C++/Winsock TCP在客户机/服务器之间发送/Recv问题

Question

案例A：

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

服务器接收到这3条消息并发送最后一个答案，但是client上的recv失败，SOCKET_ERROR值为10060。使这种情况起作用的唯一方法是在客户端上的最后一次发送之后添加一个shutdown(...，SD_SEND)。

为什么A例会有这种行为？为什么只有在添加shutdown（）命令时才工作？

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

#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;
}

提前感谢！！！！尼古拉斯·米兰达S。

Answer 1

当客户机等待响应时，服务器被阻塞在recv调用中，因此无法发送任何内容。您的接收方超时为1秒，因此1秒后客户机将生成超时错误(WSAETIMEDOUT==10060)。

关闭时，只指定SD_SEND，这样连接就不会关闭，但会导致服务器退出recv，从而能够发送响应。

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，看看在调用recv之前如何“查看”是否有可用的数据。

下面是一个使用SELECT的示例：

    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，则存在超时。否则，fd_set中的一个套接字已经准备好了数据。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，接收第三条消息，或者某个超时而没有接收任何内容，或者消息本身中的一些内容），从而导致服务器发送响应。您可以使用第二个线程作为响应，但这超出了我在一个简短的回答中所能显示的范围。