int select (int maxfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);

select系統呼叫是用來讓我們的程式監視多個檔案控制代碼(socket 控制代碼)的狀態變化的。程式會停在select這裡等待，直到被監視的檔案控制代碼有一個或多個發生了狀態改變

有一片博文寫得非常詳細易理解http://blog.csdn.net/lingfengtengfei/article/details/12392449。推薦大家看看，這裡就不說了。

主要貼程式碼，參考的也是別人的程式碼，但是發現有BUG，努力修正後實現多臺客戶段與一臺伺服器通訊：在非阻塞模式下，伺服器和客戶端可以自由發訊息，不必等待回答，目前伺服器發的訊息，所有客戶端都會收到此訊息。讀者可以自己改一下，讓伺服器與指定的客戶端通訊(可以通過鍵盤輸入要通訊的客戶端編號來控制，或者用棧或佇列來儲存客戶端編號，伺服器在分別傳送訊息)：

伺服器端程式碼：

#include<stdio.h>
#include<stdlib.h>
#include<netinet/in.h>
#include<sys/socket.h>
#include<arpa/inet.h>
#include<string.h>
#include<unistd.h>
#define BACKLOG 5     //完成三次握手但沒有accept的佇列的長度
#define CONCURRENT_MAX 8   //應用層同時可以處理的連線
#define SERVER_PORT 11332
#define BUFFER_SIZE 1024
#define QUIT_CMD ".quit"
int client_fds[CONCURRENT_MAX];
int main(int argc, const char * argv[])
{
    char input_msg[BUFFER_SIZE];
    char recv_msg[BUFFER_SIZE];
    //本地地址
    struct sockaddr_in server_addr;
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(SERVER_PORT);
    server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    bzero(&(server_addr.sin_zero), 8);
    //建立socket
    int server_sock_fd = socket(AF_INET, SOCK_STREAM, 0);
    if(server_sock_fd == -1)
    {
    	perror("socket error");
    	return 1;
    }
    //繫結socket
    int bind_result = bind(server_sock_fd, (struct sockaddr *)&server_addr, sizeof(server_addr));
    if(bind_result == -1)
    {
    	perror("bind error");
    	return 1;
    }
    //listen
    if(listen(server_sock_fd, BACKLOG) == -1)
    {
    	perror("listen error");
    	return 1;
    }
    //fd_set
    fd_set server_fd_set;
    int max_fd = -1;
    struct timeval tv;  //超時時間設定
    while(1)
    {
    	tv.tv_sec = 20;
    	tv.tv_usec = 0;
    	FD_ZERO(&server_fd_set);
    	FD_SET(STDIN_FILENO, &server_fd_set);
    	if(max_fd <STDIN_FILENO)
    	{
    		max_fd = STDIN_FILENO;
    	}
        //printf("STDIN_FILENO=%d\n", STDIN_FILENO);
	//伺服器端socket
        FD_SET(server_sock_fd, &server_fd_set);
       // printf("server_sock_fd=%d\n", server_sock_fd);
        if(max_fd < server_sock_fd)
        {
        	max_fd = server_sock_fd;
        }
	//客戶端連線
        for(int i =0; i < CONCURRENT_MAX; i++)
        {
        	//printf("client_fds[%d]=%d\n", i, client_fds[i]);
        	if(client_fds[i] != 0)
        	{
        		FD_SET(client_fds[i], &server_fd_set);
        		if(max_fd < client_fds[i])
        		{
        			max_fd = client_fds[i];
        		}
        	}
        }
        int ret = select(max_fd + 1, &server_fd_set, NULL, NULL, &tv);
        if(ret < 0)
        {
        	perror("select 出錯\n");
        	continue;
        }
        else if(ret == 0)
        {
        	printf("select 超時\n");
        	continue;
        }
        else
        {
        	//ret 為未狀態發生變化的檔案描述符的個數
        	if(FD_ISSET(STDIN_FILENO, &server_fd_set))
        	{
        		printf("傳送訊息：\n");
        		bzero(input_msg, BUFFER_SIZE);
        		fgets(input_msg, BUFFER_SIZE, stdin);
        		//輸入“.quit"則退出伺服器
        		if(strcmp(input_msg, QUIT_CMD) == 0)
        		{
        			exit(0);
        		}
        		for(int i = 0; i < CONCURRENT_MAX; i++)
        		{
        			if(client_fds[i] != 0)
        			{
        				printf("client_fds[%d]=%d\n", i, client_fds[i]);
        				send(client_fds[i], input_msg, BUFFER_SIZE, 0);
        			}
        		}
        	}
        	if(FD_ISSET(server_sock_fd, &server_fd_set))
        	{
        		//有新的連線請求
        		struct sockaddr_in client_address;
        		socklen_t address_len;
        		int client_sock_fd = accept(server_sock_fd, (struct sockaddr *)&client_address, &address_len);
        		printf("new connection client_sock_fd = %d\n", client_sock_fd);
        		if(client_sock_fd > 0)
        		{
        			int index = -1;
        			for(int i = 0; i < CONCURRENT_MAX; i++)
        			{
        				if(client_fds[i] == 0)
        				{
        					index = i;
        					client_fds[i] = client_sock_fd;
        					break;
        				}
        			}
        			if(index >= 0)
        			{
        				printf("新客戶端(%d)加入成功 %s:%d\n", index, inet_ntoa(client_address.sin_addr), ntohs(client_address.sin_port));
        			}
        			else
        			{
        				bzero(input_msg, BUFFER_SIZE);
        				strcpy(input_msg, "伺服器加入的客戶端數達到最大值,無法加入!\n");
        				send(client_sock_fd, input_msg, BUFFER_SIZE, 0);
        				printf("客戶端連線數達到最大值，新客戶端加入失敗 %s:%d\n", inet_ntoa(client_address.sin_addr), ntohs(client_address.sin_port));
        			}
        		}
        	}
        	for(int i =0; i < CONCURRENT_MAX; i++)
        	{
        		if(client_fds[i] !=0)
        		{
        			if(FD_ISSET(client_fds[i], &server_fd_set))
        			{
        				//處理某個客戶端過來的訊息
        				bzero(recv_msg, BUFFER_SIZE);
        				long byte_num = recv(client_fds[i], recv_msg, BUFFER_SIZE, 0);
        				if (byte_num > 0)
        				{
        					if(byte_num > BUFFER_SIZE)
        					{
        						byte_num = BUFFER_SIZE;
        					}
        					recv_msg[byte_num] = '\0';
        					printf("客戶端(%d):%s\n", i, recv_msg);
        				}
        				else if(byte_num < 0)
        				{
        					printf("從客戶端(%d)接受訊息出錯.\n", i);
        				}
        				else
        				{
        					FD_CLR(client_fds[i], &server_fd_set);
        					client_fds[i] = 0;
        					printf("客戶端(%d)退出了\n", i);
        				}
        			}
        		}
        	}
        }
    }
    return 0;
}
 

