使用select函数能够以非堵塞的方式和多个socket通信。程序仅仅是演示select函数的使用，功能很easy，即使某个连接关闭以后也不会改动当前连接数。连接数达到最大值后会终止程序。

1. 程序使用了一个数组fd_A，通信開始后把须要通信的多个socket描写叙述符都放入此数组。

2. 首先生成一个叫sock_fd的socket描写叙述符，用于监听port。

3. 将sock_fd和数组fd_A中不为0的描写叙述符放入select将检查的集合fdsr。

4. 处理fdsr中能够接收数据的连接。假设是sock_fd，表明有新连接增加。将新增加连接的socket描写叙述符放置到fd_A。

这部分代码实现逻辑不错，只是有点bug，对套接字缓存未做处理完整。

#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 MYPORT 1234    // the port users will be connecting to

#define BACKLOG 5     // how many pending connections queue will hold

#define BUF_SIZE 200

int fd_A[BACKLOG];    // accepted connection fd

int conn_amount;    // current connection amount

void showclient()

{

    int i;

    printf("client amount: %d\n", conn_amount);

    for (i = 0; i < BACKLOG; i++) {

        printf("[%d]:%d  ", i, fd_A[i]);

    }

    printf("\n\n");

}

int main(void)

{

    int sock_fd, new_fd;  // listen on sock_fd, new connection on new_fd

    struct sockaddr_in server_addr;    // server address information

    struct sockaddr_in client_addr; // connector's address information

    socklen_t sin_size;

    int yes = 1;

    char buf[BUF_SIZE];

    int ret;

    int i;

    if ((sock_fd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {

        perror("socket");

        exit(1);

    }

    if (setsockopt(sock_fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1) {

        perror("setsockopt");

        exit(1);

    }

    server_addr.sin_family = AF_INET;         // host byte order

    server_addr.sin_port = htons(MYPORT);     // short, network byte order

    server_addr.sin_addr.s_addr = INADDR_ANY; // automatically fill with my IP

    memset(server_addr.sin_zero, '\0', sizeof(server_addr.sin_zero));

    if (bind(sock_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) == -1) {

        perror("bind");

        exit(1);

    }

    if (listen(sock_fd, BACKLOG) == -1) {

        perror("listen");

        exit(1);

    }

    printf("listen port %d\n", MYPORT);

    fd_set fdsr;

    int maxsock;

    struct timeval tv;

    conn_amount = 0;

    sin_size = sizeof(client_addr);

    maxsock = sock_fd;

    while (1) {

        // initialize file descriptor set

        FD_ZERO(&fdsr);

        FD_SET(sock_fd, &fdsr);

        // timeout setting

        tv.tv_sec = 30;

        tv.tv_usec = 0;

        // add active connection to fd set

        for (i = 0; i < BACKLOG; i++) {

            if (fd_A[i] != 0) {

                FD_SET(fd_A[i], &fdsr);

            }

        }

        ret = select(maxsock + 1, &fdsr, NULL, NULL, &tv);

        if (ret < 0) {

            perror("select");

            break;

        } else if (ret == 0) {

            printf("timeout\n");

            continue;

        }

        // check every fd in the set

        for (i = 0; i < conn_amount; i++) {

            if (FD_ISSET(fd_A[i], &fdsr)) {

                ret = recv(fd_A[i], buf, sizeof(buf), 0);

                if (ret <= 0) {        // client close

                    printf("client[%d] close\n", i);

                    close(fd_A[i]);

                    FD_CLR(fd_A[i], &fdsr);

                    fd_A[i] = 0;

                } else {        // receive data

                    if (ret < BUF_SIZE)

                        memset(&buf[ret], '\0', 1);

                    printf("client[%d] send:%s\n", i, buf);

                }

            }

        }

        // check whether a new connection comes

        if (FD_ISSET(sock_fd, &fdsr)) {

            new_fd = accept(sock_fd, (struct sockaddr *)&client_addr, &sin_size);

            if (new_fd <= 0) {

                perror("accept");

                continue;

            }

            // add to fd queue

            if (conn_amount < BACKLOG) {

                fd_A[conn_amount++] = new_fd;

                printf("new connection client[%d] %s:%d\n", conn_amount,

                        inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));

                if (new_fd > maxsock)

                    maxsock = new_fd;

            }

            else {

                printf("max connections arrive, exit\n");

                send(new_fd, "bye", 4, 0);

                close(new_fd);

                break;

            }

        }

        showclient();

