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#1
October 1st, 2002, 01:28 AM
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implementing high-level checksum
what i'm trying to do is to implement a (very) high-level checksum for my TCP client app written in C...
here's the data format for the packets i'm sending out from the client: packet1-1033452719-878475\ what i want to do is to do a sum of the ASCII values of each indivual character (using ord())... so it would go something like this: sum = ord('p') + ord('a') + ord('c') + ... + ord(5) then after getting the checksum, i want to add it at the end of the packet too, like so: packet1-1033452719-878475-checksum\ how would i go about doing this? most importantly is how i can iterate over each character of the character array "packet1-1033452719-878475\" and perform the summation thanks!
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#2
October 1st, 2002, 01:39 AM
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How about some code like this:
Code:
unsigned char compute_checksum(unsigned char *buf) {
unsigned char *ch, result = 0;
/* buf is assumed to be null terminated */
for (ch = buf; *ch; ch++)
result += (*ch);
return result;
}
....
unsigned char checksum;
....
....
checksum = compute_checksum(buffer);
/* add checksum to end of buffer */
buffer[len] = checksum;
buffer[len+1] = '\0'; /* buffer assumed to be at least len+1 chars */
Please note that I typed this code off the top of my head, so it may have a few syntax errors. This isn't even a very good checksum algorithm, it just adds the ascii value of the chars up. You'd probably want to do a standard 16 or 32 bit CRC algorithm to ensure more reliability. You can probably google for those  Hope this helps!
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#3
October 1st, 2002, 01:48 AM
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that was fast... thanks man!
it looks good and now i'm gonna see if it's worthwhile to implement a checksum... i just realised that if i do the checksum computation, it may reduce my packet sending rate which i'm trying to get to be as accurate as possible thanks again!
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