/*
  * is_tar() -- figure out whether file is a tar archive.
  *
  * Stolen (by the author of the file utility!) 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

  * $Id: is_tar.c,v 1.2 2006/06/17 21:00:44 tkojm Exp $

  *
  * Comments changed and some code/comments reformatted
  * for file command by Ian Darwin.
  */
 
 #if HAVE_CONFIG_H
 #include "clamav-config.h"
 #endif
 
 #include <string.h>
 #include <ctype.h>
 #include <sys/types.h>
 #include "is_tar.h"
 
 #include "others.h"
 
 #define	isodigit(c)	( ((c) >= '0') && ((c) <= '7') )
 
 static int from_oct(int digs, char *where);
 
 /*
  * 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(unsigned char *buf, unsigned int nbytes)

 {
 	union record *header = (union record *)buf;
 	int	i;
 	int	sum, recsum;
 	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 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).
  */
 static int from_oct(int digs, 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;
 }