    }

    // close other connections

    for (i = 0; i < BACKLOG; i++) {

        if (fd_A[i] != 0) {

            close(fd_A[i]);

        }

    }

    exit(0);

}

我这里做改动（大概逻辑）

    for(i=0;i<BACKLOG;i++)

    {

        tcpSockIndex[i] = -1;

    }

	while(1)

	{

		FD_ZERO(&readfds);

		FD_SET(tcpSock, &readfds);

        maxfdp = maxfdp>tcpSock?maxfdp:tcpSock;

        for(i=0;i<BACKLOG;i++)

        {

//            AB_LOG("FD_SET tcpSockIndex[%d] = %d.\n", i, tcpSockIndex[i]);

            if(-1 != tcpSockIndex[i])

            {

                FD_SET(tcpSockIndex[i], &readfds);

                maxfdp = maxfdp>tcpSockIndex[i]?

maxfdp:tcpSockIndex[i];

            }

        }

        timeout.tv_sec = SELECT_TIME_OUT_TM;

        timeout.tv_usec = 0;        

		ret = select(maxfdp+1, &readfds, NULL, NULL, &timeout);

		if(ret < 0)

		{

			AB_PERROR("select error!\n");

			return ;

		}

        else if(0 == ret)

        {

			AB_PERROR("select time out!\n");

        }

        //处理client发送的报文

        for(i=0; i<BACKLOG; i++)

        {

            if ( -1 != tcpSockIndex[i] &&

                 FD_ISSET(tcpSockIndex[i], &readfds))

            {

	            AB_LOG("--- tcp client ---.\n");

                pthread_t pthd2;

                TCP_SOCK_T * pTcpSock = NULL;

                pTcpSock = (TCP_SOCK_T  *)malloc(sizeof(TCP_SOCK_T));

                pTcpSock->sock = tcpSockIndex[i];

                pthread_create(&pthd2, NULL,  bc_sock_handle_client_data, (void *)pTcpSock);

                #if 0

                bc_sock_handle_client_data(tcpSockIndex[i]);

            	ret = read(tcpSockIndex[i], NULL,  0);

                AB_LOG("close tcpSockIndex[%d] = %d, ret = %d.\n", i, tcpSockIndex[i], ret );

                //关闭client连接的套接字

                if(-1 == tcpSockIndex[i]) close(tcpSockIndex[i]);

                #endif

                //清空client字符集

                FD_CLR(tcpSockIndex[i], &readfds);

                tcpSockIndex[i] = -1;

                tcpClientConnNum --;

            }

        }

        //获取client连接过来的套接字

        if(FD_ISSET(tcpSock, &readfds))

	    {

	        AB_LOG("--- tcp server ---.\n");

            if((tcpSockClient = accept(tcpSock,

                    (struct sockaddr*)&chiAddr, &cliLen)) <= 0 )

            {

                AB_PERROR("BCHV accept socket error: %s(errno: %d).\n",strerror(errno), errno);

                continue;

            }

            //在套接字数组中找出一个可用加入的位置。

for(i=0,tcpSockFlag=0; i<BACKLOG; i++)

            {

                AB_LOG("tcpSockIndex[%d] = %d.\n", i, tcpSockIndex[i]);

                if(-1 == tcpSockIndex[i])

                {

                    tcpSockIndex[i] = tcpSockClient;

                    tcpClientConnNum ++;

                    AB_LOG("new connection client[%d] %08X:%d.\n", tcpClientConnNum,

                            chiAddr.sin_addr.s_addr, ntohs(chiAddr.sin_port));

                    //错误打印

                    //AB_LOG("new connection client[%d] %s.\n", tcpClientConnNum,

                    //        inet_ntoa(chiAddr.sin_addr));

                    #if 0

                    if (sock_c > maxfdp)

                        maxfdp = sock_c;

                    #endif

                    tcpSockFlag = 1;

                    break;

                }

            }

            //超过最大连接请求。能够发送client断开连接

            if(0 == tcpSockFlag)

            {

                AB_LOG("max connections arrive, exit\n");

                send(tcpSockClient, "bye", 4, 0);

                close(tcpSockClient);

            }

        }

	}

    //关闭全部client套接字

    for(i=0; i<BACKLOG; i++)

    {

        if(-1 != tcpSockIndex[i])

        {

            close(tcpSockIndex[i]);

        }

    }

文章摘自 http://www.cnblogs.com/faraway/archive/2009/03/06/1404449.html