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is_tar.c

/*
 * 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"

#ifndef lint
FILE_RCSID("@(#)$Id: is_tar.c,v 1.25 2004/09/11 19:15:57 christos Exp $")
#endif

#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 */

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.
       */
      switch (is_tar(buf, nbytes)) {
      case 1:
              if (file_printf(ms, (ms->flags & MAGIC_MIME) ?
                "application/x-tar" : "tar archive") == -1)
                  return -1;
            return 1;
      case 2:
            if (file_printf(ms, (ms->flags & MAGIC_MIME) ?
                "application/x-tar, POSIX" : "POSIX tar archive") == -1)
                  return -1;
            return 1;
      default:
            return 0;
      }
}

/*
 * 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.
 */
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 (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).
 */
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;
}

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