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is_tar.c

/*
 * 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
 * $Id: is_tar.c,v 1.13 2000/08/05 17:36:48 christos Exp $
 *
 * Comments changed and some code/comments reformatted
 * for file command by Ian Darwin.
 */

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

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

static int from_oct __P((int, char *));   /* 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(unsigned char *buf, int size)
{
      union record *header = (union record *)buf;
      int   i;
      int   sum, recsum;
      char  *p;

      if (size < 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(digs, where)
      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;
}

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