client                Server
send(...);----------->While(recv(...)>0){
send(...);-----------> print(message);
send(...);----------->}
recv(...);----------->Send(...); 

服务器收到 3 条消息并发送最后一个应答，但客户端的接收失败，并出现 SOCKET_ERROR，WSAGetLastError() 值为 10060。使这种情况起作用的唯一方法是在客户端最后一次发送之后添加 shutdown(...,SD_SEND)。​​

为什么案例A有这种行为？以及为什么只有在我添加 shutdown() 命令时它才有效？

但如果我这样做:

案例 B:

Client                Server
send(...);----------->While(recv(...)>0){
recv(...);-----------> send(...);
send(...);-----------> ...
recv(...);-----------> ...
send(...);-----------> ...
recv(...);----------->}

它工作正常，服务器/客户端接收和发送每条消息。

这是案例 A 的代码:客户:

#include <iostream>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

// Need to link with Ws2_32.lib, Mswsock.lib, and Advapi32.lib
#pragma comment (lib, "Ws2_32.lib")
#pragma comment (lib, "Mswsock.lib")
#pragma comment (lib, "AdvApi32.lib")

#define DEFAULT_BUFLEN 1024
#define DEFAULT_PORT "27015"

int main() {
WSADATA wsaData;
SOCKET ConnectSocket = INVALID_SOCKET;
ADDRINFOA *ptr = NULL, *result = NULL, hints;
char *ans, *sendbuf = "message\0";
char recvbuf[1024];
int iResult;
int recvbuflen = DEFAULT_BUFLEN;
ans=new char[1024];

// Initialize Winsock
iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
printf("--> Initializing Winsock...\n*** Version: %s\n", wsaData.szDescription);
if (iResult != 0) {
    printf("*** Could not initialize Socket.\n*** Error code: %d", iResult);
    return 1;
}

ZeroMemory( &hints, sizeof(hints) );
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;

// Resolve the server address and port
iResult = getaddrinfo("127.0.0.1", DEFAULT_PORT, &hints, &result);
printf("--> Setting server address...\n");
printf("--> local ip: 127.0.0.1 at port: %s...\n",DEFAULT_PORT);
if ( iResult != 0 ) {
    printf("*** Error in setting server address.\n*** Error code: %d", iResult);
    WSACleanup();
    return 1;
}

// Attempt to connect to an address until one succeeds
for(ptr=result; ptr != NULL ;ptr=ptr->ai_next) {
    // Create a SOCKET for connecting to server
    printf("--> Creating client socket object...\n");
    ConnectSocket = socket(ptr->ai_family, ptr->ai_socktype, ptr->ai_protocol);
    if (ConnectSocket == INVALID_SOCKET) {
        printf("*** Error creating socket.\n*** Error code: %d\n",WSAGetLastError());
        freeaddrinfo(result);
        WSACleanup();
        return 1;
    }
    if(setsockopt(ConnectSocket, SOL_SOCKET, SO_RCVTIMEO, (char *)new int(1000), sizeof(int))){
        WSACleanup();
        //strcpy(recvbuf, "EX_95");
        return -5;    //    Error setting recv timeout.
    }
    // Connect to server.
    iResult = connect( ConnectSocket, ptr->ai_addr, (int)ptr->ai_addrlen);
    if (iResult == SOCKET_ERROR) {
        closesocket(ConnectSocket);
        ConnectSocket = INVALID_SOCKET;
        continue;
    }
    printf("*** Client ready *** \n\n");
    break;
}

freeaddrinfo(result);

if (ConnectSocket == INVALID_SOCKET) {
    printf("*** Unable to connect to server!\n");
    WSACleanup();
    return 1;
}

