单个 TCP 服务器和多个客户端 - 服务器似乎只与最后连接的客户端通信

Question

我正在使用 select() 函数并尝试运行多个客户端。每个客户端都是独立的进程。但服务器似乎只对最后连接的客户端感兴趣。

这是我的客户.c

#include <stdio.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <string.h>
#include "mpi.h"
#define PORT 8888

int main(int argc, char const *argv[])
{
    /* Starting the MPI verion from here */

    // Initialize the MPI environment
    MPI_Init(NULL, NULL);

    // Get the number of generators to be run in parallel
    int number_of_clients;
    MPI_Comm_size(MPI_COMM_WORLD, &number_of_clients);

    // Get the rank of each generator
    int client_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &client_rank);

    struct sockaddr_in address;
    int sock = 0, valread;
    struct sockaddr_in serv_addr;
    char *message1 = "Hello from client";
    char *message2 = "I am disconnecting";
    char buffer[1024] = {0};
    if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        printf("\n Socket creation error \n");
        return -1;
    }

    memset(&serv_addr, '0', sizeof(serv_addr));

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(PORT);

    // Convert IPv4 and IPv6 addresses from text to binary form
    if(inet_pton(AF_INET, "127.0.0.1", &serv_addr.sin_addr)<=0) 
    {
        printf("\nInvalid address/ Address not supported \n");
        return -1;
    }

    if (connect(sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
    {
        printf("\nConnection Failed \n");
        return -1;
    }
    send(sock , message1 , strlen(message1) , 0 );
    printf("Hello message sent from client_rank %d\n",client_rank);
    valread = read( sock , buffer, 1024);


    buffer[valread] = '\0';
    printf("The message received from server by client_rank %d is %s\n", client_rank, buffer );
    send(sock, message2, strlen(message2), 0);
    printf("The message sent again from client_rank %d is %s\n", client_rank, message2);
    close(sock);
    // Finalize the MPI environment.
    MPI_Finalize();
    /*Ending the parallel version here*/

    return 0;
}

这是我的服务器.c

#include <unistd.h>
#include <stdio.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <string.h>
#include <arpa/inet.h>
#include <errno.h> 
#include <unistd.h>   //close 
#include <sys/types.h> 
#include <sys/time.h> //FD_SET, FD_ISSET, FD_ZERO macros 

#define PORT 8888

int main(int argc, char const *argv[])
{
    int totalconnections = 0;
    int server_fd, new_socket, valread;
    struct sockaddr_in address, cli_addr;;
    int opt = 1;
    int addrlen = sizeof(address);
    char buffer[1024] = {0};
    char *hello = "Hello from server";

    socklen_t clilen;
    int closed_connections = 0;
    int client_socket[3] , max_clients = 2 , activity, i , sd, max_sd;
    int connected_clients = 0;  
    fd_set readfds;

    //initialise all client_socket[] to 0 so not checked 
    for (i = 0; i < max_clients; i++)  
    {  
        client_socket[i] = 0;  
    } 

    // Creating socket file descriptor
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0)
    {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }

    // Forcefully attaching socket to the port 8080
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR,(char *)&opt, sizeof(opt)) < 0)
    {
        perror("setsockopt");
        exit(EXIT_FAILURE);
    }
    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;
    address.sin_port = htons( PORT );

    // Bind the server to the port and address
    if (bind(server_fd, (struct sockaddr *)&address,sizeof(address))<0)
    {
        perror("Binding Error : ");
        exit(EXIT_FAILURE);
    }
    //listen to the connections
    if (listen(server_fd, 5) < 0)
    {
        perror("Listen Error : ");
        exit(EXIT_FAILURE);
    }

    while(1)  
    {  
        //clear the socket set 
        FD_ZERO(&readfds);  

        //add master socket to set 
        FD_SET(server_fd, &readfds);  
        max_sd = server_fd;  

        //add child sockets to set 
        for ( i = 0 ; i < max_clients ; i++)  
        {  
            //socket descriptor 
            sd = client_socket[i];                 
            //if valid socket descriptor then add to read list 
            if(sd > 0)  
                FD_SET( sd , &readfds);                  
            //highest file descriptor number, need it for the select function 
            if(sd > max_sd)  
                max_sd = sd;  
        }      
        //wait for an activity on one of the sockets , timeout is NULL, so wait indefinitely 
        activity = select( max_sd + 1 , &readfds , NULL , NULL , NULL); 

        if ((activity < 0) && (errno!=EINTR))  printf("select error"); 

