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Avoiding overflows in C

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future

Joined: 14 May 2004
Posts: 330

Avoiding overflows in C

Posted: Tue Dec 21, 2004 10:48 am

Hello,

Analysing this code:

Code:

tperiod2=tperiod1; //
tperiod1=tperiod0; //
tperiod0=wperiod0; //

predict_tmr=(tperiod0+(tperiod0-tperiod1)) + //
(tperiod0-tperiod1-(tperiod1-tperiod2)); //

All being chars ranging from 30 to 255, based in the rate of change of the last, it can predict the new sample.. but when it is growing and close to 255 it overflows, also when diminishing close to 0.

I am looking for a way to avoid overflows in this case and others.

Thank you.

SherpaDoug

Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA

Re: Avoiding overflows in C

Posted: Tue Dec 21, 2004 11:55 am

Can you offset them to longs?

Code:

predict_tmr=(int)((tperiod0+(10000+tperiod0-tperiod1)) + //
(tperiod0-tperiod1-(10000+tperiod1-tperiod2)))-20000; //

The two 10000's should force the math to be done with longs, and avoid any posibility of going negative. Just take 20000 off the end and cast back to int.
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future

Joined: 14 May 2004
Posts: 330

Posted: Tue Dec 21, 2004 3:30 pm

Well, I did the ASM way

Code:

temp=sample0-sample1; //
if(STATUSbits.N) temp=0; //
temp=sample0+temp; //
if(STATUSbits.C) temp=255; //

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