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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: is_tar.c,v 1.10 2009/10/27 23:59:37 deraadt Exp $ */
/*
 * Copyright (c) Ian F. Darwin 1986-1995.
 * Software written by Ian F. Darwin and others;
 * maintained 1995-present by Christos Zoulas and others.
 * 
 * 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 immediately at the beginning of the file, without modification,
 *    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.
 *  
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR 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 AUTHOR 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.
 */
/*
 * is_tar() -- figure out whether file is a tar archive.
 *
 * Stolen (by the author!) from the public domain tar program:
 * Public Domain version written 26 Aug 1985 John Gilmore (ihnp4!hoptoad!gnu).
 *
 * @(#)list.c 1.18 9/23/86 Public Domain - gnu
 *
 * Comments changed and some code/comments reformatted
 * for file command by Ian Darwin.
 */

#include "file.h"
#include "magic.h"
#include <string.h>
#include <ctype.h>
#include <sys/types.h>
#include "tar.h"

#define	isodigit(c)	( ((c) >= '0') && ((c) <= '7') )

private int is_tar(const unsigned char *, size_t);
private int from_oct(int, const char *);	/* Decode octal number */

static const char tartype[][32] = {
	"tar archive",
	"POSIX tar archive",
	"POSIX tar archive (GNU)",
};

protected int
file_is_tar(struct magic_set *ms, const unsigned char *buf, size_t nbytes)
{
	/*
	 * Do the tar test first, because if the first file in the tar
	 * archive starts with a dot, we can confuse it with an nroff file.
	 */
	int tar = is_tar(buf, nbytes);
	int mime = ms->flags & MAGIC_MIME;

	if (tar < 1 || tar > 3)
		return 0;

	if (mime == MAGIC_MIME_ENCODING)
		return 0;

	if (file_printf(ms, mime ? "application/x-tar" :
	    tartype[tar - 1]) == -1)
		return -1;
	return 1;
}

/*
 * Return 
 *	0 if the checksum is bad (i.e., probably not a tar archive), 
 *	1 for old UNIX tar file,
 *	2 for Unix Std (POSIX) tar file,
 *	3 for GNU tar file.
 */
private int
is_tar(const unsigned char *buf, size_t nbytes)
{
	const union record *header = (const union record *)(const void *)buf;
	int	i;
	int	sum, recsum;
	const char	*p;

	if (nbytes < sizeof(union record))
		return 0;

	recsum = from_oct(8,  header->header.chksum);

	sum = 0;
	p = header->charptr;
	for (i = sizeof(union record); --i >= 0;) {
		/*
		 * We cannot use unsigned char here because of old compilers,
		 * e.g. V7.
		 */
		sum += 0xFF & *p++;
	}

	/* Adjust checksum to count the "chksum" field as blanks. */
	for (i = sizeof(header->header.chksum); --i >= 0;)
		sum -= 0xFF & header->header.chksum[i];
	sum += ' '* sizeof header->header.chksum;	

	if (sum != recsum)
		return 0;	/* Not a tar archive */
	
	if (strcmp(header->header.magic, GNUTMAGIC) == 0) 
		return 3;		/* GNU Unix Standard tar archive */
	if (strcmp(header->header.magic, TMAGIC) == 0) 
		return 2;		/* Unix Standard tar archive */

	return 1;			/* Old fashioned tar archive */
}


/*
 * Quick and dirty octal conversion.
 *
 * Result is -1 if the field is invalid (all blank, or nonoctal).
 */
private int
from_oct(int digs, const char *where)
{
	int	value;

	while (isspace((unsigned char)*where)) {	/* Skip spaces */
		where++;
		if (--digs <= 0)
			return -1;		/* All blank field */
	}
	value = 0;
	while (digs > 0 && isodigit(*where)) {	/* Scan til nonoctal */
		value = (value << 3) | (*where++ - '0');
		--digs;
	}

	if (digs > 0 && *where && !isspace((unsigned char)*where))
		return -1;			/* Ended on non-space/nul */

	return value;
}