// Send an initial buffer
iResult = send( ConnectSocket, sendbuf, (int)strlen(sendbuf), 0 );
if (iResult == SOCKET_ERROR) {
    printf("*** Send failed: %d\n", WSAGetLastError());
    closesocket(ConnectSocket);
    WSACleanup();
    return 1;
}

printf("<-- Bytes Sent: %ld\n", iResult);

    iResult = send( ConnectSocket, sendbuf, (int)strlen(sendbuf), 0 );
if (iResult == SOCKET_ERROR) {
    printf("*** Send failed: %d\n", WSAGetLastError());
    closesocket(ConnectSocket);
    WSACleanup();
    return 1;
}

printf("<-- Bytes Sent: %ld\n", iResult);


iResult = send( ConnectSocket, sendbuf, (int)strlen(sendbuf), 0 );
if (iResult == SOCKET_ERROR) {
    printf("*** Send failed: %d\n", WSAGetLastError());
    closesocket(ConnectSocket);
    WSACleanup();
    return 1;
}

printf("<-- Bytes Sent: %ld\n", iResult);

// shutdown the connection since no more data will be sent
/*iResult = shutdown(ConnectSocket, SD_SEND);
if (iResult == SOCKET_ERROR) {
    printf("shutdown failed with error: %d\n", WSAGetLastError());
    closesocket(ConnectSocket);
    WSACleanup();
    return 1;
}*/

iResult = recv(ConnectSocket, recvbuf, recvbuflen, 0);
if ( iResult > 0 )
    printf("Bytes received: %d\n", iResult);
else if ( iResult == 0 )
    printf("Connection closed\n");
else
    printf("recv failed with error: %d\n", WSAGetLastError());


// cleanup
closesocket(ConnectSocket);
WSACleanup();
system("pause");
return 0;
}

服务器:

// Need to link with Ws2_32.lib, Mswsock.lib, and Advapi32.lib
#include <iostream>
#include <Winsock2.h>
#include <Ws2tcpip.h>
#include <string>

#pragma comment (lib, "Ws2_32.lib")
#pragma comment (lib, "Mswsock.lib")
#pragma comment (lib, "AdvApi32.lib")

const char* DEFAULT_PORT = "27015";
const int DEFAULT_BUFLEN = 1024;

using namespace std;

int main(){
//Connection Variables.
WSADATA wsaData;
ADDRINFOA *result = NULL, hints;
SOCKADDR *clientInfo = NULL;
int iResult;
//Receive Variables.
char recvBuff[DEFAULT_BUFLEN];
int recvBuffLen = DEFAULT_BUFLEN;
int iSendResult;
//Server/Client sockets.
SOCKET ListenSocket = INVALID_SOCKET; //SOCKET for the server to listen for client connections.
SOCKET ClientSocket = INVALID_SOCKET;

//Initialize Winsock
iResult = WSAStartup( MAKEWORD( 2, 2 ), &wsaData );
printf("--> Initializing Winsock...\n*** Version: %s\n", wsaData.szDescription);
if ( iResult != 0 ){
    printf("*** Could not initialize Socket.\n*** Error code: %d", iResult);
    return 1;
}
//Initialize hints allocated memory.
ZeroMemory( &hints, sizeof( hints ) );

hints.ai_family   = AF_INET;     //AF_INET is used to specify the IPv4 address family.
hints.ai_socktype = SOCK_STREAM; //SOCK_STREAM is used to specify a stream socket.
hints.ai_protocol = IPPROTO_TCP; //IPPROTO_TCP is used to specify the TCP protocol.
hints.ai_flags    = AI_PASSIVE;  //AI_PASSIVE The socket address will be used in a call to the bind function.   

printf("--> Getting address info from server...\n");
iResult = getaddrinfo( NULL, DEFAULT_PORT, &hints, &result );
if( iResult != 0 ){
    printf("*** Error in getting address info from server.\n*** Error code: %d", iResult);
    WSACleanup();
    return 2;
}

printf("--> Creating server socket object...\n");
//printf("Create socket to ip: %s at port: %s\n", inet_ntoa(((SOCKADDR_IN*)(result->ai_addr))->sin_addr),DEFAULT_PORT);
ListenSocket = socket( result->ai_family, result->ai_socktype, result->ai_protocol );
if( ListenSocket == INVALID_SOCKET ){
     printf("*** Error creating socket.\n*** Error code: %d\n",WSAGetLastError());
    freeaddrinfo( result );
    WSACleanup();
    return 3;
}

