xref: /src/usr.bin/tail/forward.c

/*	$NetBSD: forward.c,v 1.34 2024/01/14 17:37:32 christos 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/cdefs.h>
#ifndef lint
#if 0
static char sccsid[] = "@(#)forward.c	8.1 (Berkeley) 6/6/93";
#endif
__RCSID("$NetBSD: forward.c,v 1.34 2024/01/14 17:37:32 christos Exp $");
#endif /* not lint */

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

#include <limits.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "extern.h"

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

/* defines for inner loop actions */
#define	USE_SLEEP	0
#define	USE_KQUEUE	1
#define	ADD_EVENTS	2

/*
 * 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	mmap the file and step back until reach the correct offset.
 *	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, n;
	int kq=-1, action=USE_SLEEP;
	struct stat statbuf;
	struct kevent ev[2];

	switch(style) {
	case FBYTES:
		if (off == 0)
			break;
		if (S_ISREG(sbp->st_mode) && sbp->st_size > 0) {
			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) && sbp->st_size > 0) {
			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 (displaybytes(fp, off))
				return;
		}
		break;
	case RLINES:
		if (S_ISREG(sbp->st_mode) && sbp->st_size > 0) {
			if (!off) {
				if (fseek(fp, 0L, SEEK_END) == -1) {
					ierr();
					return;
				}
			} else {
				if (rlines(fp, off, sbp))
					return;
			}
		} else if (off == 0) {
			while (getc(fp) != EOF);
			if (ferror(fp)) {
				ierr();
				return;
			}
		} else {
			if (displaylines(fp, off))
				return;
		}
		break;
	default:
		break;
	}

	if (fflag) {
		kq = kqueue();
		if (kq < 0)
			xerr(1, "kqueue");
		action = ADD_EVENTS;
	}

	for (;;) {
		while ((ch = getc(fp)) != EOF)  {
			if (putchar(ch) == EOF)
				oerr();
		}
		if (ferror(fp)) {
			ierr();
			return;
		}
		(void)fflush(stdout);
		if (!fflag)
			break;

		clearerr(fp);

		switch (action) {
		case ADD_EVENTS:
			n = 0;

			memset(ev, 0, sizeof(ev));
			if (fflag == 2 && fp != stdin) {
				EV_SET(&ev[n], fileno(fp), EVFILT_VNODE,
				    EV_ADD | EV_ENABLE | EV_CLEAR,
				    NOTE_DELETE | NOTE_RENAME, 0, 0);
				n++;
			}
			EV_SET(&ev[n], fileno(fp), EVFILT_READ,
			    EV_ADD | EV_ENABLE, 0, 0, 0);
			n++;

			if (kevent(kq, ev, n, NULL, 0, NULL) == -1) {
				close(kq);
				kq = -1;
				action = USE_SLEEP;
			} else {
				action = USE_KQUEUE;
			}
			break;

		case USE_KQUEUE:
			if (kevent(kq, NULL, 0, ev, 1, NULL) == -1)
				xerr(1, "kevent");

			if (ev[0].filter == EVFILT_VNODE) {
				/* file was rotated, wait until it reappears */
				action = USE_SLEEP;
			} else if (ev[0].data < 0) {
				/* file shrank, reposition to end */
				if (fseek(fp, 0L, SEEK_END) == -1) {
					ierr();
					return;
				}
			}
			break;

		case USE_SLEEP:
			/*
			 * We pause for one second after displaying any data
			 * that has accumulated since we read the file.
			 */
                	(void) sleep(1);

			if (fflag == 2 && fp != stdin &&
			    stat(fname, &statbuf) != -1) {
				if (statbuf.st_ino != sbp->st_ino ||
				    statbuf.st_dev != sbp->st_dev ||
				    statbuf.st_rdev != sbp->st_rdev ||
				    statbuf.st_nlink == 0) {
					fp = freopen(fname, "r", fp);
					if (fp == NULL) {
						ierr();
						goto out;
					}
					*sbp = statbuf;
					if (kq != -1)
						action = ADD_EVENTS;
				} else if (kq != -1)
					action = USE_KQUEUE;
			}
			break;
		}
	}
out:
	if (fflag && kq != -1)
		close(kq);
}

/*
 * rlines -- display the last offset lines of the file.
 *
 * Non-zero return means than a (non-fatal) error occurred.
 */
static int
rlines(FILE *fp, off_t off, struct stat *sbp)
{
	off_t file_size;
	off_t file_remaining;
	char *p = NULL;
	char *start = NULL;
	off_t mmap_size;
	off_t mmap_offset;
	off_t mmap_remaining = 0;

#define MMAP_MAXSIZE  (10 * 1024 * 1024)

	if (!(file_size = sbp->st_size))
		return 0;
	file_remaining = file_size;

	if (file_remaining > MMAP_MAXSIZE) {
		mmap_size = MMAP_MAXSIZE;
		mmap_offset = file_remaining - MMAP_MAXSIZE;
	} else {
		mmap_size = file_remaining;
		mmap_offset = 0;
	}

	while (off) {
		start = mmap(NULL, (size_t)mmap_size, PROT_READ,
			     MAP_FILE|MAP_SHARED, fileno(fp), mmap_offset);
		if (start == MAP_FAILED) {
			xerr(0, "%s", fname);
			return 1;
		}

		mmap_remaining = mmap_size;
		/* Last char is special, ignore whether newline or not. */
		for (p = start + mmap_remaining - 1 ; --mmap_remaining ; )
			if (*--p == '\n' && !--off) {
				++p;
				break;
			}

		file_remaining -= mmap_size - mmap_remaining;

		if (off == 0)
			break;

		if (file_remaining == 0)
			break;

		if (munmap(start, mmap_size)) {
			xerr(0, "%s", fname);
			return 1;
		}

		if (mmap_offset >= MMAP_MAXSIZE) {
			mmap_offset -= MMAP_MAXSIZE;
		} else {
			mmap_offset = 0;
			mmap_size = file_remaining;
		}
	}

	/*
	 * Output the (perhaps partial) data in this mmap'd block.
	 */
	WR(p, mmap_size - mmap_remaining);
	file_remaining += mmap_size - mmap_remaining;
	if (munmap(start, mmap_size)) {
		xerr(0, "%s", fname);
		return 1;
	}

	/*
	 * Set the file pointer to reflect the length displayed.
	 * This will cause the caller to redisplay the data if/when
	 * needed.
	 */
	if (fseeko(fp, file_remaining, SEEK_SET) == -1) {
		ierr();
		return 1;
	}
	return 0;
}