Xref: news2.mv.net comp.os.msdos.djgpp:7992
From: "Mike Marcelais" <michmarc AT microsoft DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: djgpp/allegro - help! - quick question
Date: 28 Aug 1996 22:25:55 GMT
Organization: Microsoft Corporation
Lines: 62
Message-ID: <01bb9530$89683f60$ba27379d@michmarc2>
References: <3223B9FC DOT 5C79 AT mbnet DOT mb DOT ca> <322464ED DOT 167EB0E7 AT eng DOT umd DOT edu>
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

John Sabean <docmani AT eng DOT umd DOT edu> wrote in article
<322464ED DOT 167EB0E7 AT eng DOT umd DOT edu>...
| Alex Demko wrote:
| > 
| > ¦PALETTE *new_truepal()
| > ¦{
| > ¦  PALETTE *pal = (PALETTE *)malloc(sizeof(PALETTE));
| > ¦  unsigned char r,g,b,i=0;
| > ¦  for (r=0; r<6; r++)
| > ¦   for (g=0; g<6; g++)
| > ¦    for (b=0; b<6; b++)
| > ¦     {
| > ¦     pal[i]->r=r*12;//causes GPF here
| > ¦     pal[i]->g=g*12;
| > ¦     pal[i]->b=b*12;
| > ¦     pal[i]->filler=0;
| > ¦     i++;
| > ¦     }
| > ¦  return pal;
| > ¦}
| > 
| > After I use the above routine in my code, I get a GPF in the marked
| > area. It won't GPF the first time it hits the marked code area, but
| > it will interate the inner loop about 40 (actual number depends on
| > phase of the moons) times, then GPFs. Am I dereferencing the
| > array/struct wrong? Please help, this thing is killing me :)
| > 
| > The code compiled error free.
| > 
| > Thanks
| > 
| 
| Perhaps since you've only allocated one palette structure and
| you're trying to access more than one?

No...but close.  :-)

| 
| Look at the allegro.h file. You'll find the line:
| >typedef RGB PALLETE[PAL_SIZE];

A PALLETTE is an array of RGB's.  sizeof(PALLETTE) will be
sizeof(RGB)*PAL_SIZE so he's mallocing the right stuff.  However, the problem
is that after you malloc a PALLETTE like that you -don't- have a PALLETE *,
you have an RGB *.

Trying to access PALLETE[1] will put you past the end of the PALLETE (since
you don't have an array of PALLETTE) and is the same as (pallete +
sizeof(PALLETE)) {using byte addition}

The fix is to change the malloc line to be `RGB *pal = (RGB
*)malloc(sizeof(PALLETE));' and your problem will go away.

-- 
+------------------------+----------------------+
| Mike Marcelais         | Excel Developer and  |
| michmarc AT microsoft DOT com | Magic Rules Guru     |
+------------------------+----------------------+
| Opinions expressed in this post are mine, and |
| do not necessarily reflect those of Microsoft |
+--= Moonstone Dragon =---------------= UDIC =--+