//BIND()->Associates a local address to a socket.
printf("--> Bind listen object to local ip: 127.0.0.1 at port: %s...\n",DEFAULT_PORT);
iResult = bind( ListenSocket, result->ai_addr, result->ai_addrlen );
if( iResult == SOCKET_ERROR ){
    printf("*** Binding failed.\n*** Error code: %d\n", WSAGetLastError());
    closesocket( ListenSocket );
    freeaddrinfo( result );
    WSACleanup();
    return 4;
}
freeaddrinfo(result);

printf("*** Server ONLINE: Listening...\n\n");
iResult = listen( ListenSocket, SOMAXCONN );
if( iResult == SOCKET_ERROR ){
    printf("*** Failed start listening.\n*** Error code: %d\n",WSAGetLastError());
    closesocket( ListenSocket );
    freeaddrinfo( result );
    WSACleanup();
    return 5;
}

for(;1;){   
   ClientSocket = accept( ListenSocket,clientInfo, NULL );
   if( ClientSocket == INVALID_SOCKET ){
        printf("*** Failed accepting connection from client.\n*** Error code: %d\n",WSAGetLastError());
        continue;
   }
   else{
        printf("--> Connection accepted from client.\n");
        iResult=1;
        while(iResult > 0){
            iResult = recv( ClientSocket, recvBuff, recvBuffLen, 0 );
            if( iResult > 0 ){
                printf("--> Message received: %s\n--> Total: %d\n", recvBuff, iResult);
            }
        }
        iSendResult = send( ClientSocket, "Answer\0", DEFAULT_BUFLEN, 0 );
        if( iSendResult == SOCKET_ERROR ){
            printf("*** Sending data failed.\n*** Error code: %d\n",WSAGetLastError());
            continue;
        }
        else{
            printf("--> Sent: %d bytes\n", iSendResult);
        }
        printf("*** Closing connection... \n\n");
        iResult = shutdown( ClientSocket, SD_BOTH );
        if( iResult == SOCKET_ERROR ){
            printf("*** Shutdown client failed.\nError code: %d\n",WSAGetLastError());
            closesocket( ClientSocket );
            WSACleanup();
            return 9;
        }
  }
 }
 closesocket( ClientSocket );
 WSACleanup();
 system("pause");
 return 0;
}

提前致谢!!!!尼古拉斯·米兰达 S.

Answer 1

当客户端等待响应时，服务器在 recv 调用中被阻塞，因此无法发送任何内容。您的接收器超时为 1 秒，因此 1 秒后客户端生成超时错误 (WSAETIMEDOUT == 10060)。

当你关闭时，你只指定了SD_SEND，所以连接没有关闭，但它会导致服务器退出recv，因此能够发送响应。

注意:recv 将阻塞，直到接收到流套接字 (SOCK_STREAM) 的内容。您应该查看 select() 函数，了解如何在调用 recv 之前“查看”是否有可用数据。

这里是一个使用select的例子:

    fd_set fds;
    timeval tv;
    tv.tv_sec = 5000;
    fds.fd_count = 1;
    fds.fd_array[0] = ClientSocket;

    int select_result = select(1, &fds, NULL, NULL, &tv);

如果 select_result == 0，则超时。否则，fd_set 中的一个套接字已准备好数据。这里只有一个，它在 select 调用中被指定为 readfds。

您需要重新安排应用程序，以便您有一些事件(例如，接收到第三条消息，或者没有收到任何消息的某个超时，或者消息本身中的某些内容)导致服务器发送响应。您可以使用第二个线程进行回复，但这超出了我可以在简短回答中展示的范围。