mark@hyper.UUCP (04/07/87)
Here is a fast (table driven), general crc program in C. It can be configured
to generate the crc's used by ARC, XMODEM and the CITT crc.
No more excuses for slow code!
--------------------snip-----------snip-----------snip---------------------
/* updcrc(3), crc(1) - calculate crc polynomials
*
* Calculate, intelligently, the CRC of a dataset incrementally given a
* buffer full at a time.
*
* Usage:
* newcrc = updcrc( oldcrc, bufadr, buflen )
* unsigned int oldcrc, buflen;
* char *bufadr;
*
* Compiling with -DTEST creates a program to print the CRC of stdin to stdout.
* Compile with -DMAKETAB to print values for crctab to stdout. If you change
* the CRC polynomial parameters, be sure to do this and change
* crctab's initial value.
*
* Notes:
* Regards the data stream as an integer whose MSB is the MSB of the first
* byte recieved. This number is 'divided' (using xor instead of subtraction)
* by the crc-polynomial P.
* XMODEM does things a little differently, essentially treating the LSB of
* the first data byte as the MSB of the integer. Define SWAPPED to make
* things behave in this manner.
*
* Author: Mark G. Mendel, 7/86
* UUCP: ihnp4!umn-cs!hyper!mark, GEnie: mgm
*/
/* The CRC polynomial.
* These 4 values define the crc-polynomial.
* If you change them, you must change crctab[]'s initial value to what is
* printed by initcrctab() [see 'compile with -DMAKETAB' above].
*/
/* Value used by: CITT XMODEM ARC */
#define P 0xA001 /* the poly: 0x1021 0x1021 A001 */
#define INIT_CRC 0L /* init value: -1 0 0 */
#define SWAPPED /* bit order: undef defined defined */
#define W 16 /* bits in CRC:16 16 16 */
/* data type that holds a W-bit unsigned integer */
#if W <= 16
# define WTYPE unsigned short
#else
# define WTYPE unsigned long
#endif
/* the number of bits per char: don't change it. */
#define B 8
static WTYPE crctab[1<<B] = /* as calculated by initcrctab() */ {
0x0, 0xc0c1, 0xc181, 0x140, 0xc301, 0x3c0, 0x280, 0xc241,
0xc601, 0x6c0, 0x780, 0xc741, 0x500, 0xc5c1, 0xc481, 0x440,
0xcc01, 0xcc0, 0xd80, 0xcd41, 0xf00, 0xcfc1, 0xce81, 0xe40,
0xa00, 0xcac1, 0xcb81, 0xb40, 0xc901, 0x9c0, 0x880, 0xc841,
0xd801, 0x18c0, 0x1980, 0xd941, 0x1b00, 0xdbc1, 0xda81, 0x1a40,
0x1e00, 0xdec1, 0xdf81, 0x1f40, 0xdd01, 0x1dc0, 0x1c80, 0xdc41,
0x1400, 0xd4c1, 0xd581, 0x1540, 0xd701, 0x17c0, 0x1680, 0xd641,
0xd201, 0x12c0, 0x1380, 0xd341, 0x1100, 0xd1c1, 0xd081, 0x1040,
0xf001, 0x30c0, 0x3180, 0xf141, 0x3300, 0xf3c1, 0xf281, 0x3240,
0x3600, 0xf6c1, 0xf781, 0x3740, 0xf501, 0x35c0, 0x3480, 0xf441,
0x3c00, 0xfcc1, 0xfd81, 0x3d40, 0xff01, 0x3fc0, 0x3e80, 0xfe41,
0xfa01, 0x3ac0, 0x3b80, 0xfb41, 0x3900, 0xf9c1, 0xf881, 0x3840,
0x2800, 0xe8c1, 0xe981, 0x2940, 0xeb01, 0x2bc0, 0x2a80, 0xea41,
0xee01, 0x2ec0, 0x2f80, 0xef41, 0x2d00, 0xedc1, 0xec81, 0x2c40,
0xe401, 0x24c0, 0x2580, 0xe541, 0x2700, 0xe7c1, 0xe681, 0x2640,
