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/*
* 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 1.17 2002/07/03 18:26:38 christos Exp $
*
* Comments changed and some code/comments reformatted
* for file command by Ian Darwin.
*/
#include "file.h"
#include <string.h>
#include <ctype.h>
#include <sys/types.h>
#include "tar.h"
#ifndef lint
FILE_RCSID("@(#)$Id: is_tar.c,v 1.17 2002/07/03 18:26:38 christos Exp $")
#endif
#define isodigit(c) ( ((c) >= '0') && ((c) <= '7') )
static int from_oct(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 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;
}
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