请教大家一个问题，关于UNIX网络编程第16章非阻塞I/O中的一段代码

#include	"web.h"



int

main(int argc, char **argv)

{

	int		i, fd, n, maxnconn, flags, error;

	char	buf[MAXLINE];

	fd_set	rs, ws;



	if (argc < 5)

		err_quit("usage: web <#conns> <hostname> <homepage> <file1> ...");

	maxnconn = atoi(argv[1]);



	nfiles = min(argc - 4, MAXFILES);

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

		file[i].f_name = argv[i + 4];

		file[i].f_host = argv[2];

		file[i].f_flags = 0;

	}

	printf("nfiles = %d\n", nfiles);



	home_page(argv[2], argv[3]);



	FD_ZERO(&rset);

	FD_ZERO(&wset);

	maxfd = -1;

	nlefttoread = nlefttoconn = nfiles;

	nconn = 0;

/* end web1 */

/* include web2 */

	while (nlefttoread > 0) {

		while (nconn < maxnconn && nlefttoconn > 0) {

				/* 4find a file to read */

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

				if (file[i].f_flags == 0)

					break;

			if (i == nfiles)

				err_quit("nlefttoconn = %d but nothing found", nlefttoconn);

			start_connect(&file[i]);

			nconn++;

			nlefttoconn--;

		}



		rs = rset;

		ws = wset;

		n = Select(maxfd+1, &rs, &ws, NULL, NULL);



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

			flags = file[i].f_flags;

			if (flags == 0 || flags & F_DONE)

				continue;

			fd = file[i].f_fd;

			if (flags & F_CONNECTING &&

				(FD_ISSET(fd, &rs) || FD_ISSET(fd, &ws))) {

				n = sizeof(error);

				if (getsockopt(fd, SOL_SOCKET, SO_ERROR, &error, &n) < 0 ||

					error != 0) {

					err_ret("nonblocking connect failed for %s",

							file[i].f_name);

				}

					/* 4connection established */

				printf("connection established for %s\n", file[i].f_name);

				FD_CLR(fd, &wset);		/* no more writeability test */

				write_get_cmd(&file[i]);/* write() the GET command */



			} else if (flags & F_READING && FD_ISSET(fd, &rs)) {

				if ( (n = Read(fd, buf, sizeof(buf))) == 0) {

					printf("end-of-file on %s\n", file[i].f_name);

					Close(fd);

					file[i].f_flags = F_DONE;	/* clears F_READING */

					FD_CLR(fd, &rset);

					nconn--;

					nlefttoread--;

				} else {

					printf("read %d bytes from %s\n", n, file[i].f_name);

				}

			}

		}

	}

	exit(0);

#include	"web.h"



void

start_connect(struct file *fptr)

{

	int				fd, flags, n;

	struct addrinfo	*ai;



	ai = Host_serv(fptr->f_host, SERV, 0, SOCK_STREAM);



	fd = Socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol);

	fptr->f_fd = fd;

	printf("start_connect for %s, fd %d\n", fptr->f_name, fd);



		/* 4Set socket nonblocking */

	flags = Fcntl(fd, F_GETFL, 0);

	Fcntl(fd, F_SETFL, flags | O_NONBLOCK);



		/* 4Initiate nonblocking connect to the server. */

	if ( (n = connect(fd, ai->ai_addr, ai->ai_addrlen)) < 0) {

		if (errno != EINPROGRESS)

			err_sys("nonblocking connect error");

		fptr->f_flags = F_CONNECTING;

		FD_SET(fd, &rset);			/* select for reading and writing */

		FD_SET(fd, &wset);

		if (fd > maxfd)

			maxfd = fd;



	} else if (n >= 0)				/* connect is already done */

		write_get_cmd(fptr);	/* write() the GET command */

}

#include	"web.h"



void

write_get_cmd(struct file *fptr)

{

	int		n;

	char	line[MAXLINE];



	n = snprintf(line, sizeof(line), GET_CMD, fptr->f_name);

	Writen(fptr->f_fd, line, n);

	printf("wrote %d bytes for %s\n", n, fptr->f_name);



	fptr->f_flags = F_READING;			/* clears F_CONNECTING */



	FD_SET(fptr->f_fd, &rset);			/* will read server's reply */

	if (fptr->f_fd > maxfd)

		maxfd = fptr->f_fd;

}