386BSD 0.1 development

[unix-history] / usr / src / usr.bin / file / is_tar.c

/*
* 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 <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;
}