Another email...

[whitetiger0990]

Oooooooooooooooooh, I'm sorry. The little line you put in the post made the rest of it look like your sig...

same thing happened to me

Blarg!

*pesters*

[TheBigBasicQ]

Bleh, you should look more carefully =P. I was confused for like .06 seconds before i realised it =)

[whitetiger0990]

i did. i was confused .0721 seconds though

[Moneo]

[to me as webmaster of qbnz]

_____________

Do you know where I can go to get quick basic utility on CRC error checking?

I need a utility that can generate CRC -16 error checking.

Any help would be most appreciated!

Oracle, I got the same email from this guy Tom Cushing, to which I responded the following:

Do I know you from the Qbasicnews or QBNZ forums?

I have a real good CRC calculator but it's written in C. Regardless of the language, this is not a simple algorithm. First try posting your request on one of the forums, and also scout around on the Internet.

Another idea is to use the DOS version of PKZIP. The zipped file will have an associated CRC, although its 32 bits. You can specify "no compression" using -e0 and afterwards use PKUNZIP -v to see the zip file, which will display the 32 bit CRC for the file. Maybe you can incorporate PKZIP and PKUNZIP to get a CRC on a given file, by doing a SHELL to these programs from your main program. I did something like this years ago.

Good luck!
Edward F. Moneo

HOWEVER, the solution posted by Nathan looks pretty good and it's not to long. Has anyone tried it? The trouble is, how do you test it, i.e., what do you compare the generated CRC to?
*****


*****

[na_th_an]

Well, you generate the CRC, send the file, and the receiver takes the file, generates the CRC, and compare it with yours. If it is the same, the file is correctly received.

Same with compressed data. You calculate CRC, compress. When you decompress, calculate CRC again and compare with your previous CRC. If they are not the same, there was a error whether in compressing or decompressing.

[KiZ]

Can someone actually explain what on earth CRC does? and what is the difference between CRC-16 and CRC-32?

[na_th_an]

http://www.google.es/search?q=CRC+algori...hl=es&meta=

[oracle]

Moneo: That was the guy who emailed me

Thanks for your responses guys . I'll give him the url of this thread.

[Moneo]

Well, you generate the CRC, send the file, and the receiver takes the file, generates the CRC, and compare it with yours. If it is the same, the file is correctly received.

Same with compressed data. You calculate CRC, compress. When you decompress, calculate CRC again and compare with your previous CRC. If they are not the same, there was a error whether in compressing or decompressing.

I agree in theory, Nathan, but the fact the the same algorithm issues the same CRC bytes on the send and receive ends, does not mean that the algorithm is performing a true Cyclic Redundancy Check. Maybe the algorithm is generating CRC bytes that are nothing more than than a LRC which is an exclusive or of all the bytes in the file, or maybe it's jusy a simple hash total.

There are rules for perfoming a CRC. I don't know them because I've never had the "need to Know", but these rules or specifications exist nevertheless and they are the reason why a CRC check is so reliable for checking the integrity of data files. The ACM Journal has had many articles on the subject.

Given the above, that is why I said "how do you test it?" I should have added "how do you authenticate the CRC algorithm?"
*****

[na_th_an]

I've just looked up for the algo in the internet and this code seems to follow it. I coded a CRC algo two years ago for a communications project and it was really similar. Last but not least, It's Douglas H. Lusher's code, and I always trust a livin' QB legend's code