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。
main(){fd_set rd;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((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：
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：
forward_port;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;\}\}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;} elseprintf (“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;} elsesend (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;} elsesend (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;} elsebuf1_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;} elsebuf2_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;} elsebuf2_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;} elsebuf1_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;}
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