Simple implementation of Linux multi process CS server

server side

• Multi process realizes multi-user connection, i.e. one connection for each user. Here, the server is still used to capitalize the received string and return it to the client.
  • code implementation

  #include <stdio.h>
  #include <string.h>
  #include <netinet/in.h>
  #include <arpa/inet.h>
  #include <unistd.h>
  #include <ctype.h>
  #include <stdlib.h>
  #include <sys/wait.h>
  
  #define SERV_IP "127.0.0.1"
  #define SERV_PORT 8000
  
  void wait_child(int signo)
  {
      while(waitpid(0, NULL, WNOHANG)>0);
      return;
  }
  int main(int argc,char *argv[])
  {
      pid_t pid;//Process ID
      int sfd, cfd;//Receive the connected sfd and the cfd communicating with the client
      struct sockaddr_in serv_addr, clie_addr;//Create server and client structures
      socklen_t clie_addr_len;//Client structure length
      char buf[BUFSIZ], clie_IP[BUFSIZ];//buf stores received data
      int n , i;//Number of data read n, cycle factor i
  
      sfd = Socket(AF_INET, SOCK_STREAM, 0);//Create socket
  
      bzero(&serv_addr, sizeof(serv_addr));//Zero clearing
      serv_addr.sin_family = AF_INET;//Set protocol family to IPv4
      serv_addr.sin_addr.s_addr = htonl(INADDR_ANY); //Set the network card to any valid local network card
      //inet_pton(AF_INET, SERV_IP, &serv_addr.sin_addr.s_addr);
      serv_addr.sin_port = htons(SERV_PORT);//Set port
  
      bind(sfd, (struct sockaddr * )&serv_addr, sizeof(serv_addr));//Set socket to associate with sfd
      listen(sfd, 128);//Set the maximum number of incomplete accept. Start listening
      while(1)//Loop receive client connection
      {
          clie_addr_len = sizeof(clie_addr);//Initialize client structure length
          cfd = accept(sfd, (struct sockaddr *)&clie_addr, &clie_addr_len);//Receive client connection
          printf("client IP:%s, port:%d\n", inet_ntop(AF_INET, &clie_addr.sin_addr.s_addr, clie_IP, sizeof(clie_IP)), ntohs(clie_addr.sin_port));
          pid = fork();//Create a new process
          if(pid< 0)//error
          {
              perror("fork error");
              exit(1);
          }
          else if(pid == 0)//Subprocess
          {
              close(sfd);//Close parent process file descriptor first in child process
              break;
          }
          else if(pid>0)//Parent process
          {
              close(cfd);//Close child process file descriptor first in parent process
              signal(SIGCHLD, wait_child);
          }
      }
      if(pid == 0)//Here is the real business processing part of the subprocess
      {
          while(1)//Loop client business
          {
              n = read(cfd, buf, sizeof(buf));
              if(n == 0)//If read returns 0, the client is disconnected
              {  
                  close(cfd);//Close client file descriptor
                  return 0;
              }
              else if(n == -1)//error
              {
                  perror("read error");
                  exit(1);
              }
              else
              {
                  write(STDOUT_FILENO, buf, n);
                  for(i= 0; i< n;++i)
                  {
                      buf[i] = toupper(buf[i]);
                  }
                  write(cfd, buf, n);
              }
          }
      }
      return 0;
  }
  

client side

• Connect to the server side, send the string, and print the received string (that is, it has the same function as the nc command).
  • code implementation

  #include <stdio.h>
  #include <string.h>
  #include <unistd.h>
  #include <netinet/in.h>
  #include <arpa/inet.h>
  #define MAXLINE 8192
  #define SERV_PORT 8000
  
  int main(int argc,char *argv[])
  {
      struct sockaddr_in servaddr;
      char buf[MAXLINE];
      int sockfd, n;
  
      sockfd = socket(AF_INET, SOCK_STREAM, 0);
  
      bzero(&servaddr, sizeof(servaddr));
  
      servaddr.sin_family = AF_INET;
      inet_pton(AF_INET, "127.0.0.1", &servaddr.sin_addr);
      servaddr.sin_port = htons(SERV_PORT);
  
      connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr));
      while(fgets(buf, MAXLINE, stdin)!=NULL)
      {
          write(sockfd, buf, strlen(buf));
          n =Read(sockfd, buf, MAXLINE);
          if(n ==0)
          {
              printf("the other side has been closed.\n");
              break;
          }
          else 
              write(STDOUT_FILENO, buf, n);
      }
      close(sockfd);
      return 0;
  }
  

test result

• Multiple clients can connect to the server at the same time. Each time the server accepts a client, it creates a child process. After the client is disconnected, the child process is automatically recycled by the parent process.

Problems

• Without error handling, interference may occur when multiple users connect.