函数select()和pselect()用于IO复用，它们监视多个文件描述符的集合，判断是否有符合条件的时间发生。
1.select()函数
函数select()与之前的recv()和send()直接操作文件描述符不同。使用select()函数可以先对需要操作的文件描述符进行查询，查看是否目标文件描述符可以进行读、写或者错误操作，然后当文件描述符满足操作的条件的时候才进行真正的IO操作。
select()函数原型以及解释说明如下：

select()函数例子：
使用select函数监视标准输入是否有数据处理，设置超时时间为5s。

#include<unistd.h>
#include<sys/time.h>
#include<sys/types.h>
int main(){
    fd_set rd;//定义fd_set类型的读描述符集合
    struct timeval tv;//timeval 结构
    int retval;//select返回值
    FD_ZERO(&rd);//清空读描述符集合
    FD_SET(0,&rd);//将描述符标准输入0放入读描述符集合中
    tv.tv_sec=5;
    tv.tv_usec=0;//设置超时间为5S，select函数调用返回的时候会修改timeval的值为剩余的时间。
    retval=select(2,&rd,NULL,NULL,&tv);//调用select函数监听标准输入，fd最大描述符个数加1（这里只监听标准输入一个描述符，那么这个值就是1+1，也就是2）
//  printf("[tv.tv_sec:%d]\n",tv.tv_sec);
    if(retval==-1){
        perror("select");   
    }
    else if(retval){
        printf("Data is avaialble now\n");
    }
    else{
        printf("NO Data within five seconds\n");
    }
    return 0;       
}

2.pselect()函数
函数select()是用一种超时轮循的方式来查看文件的读写错误可操作性。在Linux下，还有一个相似的函数pselect()。

pselect()函数例子：

#include<unistd.h>
#include<sys/time.h>
#include<sys/types.h>
#include <signal.h>
int sigint_event=0;
void sigint_sig_handler(int s){//设置信号处理函数
    sigint_event++;
    signal(SIGINT,sigint_sig_handler);
}
int main(){
    int r;
    fd_set rd;
    sigset_t sigmask,orignal_sigmask;//设置新掩码和保存原始掩码
    FD_ZERO(&rd);//清空读描述符集合
    FD_SET(0,&rd);//将标准输入放入读描述符集合
    sigemptyset(&sigmask);//清空信号
    sigaddset(&sigmask,SIGINT);//将SIGINT信号加入sigmask中
    //设置信号SIG_BLOCK的掩码sigmask，并将原始的掩码保存在orignal_sigmask中
    sigprocmask(SIG_BLOCK,&sigmask,&orignal_sigmask);
    signal(SIGINT,sigint_sig_handler);//挂接信号处理函数
    for(;;){
        for(;sigint_event>0;sigint_event--){
            printf("sigint_event[%d]\n",sigint_event);
        }                r=pselect(1,&rd,NULL,NULL,NULL,&orignal_sigmask);//pselect函数IO复用
        if(r==-1){
            perror("pselect");  
        }
        else if(r){
            printf("Data is avaialble now\n");
        }
        else{
            printf("NO Data within five seconds\n");
        }
        sleep(1);
    }
    return 0;           
}

最后介绍重量级select()函数例子：

/* select使用的例子 */
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/types.h>
#include <string.h>
#include <signal.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <errno.h>

static int forward_port;

#undef max
#define max(x,y) ((x) > (y) ? (x) : (y))

static int listen_socket (int listen_port) {
    struct sockaddr_in a;
    int s;
    int yes;
    if ((s = socket (AF_INET, SOCK_STREAM, 0)) < 0) {
        perror ("socket");
        return -1;
    }
    yes = 1;
    if (setsockopt
        (s, SOL_SOCKET, SO_REUSEADDR,
         (char *) &yes, sizeof (yes)) < 0) {
        perror ("setsockopt");
        close (s);
        return -1;
    }
    memset (&a, 0, sizeof (a));
    a.sin_port = htons (listen_port);
    a.sin_family = AF_INET;
    if (bind
        (s, (struct sockaddr *) &a, sizeof (a)) < 0) {
        perror ("bind");
        close (s);
        return -1;
    }
    printf ("accepting connections on port %d\n",
            (int) listen_port);
    listen (s, 10);
    return s;
}

static int connect_socket (int connect_port,
                           char *address) {
    struct sockaddr_in a;
    int s;
    if ((s = socket (AF_INET, SOCK_STREAM, 0)) < 0) {
        perror ("socket");
        close (s);
        return -1;
    }

    memset (&a, 0, sizeof (a));
    a.sin_port = htons (connect_port);
    a.sin_family = AF_INET;
    if (!inet_aton
        (address,
         (struct in_addr *) &a.sin_addr.s_addr)) {
        perror ("bad IP address format");
        close (s);
        return -1;
    }

    if (connect
        (s, (struct sockaddr *) &a,
         sizeof (a)) < 0) {
        perror ("connect()");
        shutdown (s, SHUT_RDWR);
        close (s);
        return -1;
    }
    return s;
}

#define SHUT_FD1 {                      \
        if (fd1 >= 0) {                 \
            shutdown (fd1, SHUT_RDWR);  \
            close (fd1);                \
            fd1 = -1;                   \
        }                               \
    }

#define SHUT_FD2 {                      \
        if (fd2 >= 0) {                 \
            shutdown (fd2, SHUT_RDWR);  \
            close (fd2);                \
            fd2 = -1;                   \
        }                               \
    }

