98 lines
2.2 KiB
C
98 lines
2.2 KiB
C
/*
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* is_tar() -- figure out whether file is a tar archive.
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*
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* Stolen (by the author!) from the public domain tar program:
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* Public Domain version written 26 Aug 1985 John Gilmore (ihnp4!hoptoad!gnu).
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*
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* @(#)list.c 1.18 9/23/86 Public Domain - gnu
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* $Id: is_tar.c,v 1.17 2002/07/03 18:26:38 christos Exp $
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*
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* Comments changed and some code/comments reformatted
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* for file command by Ian Darwin.
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*/
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#include "file.h"
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#include <string.h>
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#include <ctype.h>
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#include <sys/types.h>
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#include "tar.h"
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#ifndef lint
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FILE_RCSID("@(#)$Id: is_tar.c,v 1.17 2002/07/03 18:26:38 christos Exp $")
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#endif
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#define isodigit(c) ( ((c) >= '0') && ((c) <= '7') )
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static int from_oct(int, char *); /* Decode octal number */
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/*
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* Return
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* 0 if the checksum is bad (i.e., probably not a tar archive),
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* 1 for old UNIX tar file,
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* 2 for Unix Std (POSIX) tar file.
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*/
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int
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is_tar(unsigned char *buf, int nbytes)
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{
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union record *header = (union record *)buf;
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int i;
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int sum, recsum;
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char *p;
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if (nbytes < sizeof(union record))
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return 0;
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recsum = from_oct(8, header->header.chksum);
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sum = 0;
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p = header->charptr;
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for (i = sizeof(union record); --i >= 0;) {
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/*
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* We can't use unsigned char here because of old compilers,
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* e.g. V7.
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*/
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sum += 0xFF & *p++;
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}
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/* Adjust checksum to count the "chksum" field as blanks. */
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for (i = sizeof(header->header.chksum); --i >= 0;)
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sum -= 0xFF & header->header.chksum[i];
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sum += ' '* sizeof header->header.chksum;
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if (sum != recsum)
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return 0; /* Not a tar archive */
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if (0==strcmp(header->header.magic, TMAGIC))
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return 2; /* Unix Standard tar archive */
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return 1; /* Old fashioned tar archive */
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}
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/*
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* Quick and dirty octal conversion.
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*
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* Result is -1 if the field is invalid (all blank, or nonoctal).
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*/
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static int
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from_oct(int digs, char *where)
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{
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int value;
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while (isspace((unsigned char)*where)) { /* Skip spaces */
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where++;
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if (--digs <= 0)
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return -1; /* All blank field */
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}
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value = 0;
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while (digs > 0 && isodigit(*where)) { /* Scan til nonoctal */
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value = (value << 3) | (*where++ - '0');
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--digs;
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}
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if (digs > 0 && *where && !isspace((unsigned char)*where))
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return -1; /* Ended on non-space/nul */
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return value;
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}
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