１）Select是一种多路复用IO输入输出模式，在linux的输入输出编程中通过select的轮询机制，发现可用/可读或可写的接口。
   ２）低级socket程序中有一个共同点：都是基于阻塞式的编程方式
   ３）非阻塞式是函数调用时不阻塞，不管函数执行成功与否，都会立即返回。
   ４）优点：程序效率提升
   ５）缺点：返回的结果往往是错误的类型码
   ６）解决方案：Select机制。

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２.实例操作

 

１）基于Select模式实现一个网络echo的服务程序，即客户端向服务端发送信息，服务器接收到信息后，再将信息原样转发给客户端
   ２）需要设置Select函数
   ３）若当前有新连接，则加入到客户端套接字集合，若数量过载，则断开本次连接，并发送提示信息：sorry overload
   ４）源代码

//selectsocket.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#define SERVER_PORT 5555
#define QUEUE_LENGTH 5
#define BUF_SIZE 200
int main(void)
{int server_socket, new_socket;struct sockaddr_in server_addr;  struct sockaddr_in client_addr;socklen_t sin_size;int client_socket[QUEUE_LENGTH];  int conn_num;  int yes = 1;char buf[BUF_SIZE];int ret;int i;if ((server_socket = socket(AF_INET, SOCK_STREAM, 0)) < 0){perror("socket");return 0;}if (setsockopt(server_socket, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1){perror("setsockopt");return 0;}server_addr.sin_family = AF_INET;       server_addr.sin_port = htons(SERVER_PORT);   server_addr.sin_addr.s_addr = INADDR_ANY;memset(server_addr.sin_zero, '\0', sizeof(server_addr.sin_zero));if (bind(server_socket, (struct sockaddr *)&server_addr, sizeof(server_addr)) == -1){perror("bind");return 0;}if (listen(server_socket, 5) == -1){perror("listen");return 0;}printf("listen port %d\n", SERVER_PORT);fd_set clientfdset;int maxsock;struct timeval tv;conn_num = 0;sin_size = sizeof(client_addr);maxsock = server_socket;while (1){// initialize file descriptor setFD_ZERO(&clientfdset);FD_SET(server_socket, &clientfdset);// timeout settingtv.tv_sec = 15;tv.tv_usec = 0;// add active connection to fd setfor (i = 0; i < QUEUE_LENGTH; i++){if (client_socket[i] != 0){FD_SET(client_socket[i], &clientfdset);}}ret = select(maxsock + 1, &clientfdset, NULL, NULL, &tv);if (ret < 0){perror("select");break;}else if (ret == 0){printf("waitting timeout\n");continue;}// check every fd in the setfor (i = 0; i < conn_num; i++){if (FD_ISSET(client_socket[i], &clientfdset)){ret = recv(client_socket[i], buf, sizeof(buf), 0);if (ret <= 0){      printf("client[%d] close\n", i);close(client_socket[i]);FD_CLR(client_socket[i], &clientfdset);client_socket[i] = 0;}else{   printf("Client[%d] msg:%s\n", i, buf);send(client_socket[i], buf, sizeof(buf), 0);}}}if (FD_ISSET(server_socket, &clientfdset)){new_socket = accept(server_socket, (struct sockaddr *)&client_addr, &sin_size);if (new_socket <= 0){perror("accept");continue;}if (conn_num < QUEUE_LENGTH){client_socket[conn_num++] = new_socket;printf("new client[%d] %s:%d\n", conn_num,inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));if (new_socket > maxsock)maxsock = new_socket;}else{send(new_socket, "sorry overload!", sizeof("sorry overload!"), 0);close(new_socket);break;}}}for (i = 0; i < QUEUE_LENGTH; i++){if (client_socket[i] != 0){close(client_socket[i]);}}
}