这是我创建的一个迭代服务器，用于处理基本的客户端-服务器聊天应用程序。

我正在尝试在终端窗口上运行 TCPserver 并在多个终端窗口上运行 TCPclient。

尽管我设置了 backlog 值(listen 系统调用)在服务器套接字中为 5。

我预计最多可以连接 5 个客户端(一次只能接受 1 个)。

我对 listen 系统调用中设置的 backlog 值的理解有误吗？ 请澄清。

int listen(int sockfd, int backlog);

The backlog argument defines the maximum length to which the queue of pending connections for sockfd may grow.

这里是实际的程序供引用。

TCP服务器.c

#include<stdio.h>
#include<stdlib.h>

#include<sys/types.h>
#include<sys/socket.h>

#include<netinet/in.h>
#include<unistd.h>
#define BACKLOG 5

#include <netinet/in.h>
#include <arpa/inet.h>

#include<string.h>

int main()
{
    //create the server socket
    int sd;
    sd=socket(AF_INET,SOCK_STREAM,0);
    if(sd==-1)
    {
        perror("Some error occured in creating the socket: ");   
        //Interprets the value of errno as an error message, and prints it to stderr 
        exit(EXIT_FAILURE);
    }
    else
        printf("Socket created!\n");

    //define the server address
    struct sockaddr_in server_address;
    
    server_address.sin_family = AF_INET;
    server_address.sin_port = htons(9002);
    server_address.sin_addr.s_addr = INADDR_ANY; 
    //inet_addr("192.168.137.163");//INADDR_ANY;

    //bind the socket to our specified IP and port
    int bind_status = bind(sd, (struct sockaddr *) &server_address, sizeof(server_address));
    if(bind_status == -1)
    {
        perror("An error occurred in binding the socket: " );
        exit(EXIT_FAILURE);
    }
    else
        printf("Bind Successful!\n");


    //listen for connections
    int listen_status = listen(sd, BACKLOG);    
    if(listen_status == -1)
    {
        perror("Error occured in listening: ");
        exit(EXIT_FAILURE);
    } 
    else
        printf("Server is listening!\n");

    while(1)
    {
        //Accept a connection and create a new socket for this connection
        int new_sd;
        struct sockaddr_in client_address;
        int client_address_size = sizeof(client_address);

        new_sd= accept(sd, (struct sockaddr*) &client_address, &client_address_size);
        if(new_sd==-1)
        {
            perror("Can't accept connection: ");
            exit(EXIT_FAILURE);
        }
        else
            printf("Accept successful!\nA new client has connected. He'll soon send you a message.\n (You can chat or say \"exit\" to stop chatting)\n\n");
    

        //send a message to the client
        char buffer[256] = "Welcome to the server, lets chat! \n (You can chat or say \"exit\" to stop chatting)\n";
        send(new_sd,buffer,sizeof(buffer),0);

        //start chat
        while(1)
        {
            memset(buffer,0,256);
            int n = recv(new_sd, buffer, sizeof(buffer),0);
            if(n==-1 || strcmp(buffer,"exit\n")==0 || strcmp(buffer,"exit")==0)
                break;
            printf("\nclient said: %s\n",buffer);

            memset(buffer,0,256);
            printf("Say something: ");
            fgets(buffer,256,stdin);
            n = send(new_sd, buffer, sizeof(buffer), 0);
            if(n==-1 || strcmp(buffer,"exit\n")==0)
                break;
        }

        //close the sockets
        close(new_sd);
        printf("\nConnection ended. waiting for new connection now . . .\n");
    }

    close(sd);

    return 0;
}

这是客户

TCP客户端.c

#include<stdio.h>
#include<stdlib.h>

#include<sys/types.h>
#include<sys/socket.h>

#include<netinet/in.h>

#include<unistd.h>

#include <netinet/in.h>
#include <arpa/inet.h>
#include<string.h>
int main()
{
    //create the socket
    int sd = socket(AF_INET,SOCK_STREAM,0);

    if(sd==-1)
    {
        perror("Some error occurred in creating the socket: ");
        exit(EXIT_FAILURE);
    }
    else
        printf("Socket created!\n");

    //specify an address for the socket
    struct sockaddr_in server_address;
    
    server_address.sin_family = AF_INET;
    server_address.sin_port = htons(9002);
    server_address.sin_addr.s_addr = inet_addr("127.0.0.1");//INADDR_ANY;

    //Connect to the server
    int connection_status = connect(sd,(struct sockaddr *) &server_address, sizeof(server_address));

    if(connection_status == -1)
    {
        perror("There was an error connecting to the remote socket: ");
        exit(EXIT_FAILURE);
    }
    else
        printf("Connected to the server! Waiting in the queue for the server to accept the connection...\n");
        
    char buffer[256];
    //get the connection message from the server
    recv(sd, &buffer, sizeof(buffer), 0);
    printf("Server Said: %s\n", buffer);

    //start chat
    while(1)
    {
        memset(buffer,0,256);
        printf("Say something: ");
        fgets(buffer,256,stdin);
        int n = send(sd,buffer,sizeof(buffer),0);
        if(n==-1 || strcmp(buffer,"exit\n")==0)
            break;       

        memset(buffer,0,256);
        n = recv(sd,buffer,sizeof(buffer),0);
        if(n==-1 || strcmp(buffer,"exit\n")==0)
            break;

        printf("\nServer said: %s\n",buffer);
    }

    //close the socket
    close(sd);

    return 0;
}

有人建议我检查我的系统是否启用了 syncookies。当我执行 cat/proc/sys/net/ipv4/tcp_syncookies 时，我得到 1。

最佳答案

积压是“待处理 连接队列”——一旦您接受连接，它就不再是待处理的，并从队列中移除，为另外 5 个待处理连接留出空间。

如果您想限制为 5 个连接，那么您需要计算您已接受(且未关闭)的数量。任何更多的连接尝试都会在队列中等待。

关于即使积压队列已满，客户端仍连接到 TCP 迭代服务器，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/39497530/