Xref: news2.mv.net comp.os.msdos.djgpp:2689
From: Massimiliano Mantione <mantione AT mailer DOT cefriel DOT it>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: malloc crash
Date: Fri, 12 Apr 1996 10:22:52 +0200
Organization: Computer Science Dep. - Milan University
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Message-ID: <316E12DC.5B3@mailer.cefriel.it>
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To: Jesus Canal <jesus AT matrust DOT es>
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Jesus Canal wrote:
> 
> Eli Zaretskii wrote:
> > Btw, allocating 5000 chunks of 10KB is a symptom of a very
> > memory-inefficent program (IMHO).  If a program indeed needs so many
> > allocations, it should allocate memory in a few large chunks and then
> > overload `new' with something that serves allocations off those chunks.
> >
> 
> I implemented a ZBuffer class as:
> 
>    (...)
> 
>    int      theHeight;
>    int      theWidth;
>    double **theArray
> 
>    (...)
> 
> The constructor allocated one (double *) array, theWidth items long,
> and then theWidth double arrays, theHeight items long.
> Then accessing the element (X, Y) was theArray [X][Y].
> 
> I will try to avoid the malloc crash using only a double * array:
> 
>    (...)
> 
>    int      theHeight;
>    int      theWidth;
>    double  *theArray
> 
>    (...)
> 
> and accessing the element (X,Y) as theArray [theWidth * Y + X]

I'm not a true hacker, but I think your program will also get
a small speed improvement from this change.

And if you are using C++, you could put there a:

inline double ZBufferElement(int X, int Y)
    { return theArray[theWidth * Y + X]};

and take the benefits of ``information hiding'' (actually, in
C++ the info are not hidden at all, you see them in the header
file but you ``cannot'' -should not- use them).

If (for some reason) you do not like this memory access method,
you can always use your previous one, but implementing Eli's
advice like this:

(...)
private:
   int      theHeight;
   int      theWidth;
   double  *theArray
   double **theArrayRows

public:
    inline double ZBufferElement(int X, int Y)
        { return theArrayRows[X][Y]};
(...)

and using a constructor that initializes the ``theArrayRows''
pointers to the correct address of each row in ``theArray''.

With this trivial trick, you would:
[1] Waste a bit of memory (but not so much, compared to the
    whole ZBuffer).
[2] malloc only once a great amount of memory, and then
    manage to use it as you like (it was just what Eli was
    telling you).

And, since you are using C++ in a proper way, your code will
always be the same (i.e., ZBufferElement(X,Y) in each case).

I hope this (even if trivial) helps.

All the best

  Massimiliano

P.S. To all the ``C'' fans: I know you can do *exactly* the
same things without C++, defining proper macros, and using
only them... I can do in C *everything* I do in C++, also
polimorphic classes (using function pointers).
I just like C++, and do not want to start a war.

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