How to implement multiple clients to connect to the server using linux socket

1. Introduction

In actual situations, people often encounter multiple clients connecting to the server. Since the previously introduced functions such as connect, recv, send, etc. are all blocking functions, if the resources are not fully prepared, the process calling the function will enter sleep state, which will not be able to handle I/O multiplexing. .

This article gives two methods of I/O multiplexing: fcntl(), select(). It can be seen that since Linux treats socket as a special file descriptor, this brings great convenience to users.

2. The fcntl

fcntl() function has the following characteristics:

1) Non-blocking I/O: cmd can be set to F_SETFL, Set lock to O_NONBLOCK

2) Signal-driven I/O: You can set cmd to F_SETFL and set lock to O_ASYNC.

Routine:

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/un.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <netinet/in.h>
#include <fcntl.h>
#include <unistd.h>

#define SERVPORT 3333
#define BACKLOG 10
#define MAX_CONNECTED_NO 10
#define MAXDATASIZE 100

int main()
{
  struct sockaddr_in server_sockaddr,client_sockaddr;
  int sin_size,recvbytes,flags;
  int sockfd,client_fd;
  char buf[MAXDATASIZE];
/*创建socket*/
  if((sockfd = socket(AF_INET,SOCK_STREAM,0))==-1){
    perror("socket");
    exit(1);
  }
  printf("socket success!,sockfd=%d\n",sockfd);

/*设置sockaddr结构*/
  server_sockaddr.sin_family=AF_INET;
  server_sockaddr.sin_port=htons(SERVPORT);
  server_sockaddr.sin_addr.s_addr=INADDR_ANY;
  bzero(&(server_sockaddr.sin_zero),8);

/*将本地ip地址绑定端口号*/
  if(bind(sockfd,(struct sockaddr *)&server_sockaddr,sizeof(struct sockaddr))==-1){
    perror("bind");
    exit(1);
  }
  printf("bind success!\n");

/*监听*/
  if(listen(sockfd,BACKLOG)==-1){
    perror("listen");
    exit(1);
  }
  printf("listening....\n");

/*fcntl()函数，处理多路复用I/O*/
  if((flags=fcntl( sockfd, F_SETFL, 0))<0)
      perror("fcntl F_SETFL");
    flags |= O_NONBLOCK;
    if(fcntl( sockfd, F_SETFL,flags)<0)
      perror("fcntl");
  while(1){
    sin_size=sizeof(struct sockaddr_in);
    if((client_fd=accept(sockfd,(struct sockaddr*)&client_sockaddr,&sin_size))==-1){  //服务器接受客户端的请求，返回一个新的文件描述符
      perror("accept");
      exit(1);
    }
    if((recvbytes=recv(client_fd,buf,MAXDATASIZE,0))==-1){
      perror("recv");
      exit(1);
    }
    if(read(client_fd,buf,MAXDATASIZE)<0){
      perror("read");
      exit(1);
    }
    printf("received a connection :%s",buf);

/*关闭连接*/
  close(client_fd);
  exit(1);
  }/*while*/
}

Run this Program:

[root@localhost net]# ./fcntl
socket success!,sockfd=3
bind success!
listening....
accept: Resource temporarily unavailable

You can see that when the accept resource is unavailable, the program will automatically return.

If you replace the red bold code with:

if((flags=fcntl( sockfd, F_SETFL, 0))<0)
      perror("fcntl F_SETFL");
    flags |= O_ASYNC;
    if(fcntl( sockfd, F_SETFL,flags)<0)
      perror("fcntl");

The running results are as follows:

[root@localhost net]# ./fcntl1
socket success!,sockfd = 3
bind success!
listening...

You can see that the process is waiting until until another relevant signal drives it.

3.select

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/un.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <netinet/in.h>
#define SERVPORT 3333
#define BACKLOG 10
#define MAX_CONNECTED_NO 10
#define MAXDATASIZE 100
int main()
{
  struct sockaddr_in server_sockaddr,client_sockaddr;
  int sin_size,recvbytes;
  fd_set readfd;
  fd_set writefd;
  int sockfd,client_fd;
  char buf[MAXDATASIZE];
/*创建socket*/
  if((sockfd = socket(AF_INET,SOCK_STREAM,0))==-1){
    perror("socket");
    exit(1);
  }
  printf("socket success!,sockfd=%d\n",sockfd);
/*设置sockaddr结构*/
  server_sockaddr.sin_family=AF_INET;
  server_sockaddr.sin_port=htons(SERVPORT);
  server_sockaddr.sin_addr.s_addr=INADDR_ANY;
  bzero(&(server_sockaddr.sin_zero),8);
/*将本地ip地址绑定端口号*/
  if(bind(sockfd,(struct sockaddr *)&server_sockaddr,sizeof(struct sockaddr))==-1){
    perror("bind");
    exit(1);
  }
  printf("bind success!\n");
/*监听*/
  if(listen(sockfd,BACKLOG)==-1){
    perror("listen");
    exit(1);
  }
  printf("listening....\n");
/*select*/
  FD_ZERO(&readfd);              // 将readfd 清空 
FD_SET(sockfd,&readfd);         //将sockfd加入到readfd集合中
  while(1){
  sin_size=sizeof(struct sockaddr_in);
  if(select(MAX_CONNECTED_NO,&readfd,NULL,NULL,(struct timeval（FD_ISSET(sockfd,&readfd)>0){         // FD_ISSET 这个宏判断 sockfd 是否属于可读的文件描述符。从 sockfd 中读入, 输出到标准输出上去.
      if((client_fd=accept(sockfd,(struct sockaddr *)&client_sockaddr,&sin_size))==-1){   //client_sockaddr：客户端地址
        perror("accept");
        exit(1);
      }
      if((recvbytes=recv(client_fd,buf,MAXDATASIZE,0))==-1){
        perror("recv");
        exit(1);
      }
      if(read(client_fd,buf,MAXDATASIZE)<0){
        perror("read");
        exit(1);
      }
      printf("received a connection :%s",buf);
    }/*if*/
    close(client_fd);
    }/*select*/
  }/*while*/
}
运行结果如下：
[root@localhost net]#  gcc select1.c -o select1
[root@localhost net]# ./select1
socket create success!
bind success!
listening...

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