CRC Implementation Help

[beefeater267]

Hi,

I'm new to C++, and I need to implement a CRC algorithm as a function, and i'm having trouble doing so.

The polynomial specified is : x16 + x15 + x10 + x3

Can anyone help me out with this one? I've been stuck on it for a while.

Thanks

 

[Salem]

Like this?


--
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.

 

[beefeater267]

Salem,

No, not like that. This is not a standard CRC algorithm.

 

[ArkM]

This is not a standard CRC algorithm...
There are several standards for CRCs. You want so-called XMODEM/CRC.
See, for example:

http://www.totse.com/en/technology/telecommunications/wxmodem.html

From the link above:

/*
* This function calculates the CRC used by the XMODEM/CRC Protocol
* The first argument is a pointer to the message block.
* The second argument is the number of bytes in the message block.
* The function returns an integer which contains the CRC.
* The low order 16 bits are the coefficients of the CRC.
*/
int calcrc(ptr, count)
char *ptr;
int count;
More.. {
int crc, i;

crc = 0;
while (--count >= 0) {
crc = crc ^ (int)*ptr++ << 8;
for (i = 0; i < 8; ++i)
if (crc & 0x8000)
crc = crc << 1 ^ 0x1021;
else
crc = crc << 1;
}
return (crc & 0xFFFF);
}

 

[beefeater267]

ArkM -

Thanks for the help. I'll try it out and let you know how it goes!