#include<stdio.h>
#include<stdlib.h>
#include<netinet/in.h>
#include<sys/socket.h>
#include<arpa/inet.h>
#include<string.h>
#include<unistd.h>
#define BUFFER_SIZE 1024

int main(int argc, const char * argv[])
{
    struct sockaddr_in server_addr;
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(11332);
    server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    bzero(&(server_addr.sin_zero), 8);

    int server_sock_fd = socket(AF_INET, SOCK_STREAM, 0);
    if(server_sock_fd == -1)
    {
	perror("socket error");
	return 1;
    }
    char recv_msg[BUFFER_SIZE];
    char input_msg[BUFFER_SIZE];

    if(connect(server_sock_fd, (struct sockaddr *)&server_addr, sizeof(struct sockaddr_in)) == 0)
    {
	fd_set client_fd_set;
	struct timeval tv;

	while(1)
	{
		tv.tv_sec = 20;
		tv.tv_usec = 0;
	    FD_ZERO(&client_fd_set);
	    FD_SET(STDIN_FILENO, &client_fd_set);
	    FD_SET(server_sock_fd, &client_fd_set);

	   select(server_sock_fd + 1, &client_fd_set, NULL, NULL, &tv);
		if(FD_ISSET(STDIN_FILENO, &client_fd_set))
		{
		    bzero(input_msg, BUFFER_SIZE);
		    fgets(input_msg, BUFFER_SIZE, stdin);
		    if(send(server_sock_fd, input_msg, BUFFER_SIZE, 0) == -1)
		    {
		    	perror("傳送訊息出錯!\n");
		    }
		}
		if(FD_ISSET(server_sock_fd, &client_fd_set))
		{
		    bzero(recv_msg, BUFFER_SIZE);
		    long byte_num = recv(server_sock_fd, recv_msg, BUFFER_SIZE, 0);
		    if(byte_num > 0)
		    {
			if(byte_num > BUFFER_SIZE)
			{
			    byte_num = BUFFER_SIZE;
			}
			recv_msg[byte_num] = '\0';
			printf("伺服器:%s\n", recv_msg);
		    }
		    else if(byte_num < 0)
		    {
			printf("接受訊息出錯!\n");
		    }
		    else
		    {
			printf("伺服器端退出!\n");
			exit(0);
		    }
		}
	    }
	//}
    }
    return 0;
}

 

struct timeval tv 設定為0，這樣再次呼叫select時它會立即返回，根本不會監視socket控制代碼，導致一個超時的死迴圈。
所以每次呼叫select之後都要重新給struct timeval tv 賦值。