/* 缓冲区的大小为1024 */
#define BUF_SIZE 1024
int main (int argc, char **argv) {
    int h;
    int fd1 = -1, fd2 = -1;
    char buf1[BUF_SIZE], buf2[BUF_SIZE];
    int buf1_avail, buf1_written;
    int buf2_avail, buf2_written;

    if (argc != 4) {
        fprintf (stderr,
                 "Usage\n\tfwd <listen-port> \
<forward-to-port> <forward-to-ip-address>\n");
        exit (1);
    }

    signal (SIGPIPE, SIG_IGN);

    forward_port = atoi (argv[2]);

    h = listen_socket (atoi (argv[1]));
    if (h < 0)
        exit (1);

    /* 主循环 */
    for (;;) {
        int r, nfds = 0;
        fd_set rd, wr, er;  /*定义用于读、写和错误的文件集*/
        FD_ZERO (&rd);      /*清空读文件集描述符和*/
        FD_ZERO (&wr);      /*清空写文件描述符集和*/
        FD_ZERO (&er);      /*清空错误文件描述符集和*/
        FD_SET (h, &rd);    /*将服务器的侦听套接字放入可读文件描述符集合*/
        nfds = max (nfds, h);
        if (fd1 > 0 && buf1_avail < BUF_SIZE) {
            FD_SET (fd1, &rd);
            nfds = max (nfds, fd1);
        }
        if (fd2 > 0 && buf2_avail < BUF_SIZE) {
            FD_SET (fd2, &rd);
            nfds = max (nfds, fd2);
        }
        if (fd1 > 0
            && buf2_avail - buf2_written > 0) {
            FD_SET (fd1, &wr);
            nfds = max (nfds, fd1);
        }
        if (fd2 > 0
            && buf1_avail - buf1_written > 0) {
            FD_SET (fd2, &wr);
            nfds = max (nfds, fd2);
        }
        if (fd1 > 0) {
            FD_SET (fd1, &er);
            nfds = max (nfds, fd1);
        }
        if (fd2 > 0) {
            FD_SET (fd2, &er);
            nfds = max (nfds, fd2);
        }

        r = select (nfds + 1, &rd, &wr, &er, NULL);

        if (r == -1 && errno == EINTR)
            continue;
        if (r < 0) {
            perror ("select()");
            exit (1);
        }
        if (FD_ISSET (h, &rd)) {
            unsigned int l;
            struct sockaddr_in client_address;
            memset (&client_address, 0, l =
                    sizeof (client_address));
            r = accept (h, (struct sockaddr *)
                        &client_address, &l);
            if (r < 0) {
                perror ("accept()");
            } else {
                SHUT_FD1;
                SHUT_FD2;
                buf1_avail = buf1_written = 0;
                buf2_avail = buf2_written = 0;
                fd1 = r;
                fd2 =
                    connect_socket (forward_port,
                                    argv[3]);
                if (fd2 < 0) {
                    SHUT_FD1;
                } else
                    printf ("connect from %s\n",
                            inet_ntoa
                            (client_address.sin_addr));
            }
        }
/* NB: read oob data before normal reads */
        if (fd1 > 0)
            if (FD_ISSET (fd1, &er)) {
                char c;
                errno = 0;
                r = recv (fd1, &c, 1, MSG_OOB);
                if (r < 1) {
                    SHUT_FD1;
                } else
                    send (fd2, &c, 1, MSG_OOB);
            }
        if (fd2 > 0)
            if (FD_ISSET (fd2, &er)) {
                char c;
                errno = 0;
                r = recv (fd2, &c, 1, MSG_OOB);
                if (r < 1) {
                    SHUT_FD1;
                } else
                    send (fd1, &c, 1, MSG_OOB);
            }
        if (fd1 > 0)
            if (FD_ISSET (fd1, &rd)) {
                r =
                    read (fd1, buf1 + buf1_avail,
                          BUF_SIZE - buf1_avail);
                if (r < 1) {
                    SHUT_FD1;
                } else
                    buf1_avail += r;
            }
        if (fd2 > 0)
            if (FD_ISSET (fd2, &rd)) {
                r =
                    read (fd2, buf2 + buf2_avail,
                          BUF_SIZE - buf2_avail);
                if (r < 1) {
                    SHUT_FD2;
                } else
                    buf2_avail += r;
            }
        if (fd1 > 0)
            if (FD_ISSET (fd1, &wr)) {
                r =
                    write (fd1,
                           buf2 + buf2_written,
                           buf2_avail -
                           buf2_written);
                if (r < 1) {
                    SHUT_FD1;
                } else
                    buf2_written += r;
            }
        if (fd2 > 0)
            if (FD_ISSET (fd2, &wr)) {
                r =
                    write (fd2,
                           buf1 + buf1_written,
                           buf1_avail -
                           buf1_written);
                if (r < 1) {
                    SHUT_FD2;
                } else
                    buf1_written += r;
            }
/* check if write data has caught read data */
        if (buf1_written == buf1_avail)
            buf1_written = buf1_avail = 0;
        if (buf2_written == buf2_avail)
            buf2_written = buf2_avail = 0;
/* one side has closed the connection, keep
   writing to the other side until empty */
        if (fd1 < 0
            && buf1_avail - buf1_written == 0) {
            SHUT_FD2;
        }
        if (fd2 < 0
            && buf2_avail - buf2_written == 0) {
            SHUT_FD1;
        }
    }
           return 0;
}