为什么在服务器端和客户端调用 recv 和 send 函数时使用客户端的文件描述符？

Question

TCP服务器

#include<stdio.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/ip.h>
#include<netinet/in.h>
#include<string.h>
#include<stdlib.h>

int main()
{
    int fd = socket(AF_INET, SOCK_STREAM, 0);
    if(fd == -1)
    {
        printf("socket failed!\n");
        exit(0);
    }
    printf("Enter port: ");
    int port;
    scanf("%d",&port);
    struct sockaddr_in server;
    server.sin_family = AF_INET;
    server.sin_port = htons(port);
    server.sin_addr.s_addr = INADDR_ANY;

    int bind_ret = bind(fd, (struct sockaddr*)(&server), sizeof(server));
    if(bind_ret == -1)
    {
       printf("bind failed!\n");
       exit(0);
    }

    int listen_ret = listen(fd, 10);
    if(listen_ret == -1)
    {
           printf("listen failed!\n");
           exit(0);
    }

    struct sockaddr_in client;
    int l = sizeof(client);
    int client_fd = accept(fd, (struct sockaddr*)(&client), &l);
    if(client_fd == -1)
    {
        printf("accept failed!\n");
        exit(0);
    }

    while(1)
    {
        char msg_recv[50];
        int recv_ret = recv(client_fd, msg_recv, sizeof(msg_recv),0);
        if(recv_ret == -1)
        {
            printf("recv failed!\n");
            exit(0);
        }
    
        msg_recv[recv_ret]='[=10=]';
        if(strcmp("bye",msg_recv)==0)
        {
            exit(0);
        }
    
        printf("Message recieved: %s\n",msg_recv);
    
        char msg_send[50];
        printf("Enter message: ");
        scanf(" %s",msg_send);
        int send_ret = send(client_fd, msg_send, strlen(msg_send),0);
        if(send_ret == 0)
        {
            printf("send failed!\n");
        }
    
        if(strcmp("bye",msg_send) == 0)
         exit(0);
    }
}

TCPClient

#include<stdio.h>
#include<sys/socket.h>
#include<sys/types.h>
#include<netinet/ip.h>
#include<netinet/in.h>
#include<string.h>
#include<stdlib.h>

int main()
{ int fd = socket(AF_INET, SOCK_STREAM, 0);
  if(fd == -1)
  { 
    printf("socket failed!\n");
    exit(0);
  }

  int port;
  printf("Enter port number: ");
  scanf("%d",&port);

  struct sockaddr_in client;

  client.sin_family = AF_INET;
  client.sin_port = htons(port);
  client.sin_addr.s_addr = INADDR_ANY;

  int connect_ret = connect(fd, (struct sockaddr*)(&client), sizeof(client));
  if(connect_ret == -1)
  {
    printf("connect failed!\n");
    exit(0);
  }

  while(1)
  {
    printf("Enter message: ");
    char msg_send[50];
    scanf("%s",msg_send);

    int send_ret = send(fd, msg_send, strlen(msg_send), 0);
    if(send_ret == -1)
    {
        printf("send failed!\n");
        exit(0);
    } 

    if(strcmp("bye", msg_send)==0)
    {
        exit(0);
    }

    char msg_recv[50];
    int recv_ret = recv(fd, msg_recv, sizeof(msg_recv), 0);
    if(recv_ret == -1)
    {
        printf("recv failed!\n");
        exit(0);
    }
    msg_recv[recv_ret]= '[=11=]';
    if(strcmp("bye", msg_recv) == 0)
     exit(0);
 
    printf("Message recieved: %s \n",msg_recv);
  }
 }

在上面的服务器程序中，recv和send是通过传递client_fd作为参数来调用的，而在客户端程序中，recv和send 通过传递 fd 作为参数来调用。我想知道为什么在服务器端我们没有像在客户端那样使用它自己的套接字文件描述符？

Answer 1

服务器的 fd 描述符是一个 listen()'ing 套接字。它不能执行任何 I/O，只能接收传入的客户端连接。 accept() 从 fd 的队列中拉出一个挂起的客户端连接和 returns 一个可以与该客户端执行 I/O 的新套接字描述符。

客户端的 fd 描述符是一个 connect()ing 套接字。一旦连接被接受，它就可以与服务器执行 I/O。

为什么在服务器端和客户端调用 recv 和 send 函数时使用客户端的文件描述符？

Why is the client's file descriptor used while calling the recv and send funtions on both server and client sides?

sockets

tcpclient

send

recv

tcpserver