Re: [BLACKBOX] Seasons Greetings from Linuxland

From: [at]} <Chris>
Date: Thu, 30 Dec 2010 09:51:54 +1030

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>From: BlackBox [mailto:BLACKBOX{([at]})nowhere.xy
>Sent: Wednesday, 29 December 2010 9:10 PM
>To: BLACKBOX{([at]})nowhere.xy
>Subject: [BLACKBOX] Seasons Greetings from Linuxland
....
....
>
>Even if we ignore the vagaries of the C compiler then neither
>the intention of the programmer nor the outcome of the expression
>
>digdata[digNr] = (bitdata[bitNr++] * 8) + (bitdata[bitNr++] *
>4) + (bitdata[bitNr++] * 2) + (bitdata[bitNr++]);
>
>is immediately obvious.
>
>It could be made obvious by expanding it to
>
>bitNr++;
>digdata[digNr] = (bitdata[bitNr] * 8) + (bitdata[bitNr] * 4) +
>(bitdata[bitNr] * 2) + (bitdata[bitNr]);
>
>or to
>
>digdata[digNr] = (bitdata[bitNr] * 8) + (bitdata[bitNr] * 4) +
>(bitdata[bitNr] * 2) + (bitdata[bitNr]);
>bitNr++;
>
>depending on what the writer intended.
>

No (irrespective of what the writer intended) - neither of those expansions
is equivalent to the original statement.

The problem is that he has used *four* separate instances of the ++ operator
in the one statement. He mistakenly assumed that the C compiler would
evaluate the expression from left to right. Suppose that the initial value
of bitNr was 0. What he wanted was the equivalent of:

  digdata[digNr] = (bitdata[0] * 8) + (bitdata[1] * 4) + (bitdata[2] * 2) +
bitdata[3];

Instead the compiler he used evaluated the expression from right to left and
he got the equivalent of:

  digdata[digNr] = (bitdata[3] * 8) + (bitdata[2] * 4) + (bitdata[1] * 2) +
bitdata[0];

What scares me is that if he had got lucky and chosen another compiler (or
maybe even a different optimisation setting?) he might have got the answer
he wanted - but there's no guarantee. Both compilers would have been
conforming to the rules of the language.

-----------------------

One reliable way to write what he wanted is:

  digdata[digNr] = (bitdata[bitNr] * 8) + (bitdata[bitNr+1] * 4) +
(bitdata[bitNr+2] * 2) + (bitdata[bitNr+3]);
  bitNr += 4;


Regards,
Chris Burrows

CFB Software
http://www.cfbsoftware.com

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Received on Thu Dec 30 2010 - 00:21:54 UTC