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多路復(fù)用select與poll

在UNIX/linux中有4中IO模型，分別為：
        1、 阻塞IO
        2、 非阻塞IO
        3、 IO多路復(fù)用
        4、 信號(hào)驅(qū)動(dòng)IO

這幾種IO模型，阻塞IO是長(zhǎng)用到的，并且操作相對(duì)簡(jiǎn)單，但是缺點(diǎn)在于效率低下，尤其是在，同時(shí)操作多個(gè)IO的時(shí)候，不能隨時(shí)的處理各個(gè)IO操作。而非阻塞IO可以解決這個(gè)問(wèn)題，但是同樣也存在的問(wèn)題，因?yàn)槭褂梅亲枞鸌O模型的時(shí)候，就需要對(duì)每個(gè)IO操作進(jìn)行輪訓(xùn)操作，而實(shí)際上輪空的幾率是很大的，多以非阻塞IO模型是在那種輪空幾率相對(duì)較小的的時(shí)候才會(huì)使用。因?yàn)檩営?xùn)是會(huì)占用相當(dāng)多的cpu時(shí)間片的�；�于這種考慮多路復(fù)用模型變產(chǎn)生了。

多路復(fù)用模型是對(duì)多個(gè)IO操作進(jìn)行檢測(cè)，返回可操作集合，這樣就可以對(duì)其進(jìn)行操作了。這樣就避免了阻塞IO不能隨時(shí)處理各個(gè)IO和非阻塞占用系統(tǒng)資源的確定。下面就多路復(fù)用中兩個(gè)常用的系統(tǒng)調(diào)用select和poll進(jìn)行簡(jiǎn)單的說(shuō)明。

1、 select

select函數(shù)的原型為：

#include <sys/select.h>
        #include <sys/time.h>
        #include <sys/types.h>
        #include <unistd.h>
        int select(int nfds,
                fd_set *readfds, 
                fd_set *writefds,
                fd_set *exceptfds, 
                struct timeval *timeout);

select的使用需要維護(hù)三類文件描述符集合，分別是：
        readfds：用于存放我們要檢測(cè)的可讀的文件描述符。
        writefds：用于存放我們要檢測(cè)的可寫的文件描述符。
        exceptfds：用于存放我們要檢測(cè)的是否發(fā)生異常的文件描述符。

在使用select的時(shí)候需要注意，在調(diào)用select后者幾個(gè)集合就會(huì)變化，變?yōu)槭遣僮鞯募�合，即可讀寫或發(fā)生異常，所以在調(diào)用select之前需要保存以前的集合。

系統(tǒng)為我們提供了下面四個(gè)宏來(lái)操作這些文件描述符集合：

void FD_SET(int fd, fd_set *set);
        void FD_CLR(int fd, fd_set *set);
        I int FD_ISSET(int fd, fd_set *set);
        void FD_ZERO(fd_set *set);

其中FD_SET用來(lái)將一個(gè)文件描述符加入到文件描述符集合里，F(xiàn)D_CLR是刪除，F(xiàn)D_ISSET是判斷文件描述符是否在這個(gè)集合里，而FD_ZERO是將這個(gè)集合清零。

下面我們以一個(gè)簡(jiǎn)單的例子來(lái)說(shuō)明select的使用。

#include <stdio.h>
        #include <stdlib.h>
        #include <unistd.h>
        #include <string.h>
        #include <sys/types.h>
        #include <sys/select.h>
        #include <sys/socket.h>
        #include <netinet/in.h>
        #include <arpa/inet.h>

#define N 64

int main(int argc, char *argv[])
        {
                if(argc != 2)
                {
                        printf(“format: server <server ip> <server port>”);
                        exit(0);
                }
                int i, servsock, connfd, maxfd;
                char buf[N];
                fd_set rdfd,rdfd1;

        struct sockaddr_in servaddr;

        if((servsock = socket(PF_INET, SOCK_STREAM, 0)) < 0)
                {
                        perror("socket");
                        exit(1);
                }

        memset(&servaddr, 0, sizeof(servaddr));
                servaddr.sin_family = PF_INET;
                servaddr.sin_addr.s_addr = inet_addr("192.168.1.203");
                servaddr.sin_port = htons(8889);

        if(bind(servsock, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0)
                {
                        perror("bind");
                        exit(1);
                }

        listen(servsock, 5);
                maxfd = servsock;