0x2200, 0xe2c1, 0xe381, 0x2340, 0xe101, 0x21c0, 0x2080, 0xe041,
0xa001, 0x60c0, 0x6180, 0xa141, 0x6300, 0xa3c1, 0xa281, 0x6240,
0x6600, 0xa6c1, 0xa781, 0x6740, 0xa501, 0x65c0, 0x6480, 0xa441,
0x6c00, 0xacc1, 0xad81, 0x6d40, 0xaf01, 0x6fc0, 0x6e80, 0xae41,
0xaa01, 0x6ac0, 0x6b80, 0xab41, 0x6900, 0xa9c1, 0xa881, 0x6840,
0x7800, 0xb8c1, 0xb981, 0x7940, 0xbb01, 0x7bc0, 0x7a80, 0xba41,
0xbe01, 0x7ec0, 0x7f80, 0xbf41, 0x7d00, 0xbdc1, 0xbc81, 0x7c40,
0xb401, 0x74c0, 0x7580, 0xb541, 0x7700, 0xb7c1, 0xb681, 0x7640,
0x7200, 0xb2c1, 0xb381, 0x7340, 0xb101, 0x71c0, 0x7080, 0xb041,
0x5000, 0x90c1, 0x9181, 0x5140, 0x9301, 0x53c0, 0x5280, 0x9241,
0x9601, 0x56c0, 0x5780, 0x9741, 0x5500, 0x95c1, 0x9481, 0x5440,
0x9c01, 0x5cc0, 0x5d80, 0x9d41, 0x5f00, 0x9fc1, 0x9e81, 0x5e40,
0x5a00, 0x9ac1, 0x9b81, 0x5b40, 0x9901, 0x59c0, 0x5880, 0x9841,
0x8801, 0x48c0, 0x4980, 0x8941, 0x4b00, 0x8bc1, 0x8a81, 0x4a40,
0x4e00, 0x8ec1, 0x8f81, 0x4f40, 0x8d01, 0x4dc0, 0x4c80, 0x8c41,
0x4400, 0x84c1, 0x8581, 0x4540, 0x8701, 0x47c0, 0x4680, 0x8641,
0x8201, 0x42c0, 0x4380, 0x8341, 0x4100, 0x81c1, 0x8081, 0x4040,
} ;
WTYPE
updcrc( icrc, icp, icnt )
WTYPE icrc;
unsigned char *icp;
int icnt;
{
register WTYPE crc = icrc;
register unsigned char *cp = icp;
register int cnt = icnt;
while( cnt-- ) {
#ifndef SWAPPED
crc = (crc<<B) ^ crctab[(crc>>(W-B)) ^ *cp++];
#else
crc = (crc>>B) ^ crctab[(crc & ((1<<B)-1)) ^ *cp++];
#endif SWAPPED
}
return( crc );
}
#ifdef MAKETAB
#include <stdio.h>
main()
{
initcrctab();
}
initcrctab()
{
register int b, i;
WTYPE v;
for( b = 0; b <= (1<<B)-1; ++b ) {
#ifndef SWAPPED
for( v = b<<(W-B), i = B; --i >= 0; )
v = v & ((WTYPE)1<<(W-1)) ? (v<<1)^P : v<<1;
#else
for( v = b, i = B; --i >= 0; )
v = v & 1 ? (v>>1)^P : v>>1;
#endif
crctab[b] = v;
printf( "0x%lx,", v & ((1L<<W)-1L));
if( (b&7) == 7 )
printf("\n" );
else
printf(" ");
}
}
#endif
#ifdef TEST
#include <stdio.h>
#include <fcntl.h>
#define MAXBUF 4096
main( ac, av )
int ac; char **av;
{
int fd;
int nr;
int i;
char buf[MAXBUF];
WTYPE crc, crc2;
fd = 0;
if( ac > 1 )
if( (fd = open( av[1], O_RDONLY )) < 0 ) {
perror( av[1] );
exit( -1 );
}
crc = crc2 = INIT_CRC;
while( (nr = read( fd, buf, MAXBUF )) > 0 ) {
crc = updcrc( crc, buf, nr );
}
if( nr != 0 )
perror( "reading" );
else {
printf( "%lx\n", crc );
}
#ifdef MAGICCHECK
/* tack one's complement of crc onto data stream, and
continue crc calculation. Should get a constant (magic number)
dependent only on P, not the data.
*/
crc2 = crc ^ -1L;
for( nr = W-B; nr >= 0; nr -= B ) {
buf[0] = (crc2 >> nr);
crc = updcrc(crc, buf, 1);
}
/* crc should now equal magic */
buf[0] = buf[1] = buf[2] = buf[3] = 0;
printf( "magic test: %lx =?= %lx\n", crc, updcrc(-1, buf, W/B));
#endif MAGICCHECK
}
#endif
--
Mark G. Mendel, ihnp4!umn-cs!hyper!mark, Network Systems Corporation
All opinions expressed herein, even the most arbitrary, are defended by my
employer with religious fervor.