并发服务器就是能够同时处理多个客户端请求的服务器。并发服务器可通过多进程或者多线程程序来实现，这里采用多进程程序。

服务器

#include<stdio.h>  #include<stdlib.h>  #include<string.h>  #include<unistd.h>  #include<sys/types.h>  #include<sys/socket.h>  #include<netinet/in.h>/*定义了struct sockaddr_in*/  void main()  {      int sockfd;      /*创建socket*/      sockfd=socket(AF_INET,SOCK_STREAM,0);      if(sockfd==-1)      {          printf("create socket false");          exit(1);          }      /*绑定地址*/      struct sockaddr_in server_addr;      server_addr.sin_family=AF_INET;      server_addr.sin_port=htons(5000);      server_addr.sin_addr.s_addr=htonl(INADDR_ANY);      if(bind(sockfd,(const struct sockaddr *)(&server_addr),sizeof(server_addr))==-1)      {          printf("bind false");          exit(1);          }      /*监听端口*/      listen(sockfd,3);      /*父进程等待连接，成功连接后创建子进程收发数据*/      int newfd;      struct sockaddr_in client_addr;      int addrlen=sizeof(client_addr);      char buf[128];      pid_t pid;      while(1)      {          newfd=accept(sockfd,(struct sockaddr *)(&client_addr),&addrlen);          pid=fork();          if(pid==0)          {              while(1)              {                  recv(newfd,buf,128,0);                  printf("the message is from :%s\n",inet_ntoa(client_addr.sin_addr));                  printf("the message received is %s\n",buf);                  if(strcmp(buf,"exit")==0)                  {                      break;                    }                }              close(newfd);              close(sockfd);              exit(0);          }      }      /*结束连接*/      close(newfd);      close(sockfd);  }

客户端

#include<stdio.h>  #include<stdlib.h>  #include<string.h>  #include<unistd.h>  #include<sys/types.h>  #include<sys/socket.h>  #include<netinet/in.h>/*定义了struct sockaddr_in*/  void main()  {      /*创建socket*/      int sockfd=socket(AF_INET,SOCK_STREAM,0);      if(sockfd==-1)      {          printf("create socket false");          exit(1);          }      /*连接服务器*/      struct sockaddr_in server_addr;      server_addr.sin_family=AF_INET;      server_addr.sin_port=htons(5000);      server_addr.sin_addr.s_addr=inet_addr("192.168.137.246");      if(connect(sockfd,(const struct sockaddr *)(&server_addr),sizeof(server_addr))==-1)      {          printf("connect false");          exit(1);          }      /*收发数据*/      printf("please enter your message,and exit for exit\n");      char buf[128];      while(1)      {          scanf("%s",buf);          send(sockfd,buf,128,0);          if(strcmp(buf,"exit")==0)              break;      }      /*结束连接*/      close(sockfd);  }