is_tar.c

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/*
 * is_tar() -- figure out whether file is a tar archive.
 *
 * Stolen (by the author!) from the public domain tar program:
 * Pubic 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 <stdlib.h>
#include <unistd.h>
#include <ctype.h>
#include <sys/types.h>
#include "tar.h"

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

long from_oct();		/* Decode octal number */

/*
 * 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.
 */
int is_tar(header)
     register union record *header;
{
	register int i;
	register long sum, recsum;
	register char *p;

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

	sum = 0;
	p = header->charptr;
	for (i = sizeof(*header); --i >= 0;) {
		/*
		 * We can't 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 (0 == strcmp(header->header.magic, TMAGIC))
		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).
 */
long from_oct(digs, where)
     register int digs;
     register char *where;
{
	register long value;

	while (isspace(*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(*where))
		return -1;	/* Ended on non-space/nul */

	return value;
}