/* TCPmechod.c - main, echo */#include <sys/types.h>#include <sys/socket.h>#include <sys/time.h>#include <netinet/in.h>#include <unistd.h>#include <string.h>#include <stdio.h>#define	QLEN		  32	/* maximum connection queue length	*/#define	BUFSIZE		4096extern int	errno;int		errexit(const char *format, ...);int		passiveTCP(const char *service, int qlen);int		echo(int fd);/*------------------------------------------------------------------------ * main - Concurrent TCP server for ECHO service *------------------------------------------------------------------------ */intmain(int argc, char *argv[]){	char	*service = "echo";	/* service name or port number	*/	struct sockaddr_in fsin;	/* the from address of a client	*/	int	msock;			/* master server socket		*/	fd_set	rfds;			/* read file descriptor set	*/	fd_set	afds;			/* active file descriptor set	*/	unsigned int	alen;		/* from-address length		*/	int	fd, nfds;		switch (argc) {	case	1:		break;	case	2:		service = argv[1];		break;	default:		errexit("usage: TCPmechod [port]\n");	}	msock = passiveTCP(service, QLEN);	nfds = getdtablesize();	FD_ZERO(&afds);	FD_SET(msock, &afds);	while (1) {		memcpy(&rfds, &afds, sizeof(rfds));		if (select(nfds, &rfds, (fd_set *)0, (fd_set *)0,				(struct timeval *)0) < 0)			errexit("select: %s\n", strerror(errno));		if (FD_ISSET(msock, &rfds)) {			int	ssock;			alen = sizeof(fsin);			ssock = accept(msock, (struct sockaddr *)&fsin,				&alen);			if (ssock < 0)				errexit("accept: %s\n",					strerror(errno));			FD_SET(ssock, &afds);		}		for (fd=0; fd<nfds; ++fd)			if (fd != msock && FD_ISSET(fd, &rfds))				if (echo(fd) == 0) {					(void) close(fd);					FD_CLR(fd, &afds);				}	}}/*------------------------------------------------------------------------ * echo - echo one buffer of data, returning byte count *------------------------------------------------------------------------ */intecho(int fd){	char	buf[BUFSIZ];	int	cc;	cc = read(fd, buf, sizeof buf);	if (cc < 0)		errexit("echo read: %s\n", strerror(errno));	if (cc && write(fd, buf, cc) < 0)		errexit("echo write: %s\n", strerror(errno));	return cc;}