                FD_ZERO(&rdfd1);
                FD_SET(0, &rdfd1);
                FD_SET(servsock, &rdfd1);

        for(;;)
                {

                        rdfd = rdfd1;
                        if(select(maxfd+1, &rdfd, NULL, NULL, NULL) < 0)
                        {
                                perror("select");
                                exit(1);
                        }

                for(i = 0; i <= maxfd; i++)
                        {
                                if(FD_ISSET(i, &rdfd))
                                {
                                        if( i == STDIN_FILENO)
                                        {
                                                fgets(buf, N, stdin);
                                                printf("%s", buf);
                                        }
                                        else if(i == servsock)
                                        {
                                                connfd = accept(servsock, NULL, NULL);
                                                printf("New Connection %d is comming\n", connfd);
                                                send(connfd, buf, strlen(buf), 0);
                                                sleep(1);
                                                close(connfd);
                                        }
                                }
                        }
                }

        return 0;
        }

上面這段程序在select會(huì)阻塞，這個(gè)時(shí)候我們可以定義一個(gè)時(shí)間結(jié)構(gòu)體變量，如下：

struct timeval timeout；
        timeval的原型為：
        struct timeval {
                                long        tv_sec;        /* seconds */
                                long        tv_usec;        /* microseconds */
        };

這個(gè)變量可以提供一個(gè)微秒級(jí)的時(shí)間，我們可以使用這個(gè)作為超時(shí)處理。使用方式就是把這個(gè)變量作為select系統(tǒng)調(diào)用的后一個(gè)參數(shù)。

2、 poll

poll的原型為：

#include <poll.h>
        int poll(struct pollfd *fds, nfds_t nfds, int timeout);

poll的使用不需要分別維護(hù)幾個(gè)表，只需要維護(hù)一個(gè)結(jié)構(gòu)體數(shù)組，這個(gè)結(jié)構(gòu)體為：

struct pollfd {
                                int        fd;        /* file descriptor */
                                short events;        /* requested events */
                                short revents;        /* returned events */
        };

這個(gè)結(jié)構(gòu)體幾個(gè)成員分別是我們要監(jiān)測(cè)的文件描述符，和我們請(qǐng)求的事件及后返回的事件。

poll事件的類型為：

POLLIN There is data to read.
        POLLPRI There is urgent data to read .
        POLLOUT Writing now will not block.
        POLLERR Error condition (output only).
        POLLHUP Hang up (output only).
        POLLNVAL Invalid request: fd not open (output only).

上面這些事件下面三種在events中沒(méi)有用的，只能在revents中才能出現(xiàn)。我們可以調(diào)用poll并且查看每個(gè)文件描述符的返回狀態(tài)進(jìn)行相應(yīng)的操作。

下面以一個(gè)簡(jiǎn)單的實(shí)力說(shuō)明poll的使用：

#include <stdio.h>
        #include <stdlib.h>
        #include <unistd.h>
        #include <poll.h>
        #include <sys/socket.h>
        #include <netinet/in.h>
        #include <arpa/inet.h>
        #include <sys/types.h>
        #include <errno.h>
        #include <string.h>

#define OPEN_MAX openmax
        #define BUFSIZE 1024

int main(int argc, char **argv)
        {
                if(argc != 3)
                {
                        printf("format: server-poll <server ip> <port>");
                        exit(0);
                }

        int openmax = getdtablesize();

        int listensock, connsock;
                struct sockaddr_in servaddr;
                struct pollfd fds[OPEN_MAX];
                int i, count = 0, ready, reval;
                char readbuf[BUFSIZE];

        for(i = 0; i < OPEN_MAX; i++)
                {
                        fds[i].fd = -1;
                }

        if((listensock = socket(AF_INET, SOCK_STREAM, 0)) < 0)
                {
                        perror("socket");
                        exit(0);
                }

        memset(&servaddr, 0, sizeof(servaddr));
                servaddr.sin_family = AF_INET;
                servaddr.sin_addr.s_addr = inet_addr(argv[1]);
                servaddr.sin_port = htons(atoi(argv[2]));

        if(bind(listensock, (struct sockaddr*)&servaddr, sizeof(servaddr)) < 0)
                {
                        perror("bind");
                        exit(1);
                }

        listen(listensock, 5);

        fds[0].fd = 0;
                fds[0].events = POLLIN;
                count++;
                fds[1].fd = listensock;
                fds[1].events = POLLIN;
                count++;

                for(;;)
                {
                        memset(readbuf, 0, BUFSIZE);
                        ready = poll(fds, count, 0);
                        for(i = 0; i < count; i++)
                        {
                                if(fds[i].revents & POLLIN)
                                {
                                        if(fds[i].fd == 0)
                                        {
                                                fgets(readbuf, BUFSIZE, stdin);
                                                printf("read buf:%s\n", readbuf);
                                        }
                                        else if(fds[i].fd == listensock)
                                        {
                                                connsock = accept(listensock, NULL, NULL);
                                                printf("client connect\n");
                                                fds[count].fd = connsock;
                                                fds[count].events = POLLIN;
                                                count++;
                                                }
                                                else
                                                {
                                                        reval = recv(fds[i].fd, readbuf, BUFSIZE, 0);
                                                        printf("recv buf:%s\n", readbuf);
                                                        send(fds[i].fd, readbuf, reval, 0);
                                                }
                                        }
                                }
                       }
                }
                return 0;
        }