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From: mlinar@poisson.usc.edu (Mitch Mlinar)
Newsgroups: comp.lang.c
Subject: Re: Another portable code question
Message-ID: <3127@oberon.USC.EDU>
Date: Tue, 30-Jun-87 14:56:05 EDT
Article-I.D.: oberon.3127
Posted: Tue Jun 30 14:56:05 1987
Date-Received: Thu, 2-Jul-87 01:28:39 EDT
References: <16673@cca.CCA.COM> <1763@ttrdc.UUCP> <16769@cca.CCA.COM> <1019@killer.UUCP> <225@sugar.UUCP> <4761@columbia.UUCP>
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Reply-To: mlinar@poisson.usc.edu.UUCP (Mitch Mlinar)
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In article <4761@columbia.UUCP> francus@cheshire.columbia.edu.UUCP (Yoseff Francus) writes:
>In article <225@sugar.UUCP> peter@sugar.UUCP (Peter DaSilva) writes:
>>> There it is in a nutshell, you really don't need the * unless the function
>>> is declared int (**func)();, then you have pointer to pointer to a function
>>> returning an integer.
>>
>
>True ususally you want to pass the address of a strucuture.  But what if
>you change the values of the structure in the function, and DON'T want
>to know about the changes after returning from the function????
>
>Yoseff
>

Maybe I am not too familiar with the BIG frame architectures, but I am
somewhat familiar with items of the Sun/uVaxII genre on down.  The issue
actually boils down to efficiency of structures and the simple fact is:

Operations on static variables are ALWAYS faster than operations on auto
variables.

Now, this could just be a weakness in the uP instruction set or in the C
compilers themselves, but I have yet to see the reverse.  If you insist
upon doing big structs as "auto" (stack) variables, then there is probably
not much difference in speed - both passing a structure on the stack and
copying to a local "auto" are slow.  However, using stack operations on
these machines is MUCH slower than passing a pointer and copying to a
local static area.  Secondly, MANY C compilers still do not support pushing
anything other than simple arguments on the stack.  Pushing whole structures
is NOT portable in my book.

As it turns out, almost ALL efficient C compilers handle static memory
allocation much better than stack, too.  So, you are not saving any REAL
memory, although swap decreases.  (Use of static is REQUIRED in our Sun 3.0
compilers here at USC, ANY auto variable over 16k-bytes in size gets lunched
on the second recursive entry.  It took me 2 weeks and head bashing with
the also buggie dbxtool to figure this one out.  By breaking things up to
<16k-bytes and using static, the code zooms!)

-Mitch