/*
  * 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 "clamav.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(const 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;

 }