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rcsid[] and sccsid[] and copyright[] are essentially unmaintained (and
unmaintainable).  these days, people use source.  these id's do not provide
any benefit, and do hurt the small install media
(the 33,000 line diff is essentially mechanical)
ok with the idea millert, ok dms

/*	$OpenBSD: forward.c,v 1.26 2009/10/27 23:59:44 deraadt Exp $	*/
/*	$NetBSD: forward.c,v 1.7 1996/02/13 16:49:10 ghudson Exp $	*/

/*-
 * Copyright (c) 1991, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Edward Sze-Tyan Wang.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/event.h>

#include <err.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

#include "extern.h"

static int rlines(FILE *, off_t, struct stat *);

/*
 * forward -- display the file, from an offset, forward.
 *
 * There are eight separate cases for this -- regular and non-regular
 * files, by bytes or lines and from the beginning or end of the file.
 *
 * FBYTES	byte offset from the beginning of the file
 *	REG	seek
 *	NOREG	read, counting bytes
 *
 * FLINES	line offset from the beginning of the file
 *	REG	read, counting lines
 *	NOREG	read, counting lines
 *
 * RBYTES	byte offset from the end of the file
 *	REG	seek
 *	NOREG	cyclically read characters into a wrap-around buffer
 *
 * RLINES
 *	REG	step back until the correct offset is reached.
 *	NOREG	cyclically read lines into a wrap-around array of buffers
 */
void
forward(FILE *fp, enum STYLE style, off_t off, struct stat *sbp)
{
	int ch;
	struct stat nsb;
	int kq, queue;
	struct kevent ke;

	switch(style) {
	case FBYTES:
		if (off == 0)
			break;
		if (S_ISREG(sbp->st_mode)) {
			if (sbp->st_size < off)
				off = sbp->st_size;
			if (fseeko(fp, off, SEEK_SET) == -1) {
				ierr();
				return;
			}
		} else while (off--)
			if ((ch = getc(fp)) == EOF) {
				if (ferror(fp)) {
					ierr();
					return;
				}
				break;
			}
		break;
	case FLINES:
		if (off == 0)
			break;
		for (;;) {
			if ((ch = getc(fp)) == EOF) {
				if (ferror(fp)) {
					ierr();
					return;
				}
				break;
			}
			if (ch == '\n' && !--off)
				break;
		}
		break;
	case RBYTES:
		if (S_ISREG(sbp->st_mode)) {
			if (sbp->st_size >= off &&
			    fseeko(fp, -off, SEEK_END) == -1) {
				ierr();
				return;
			}
		} else if (off == 0) {
			while (getc(fp) != EOF)
				;
			if (ferror(fp)) {
				ierr();
				return;
			}
		} else {
			if (bytes(fp, off))
				return;
		}
		break;
	case RLINES:
		if (S_ISREG(sbp->st_mode)) {
			if (!off) {
				if (fseeko(fp, (off_t)0, SEEK_END) == -1) {
					ierr();
					return;
				}
			} else if (rlines(fp, off, sbp) != 0)
				lines(fp, off);
		} else if (off == 0) {
			while (getc(fp) != EOF)
				;
			if (ferror(fp)) {
				ierr();
				return;
			}
		} else {
			if (lines(fp, off))
				return;
		}
		break;
	}

	kq = -1;
kq_retry:
	if (fflag && ((kq = kqueue()) >= 0)) {
		EV_SET(&ke, fileno(fp), EVFILT_READ,
		    EV_ENABLE | EV_ADD | EV_CLEAR,
		    0,
		    0, NULL);
		if (kevent(kq, &ke, 1, NULL, 0, NULL) < 0) {
			close(kq);
			kq = -1;
		} else if (S_ISREG(sbp->st_mode)) {
			EV_SET(&ke, fileno(fp), EVFILT_VNODE,
			    EV_ENABLE | EV_ADD | EV_CLEAR,
			    NOTE_DELETE | NOTE_RENAME | NOTE_TRUNCATE,
			    0, NULL);
			if (kevent(kq, &ke, 1, NULL, 0, NULL) < 0) {
				close(kq);
				kq = -1;
			}
		}
	}

	for (;;) {
		while (!feof(fp) && (ch = getc(fp)) != EOF)
			if (putchar(ch) == EOF)
				oerr();
		if (ferror(fp)) {
			ierr();
			if (kq != -1)
				close(kq);
			return;
		}
		(void)fflush(stdout);
		if (!fflag)
			break;
		clearerr(fp);
		queue = 1;
		if (kq < 0 || kevent(kq, NULL, 0, &ke, 1, NULL) <= 0) {
			queue = 0;
			sleep(1);
		} else if (ke.filter == EVFILT_READ) {
			continue;
		} else if ((ke.fflags & NOTE_TRUNCATE) == 0) {
			/*
			 * File was renamed or deleted.
			 *
			 * Continue to look at it until a new file reappears
			 * with the same name. 
			 * Fall back to the old algorithm for that.
			 */
			close(kq);
			kq = -1;
		}

		if (is_stdin || stat(fname, &nsb) != 0)
			continue;
		/* Reopen file if the inode changes or file was truncated */
		if (nsb.st_ino != sbp->st_ino) {
			warnx("%s has been replaced, reopening.", fname);
			if ((fp = freopen(fname, "r", fp)) == NULL) {
				ierr();
				if (kq >= 0)
					close(kq);
				return;
			}
			(void)memcpy(sbp, &nsb, sizeof(nsb));
			goto kq_retry;
		} else if ((queue && (ke.fflags & NOTE_TRUNCATE)) ||
		    (!queue && nsb.st_size < sbp->st_size)) {
			warnx("%s has been truncated, resetting.", fname);
			fpurge(fp);
			rewind(fp);
		}
		(void)memcpy(sbp, &nsb, sizeof(nsb));
	}
	if (kq >= 0)
		close(kq);
}

/*
 * rlines -- display the last offset lines of the file.
 */
static int
rlines(FILE *fp, off_t off, struct stat *sbp)
{
	off_t pos;
	int ch;

	pos = sbp->st_size;
	if (pos == 0)
		return (0);

	/*
	 * Position before char.
	 * Last char is special, ignore it whether newline or not.
	 */
	pos -= 2;
	ch = EOF;
	for (; off > 0 && pos >= 0; pos--) {
		/* A seek per char isn't a problem with a smart stdio */
		if (fseeko(fp, pos, SEEK_SET) == -1) {
			ierr();
			return (1);
		}
		if ((ch = getc(fp)) == '\n')
			off--;
		else if (ch == EOF) {
			if (ferror(fp)) {
				ierr();
				return (1);
			}
			break;
		}
	}
	/* If we read until start of file, put back last read char */
	if (pos < 0 && off > 0 && ch != EOF && ungetc(ch, fp) == EOF) {
		ierr();
		return (1);
	}

	while (!feof(fp) && (ch = getc(fp)) != EOF)
		if (putchar(ch) == EOF)
			oerr();
	if (ferror(fp)) {
		ierr();
		return (1);
	}

	return (0);
}