#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <libgen.h> //basename
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <assert.h>
#include <errno.h>


int main(int argc, char *argv[]){
    int i, maxi;
    char buf[1024];
    fd_set read_fds, all_read_fds;
    fd_set exception_fds, all_exception_fds;
    int nready, connectfd[FD_SETSIZE];  
    int connfd,sockfd,maxfd;
    
    struct sockaddr_in client_address;
    socklen_t client_addrlen = sizeof(client_address);
    
    if(argc <= 2){ 
        printf("usage: %s ip_address port_number\n", basename(argv[0]));
        return 1;
    }   

    const char *ip = argv[1];
    int port = atoi(argv[2]);
    
    int ret = 0;
    struct sockaddr_in address;
    bzero(&address, sizeof(address));
    address.sin_family = AF_INET;
    inet_pton(AF_INET, ip, &address.sin_addr);
    address.sin_port = htons(port);

    int listenfd = socket(AF_INET, SOCK_STREAM, 0);
    assert(listenfd >= 0);
    ret = bind(listenfd, (struct sockaddr *)&address, sizeof(address));
    assert(ret != -1);
    ret = listen(listenfd, 5);
    assert(ret != -1);

    maxfd = listenfd;
    maxi = -1;
    for(i= 0; i < FD_SETSIZE; i++)
        connectfd[i] = -1;
    FD_ZERO(&all_read_fds);
    FD_ZERO(&all_exception_fds);
    FD_SET(listenfd, &all_read_fds);
    //do not care about the OOB of listen socket

    while(1){
        read_fds = all_read_fds;
        exception_fds = all_exception_fds;
        memset(buf, 0, sizeof(buf));
        nready = select(maxfd + 1, &read_fds, NULL, &exception_fds, NULL);
        //new client connection coming
        if(FD_ISSET(listenfd, &read_fds)){
            connfd = accept(listenfd, (struct sockaddr *)&client_address, &client_addrlen);
            if(connfd < 0){
                perror("accept failed\n");
                exit(-1);
            }

            for(i=0; i < FD_SETSIZE; i++)
                if(connectfd[i] < 0){
                    connectfd[i] = connfd;
                    break;
                }
            if( i == FD_SETSIZE){
                printf("too many clients..\n");
                exit(-1);
            }
            // add new descriptor to the read set and exception set
            FD_SET(connfd, &all_read_fds);
            FD_SET(connfd, &all_exception_fds);
            if(connfd > maxfd)
                maxfd = connfd;
            if(i > maxi)
                maxi = i;// update the max index in connectfd[]
            //we complete the listen socktet processing
            if(--nready <= 0)
                continue;
        }
        for(i = 0; i <= maxi; i++){
            //check all the connected clients for data
            if((sockfd = connectfd[i]) < 0)
                continue;
            if(FD_ISSET(sockfd, &read_fds)){
                ret = recv(sockfd, buf, sizeof(buf) - 1, 0);
                if(ret <= 0)
                    break;//connection closed
                // process logic
                printf("get %d bytes of normal data:%s\n", ret, buf);
            //we complete the listen socktet processing
            if(--nready <= 0)
                continue;
        }
        for(i = 0; i <= maxi; i++){
            //check all the connected clients for data
            if((sockfd = connectfd[i]) < 0)
                continue;
            if(FD_ISSET(sockfd, &read_fds)){
                ret = recv(sockfd, buf, sizeof(buf) - 1, 0);
                if(ret <= 0)
                    break;//connection closed
                // process logic
                printf("get %d bytes of normal data:%s\n", ret, buf);
            }else if(FD_ISSET(sockfd, &exception_fds)){
                //handle exceptions
                //get out of band data 
                ret = recv(connfd, buf, sizeof(buf) - 1, MSG_OOB);
                if(ret <= 0)
                    break;
                printf("get %d bytes of OOB data: %s", ret, buf);

            }
            FD_CLR(sockfd, &all_read_fds);
            FD_CLR(sockfd, &all_exception_fds);
            //after we deal with this client
            close(sockfd);
            if(--nready <= 0)
                break;
        }
    }

    close(listenfd);
    return 0;
}

参考：《高性能服务器编程》和《UNP》