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Date: 12 Mar 88 21:05:56 GMT
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Subject:  Re: Pascal --> C question

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From:         "Richard A. O'Keefe" <ok@quintus.uucp>
Subject:      Re: Pascal --> C question
Comments: To: info-c@BRL-SMOKE.arpa
To:           Vic Kapella <SXJVK@ALASKA>

In article <3353@psuvax1.psu.edu>, schwartz@gondor.cs.psu.edu (Scott Schwartz)
 writes:
> The "packed" thing has a special meaning.  I can't quote you from the
> ANSI or ISO standards offhand, but the idea is that the compiler is
> supposed to arrange that the packed structure takes up a minimal amount
> of space (probably subject to some alignment restrictions).  So in the
> example I gave each boolean would take up one bit, and there would be
> 1024 of them, the whole array taking up 128 bytes.

The trouble is that a Pascal compiler is absolutely free to ignore
'packed' entirely.  When you say
    var a: array [0.1023] of boolean;
you have no guarantee at all that the compiler won't use 1024 "words".
To quote the Berkeley Pascal manual page:
    "The keyword 'packed' is recognized but has no effect."
You would be lucky to find a Pascal compiler which will pack array
elements down to the bit level.  Byte level, yes.

The situation is better in C for two reasons.
(1) At least you can say
    char a[1024];
    and get nothing worse than byte packing.

(2) You can write the bit-extraction operations as macros, and use them
    over and over again on different arrays.

And of course there are those C compilers which have the "inline"
processor, so that you can have
    get_bit(array, index)
and    put_bit(array, index, value)
turned into your machine's instructions for bit access.  (Yay, Sun!)