        //If something happened on the master socket ,then its an incoming connection 
        if (FD_ISSET(server_fd, &readfds))  
        {  
            if ((new_socket = accept(server_fd, (struct sockaddr *)&address,(socklen_t*)&addrlen))<0)  
            {  
                perror("accept");  
                exit(EXIT_FAILURE);  
            }  

            //Print out the details of the new connection 
            printf("New connection , socket fd is %d , ip is : %s , port : %d\n" , new_socket , inet_ntoa(address.sin_addr) , ntohs(address.sin_port));   

            //add new socket to array of sockets 
            for (i = 0; i < max_clients; i++)  
            {  
                //if position is empty 
                if( client_socket[i] == 0 )  
                {  
                    client_socket[i] = new_socket;                         
                    break;  
                }  
            }   
        }  

        //else its some IO operation on some other socket
        else
        {
            for (i = 0; i < max_clients; i++)  
            {  
                sd = client_socket[i];  

                if (FD_ISSET( sd , &readfds))  
                {  
                    //Read the incoming message. If valread = 0, then that client disconnected
                    valread = read( sd , buffer, 1024) ;
                    if(valread == 0) 
                    {  
                        //Somebody disconnected , get his details and print 
                        getpeername(sd , (struct sockaddr*)&address ,(socklen_t*)&addrlen);  
                        printf("Host disconnected , ip %s , port %d \n" ,inet_ntoa(address.sin_addr) , ntohs(address.sin_port));  

                        //Increase the closed connection by 1    
                        closed_connections++;  
                        client_socket[i] = 0;  
                    }  

                    //Send message to the client that sent the message
                    else
                    {  

                        buffer[valread] = '\0'; 
                        getpeername(sd , (struct sockaddr*)&address ,(socklen_t*)&addrlen);
                        printf("Message received from client connected to port %d is %s \n",ntohs(address.sin_port), buffer );
                        send(sd , hello , strlen(hello) , 0 );
                        printf("Hello message sent to client connected on port %d\n", ntohs(address.sin_port));
                    }  
                }  
            }            
        }

        //If all the clients disconnected then break out of while loop
        if(closed_connections == max_clients) break;
    }
    close(server_fd);
    return 0;
}

编译client.c

gcc client.c -o client

编译 server.c

mpicc server.c -o server

当我使用命令 ./client 和 mpirun -np 2 ./client 运行服务器和客户端时，我在 服务器上得到以下输出侧面

 New connection , socket fd is 4 , ip is : 127.0.0.1 , port : 57448
    New connection , socket fd is 5 , ip is : 127.0.0.1 , port : 57450
    Message received from client connected to port 57450 is Hello from client 
    Hello message sent to client connected on port 57450
    Message received from client connected to port 57450 is Hello from client 
    Hello message sent to client connected on port 57450
    Host disconnected , ip 127.0.0.1 , port 57448 
    Host disconnected , ip 127.0.0.1 , port 57450 

客户端的输出是：

Hello message sent from client_rank 0
Hello message sent from client_rank 1
The message received from server by client_rank 0 is Hello from server
The message received from server by client_rank 1 is Hello from server

我不知道，为什么服务器不关心旧客户端。

我还注意到的第二个问题是，即使我使用了 select() 函数，服务器也没有收到客户端发送的 message2。我认为由于我使用了 select 功能，服务器应该跟踪客户端发送的任何发送或断开请求。

我们将不胜感激。

Answer 1

您的程序中存在竞争条件。

客户端send()第二条消息，然后close()立即套接字。

在服务器端，当一条消息被读取时，它总是发回给客户端。

这意味着如果客户端 close() 套接字在服务器（完全）发送其回复之前，SIGPIPE 可能会发生。

Answer 2

address 始终是最近接受的客户端。您永远不会将其设置回您刚刚从中读取消息的客户端的地址。您可以使用getpeername() 来做到这一点，但最好只打印实际的套接字 FD 而不是端口。

无论如何，消息总是错误的。它们没有连接到到那个端口，它们是从那个端口连接的。

所以这里没有实际问题#1 需要解决。只是双重误导信息。

您提到的第二个问题是由于您未能对send() 和recv() 函数进行错误检查而引起的。所以你没有证据表明他们没有失败，所以没有理由期望完美的操作。通过错误检查再试一次。可能很有趣。