crc algorithm

Peter Otten __peter__ at web.de
Wed Sep 3 09:19:29 CEST 2014


dream4soul at gmail.com wrote:

> On Tuesday, September 2, 2014 9:24:54 PM UTC+3, Peter Otten wrote:
>> dream4soul at gmail.com wrote:
>> 
>> 
>> 
>> > I have trouble to implement crc algorithm in python 3.3
>> 
>> > 
>> 
>> > c version work  perfect. I try to use bytes, int and c_types without
>> > any
>> 
>> > success can some who help me:
>> 
>> 
>> 
>> ctypes is for interfacing with C; don't use it in regular code.
>> 
>> 
>> 
>> > c version:
>> 
>> > 
>> 
>> > unsigned short calc_crc(const void *p_dat, int l_dat){
>> 
>> >         unsigned char *dat_ptr;
>> 
>> >         int loopc;
>> 
>> >         unsigned short crc_dat;
>> 
>> >         unsigned char c_work;
>> 
>> > 
>> 
>> >         dat_ptr = (unsigned char*)p_dat;
>> 
>> >         crc_dat = 0x0000;
>> 
>> >         for (; l_dat > 0; l_dat--)
>> 
>> >         {
>> 
>> >                 c_work = *(dat_ptr++);
>> 
>> >                 for (loopc = 0; loopc < 8; loopc++)
>> 
>> >                 {
>> 
>> >                         if ((((unsigned char )(crc_dat & 0x0001)) ^
>> 
>> >                         (c_work & 0x01)) == 0x01)
>> 
>> >                         {
>> 
>> >                                 crc_dat >>=1 ;
>> 
>> >                                 crc_dat ^=0x8408;
>> 
>> >                         } else {
>> 
>> >                                 crc_dat >>=1;
>> 
>> > 
>> 
>> >                         }
>> 
>> >                         c_work >>=1;
>> 
>> >                 }
>> 
>> >         }
>> 
>> >         return(crc_dat);
>> 
>> > }
>> 
>> 
>> 
>> A near-literal translation would be:
>> 
>> 
>> 
>> def calc_crc(data):
>> 
>>     crc = 0
>> 
>>     for work in data:
>> 
>>         for i in range(8):
>> 
>>             if (crc & 1) ^ (work & 1):
>> 
>>                 crc >>= 1
>> 
>>                 crc ^= 0x8408
>> 
>>             else:
>> 
>>                 crc >>= 1
>> 
>>             work >>= 1
>> 
>>     return crc
>> 
>> 
>> 
>> I don't see any operation where the "unboundedness" of Python's integer
>> type
>> 
>> could be a problem -- but no guarantees.
> 
> this doesn't work
> 
> calc_crc(b'\x00\x00\x34\x35\x38\x35')
> rsult 0x9f41 , but c function gives us 0x8c40

Are you sure? I get 0x9f41 with the C version you posted:

$ cat crc.c
#include <stdio.h>

unsigned short calc_crc(const void *p_dat, int l_dat){
        unsigned char *dat_ptr;
        int loopc;
        unsigned short crc_dat;
        unsigned char c_work;

        dat_ptr = (unsigned char*)p_dat;
        crc_dat = 0x0000;
        for (; l_dat > 0; l_dat--)
        {
                c_work = *(dat_ptr++);
                for (loopc = 0; loopc < 8; loopc++)
                {
                        if ((((unsigned char )(crc_dat & 0x0001)) ^ (c_work 
& 0x01)) == 0x01)
                        {
                                crc_dat >>=1 ;
                                crc_dat ^=0x8408;
                        } else {
                                crc_dat >>=1;

                        }
                        c_work >>=1;
                }
        }
        return(crc_dat);
}

main()
{
  unsigned char data[] = "\x00\x00\x34\x35\x38\x35";
  unsigned short crc = calc_crc(data, 6);
  printf("%x\n", crc);
}
$ gcc crc.c
$ ./a.out 
9f41





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