/* * 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 * $Id: is_tar.c,v 1.1.1.2 1997/03/18 17:58:48 mpp Exp $ * * Comments changed and some code/comments reformatted * for file command by Ian Darwin. */ #include #include #include #include "tar.h" #define isodigit(c) ( ((c) >= '0') && ((c) <= '7') ) #if defined(__STDC__) || defined(__cplusplus) static int from_oct(int, char*); /* Decode octal number */ #else static int 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, nbytes) unsigned char *buf; int nbytes; { register union record *header = (union record *)buf; register int i; register int sum, recsum; register 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(digs, where) register int digs; register char *where; { register int 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; }