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
 * is_tar.c,v 1.2 1993/06/10 00:38:12 jtc Exp
 *
 * Comments changed and some code/comments reformatted
 * for file command by Ian Darwin.
 */

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

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

#if     defined(__STDC__) || defined(__cplusplus)
static long from_oct(int, char*);       /* Decode octal number */
#else
static long from_oct();
#endif

/*
 * 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(buf)
unsigned char *buf;
{
        register union record *header = (union record *)buf;
        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(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 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;
}