Path: chuka.playstation.co.uk!scea!sumner
From: sumner@austin.metrowerks.com (Joel Sumner)
Newsgroups: scee.yaroze.programming.codewarrior,scea.yaroze.programming.codewarrior,scea.yaroze.freetalk,scee.yaroze.freetalk
Subject: Pointer usage (Was Re: Now I'm getting REALLY mad!)
Followup-To: scee.yaroze.programming.codewarrior
Date: Fri, 19 Dec 1997 12:01:20 -0600
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Xref: chuka.playstation.co.uk scee.yaroze.programming.codewarrior:120 scea.yaroze.programming.codewarrior:262 scea.yaroze.freetalk:194

Hi Guilio,
  I'm glad you posted your source for this.  I think I know what's going
on. This may be helpful for others having this problem.

In article <349A9CA1.7E336240@tin.it>, Giulio <vanderik@tin.it> wrote:

>typedef struct SpriteType
>{
>  GsSPRITE *SprBody            ; // Body of the sprite using library
>  int       SprOld_X,SprOld_Y  ; // Old sprite's position
>  u_char    AnimClock          ; // Animation clock
>  u_char    AnimSpeed          ; // Animation speed
>  u_char    MotionClock        ; // Motion clock
>  u_char    MotionSpeed        ; // Motion speed
>  u_char    CurrentFrame       ; // Current frame being displayed
>  u_char    NumFrames          ; // Total number of frames
>  u_char    State              ; // State of the sprite (Dead, Alive...)
>  u_char    Hits               ; // How many hits the sprite has got    
>  u_char    Doing              ; // What the sprite is doing
>  u_char    Type               ; // What kind of sprite is
>                                 // being processed
>  // Other sprite related variables....
>} Sprite, *SpritePtr ;
> 
>Sprite PlayerOne [1] ;    // Ugly struct of player one that 
>                          // I must use to make my program run
>                          // using PlayerOne as a pointer
>
>// SpritePtr PlayerOne ;  // Right pointer def that it's not usable
>// Sprite *PlayerOne ;    // Eh,eh too easy to be able to use this


I think I see your original problem.  Defining a *pointer* to a type, does
not allocate space for the type itself. When you do the following

SpritePtr   PlayerOne;

You have allocated space for a POINTER to a structure. Not the structure
itself.  You can't use this pointer until you tell it to point to something
(a SpriteType structure).  So how do you make space for a SpriteType
structure? There are two ways.

1) Have the compiler generate space for it.

The following code sequence will do this

// allocate space for two SpriteType objects
Sprite   gSprite[2];

// set the PlayerOne to point to an area in memory that holds a Sprite object
SpritePtr   PlayerOne=&gSprite[0];

// set the PlayerTwo pointer to point to an area in memory that holds a
Sprite object
SpritePtr   PlayerTwo=&gSprite[1];

2) Allocate it at runtime.
The above sequence works because you know at compile-time how many players
you are going to support (max two).  However, maybe you only want to
allocate space for the second player if they have selected a multi-player
game.  You can allocate memory  at runtime using the "malloc()" library
function.

// Allocate space for the POINTERs to the SPRITE objects, not the SPRITE
objects themselves.
SpritePtr   PlayerOne;
SpritePtr   PlayerTwo;

PlayerOne=(SpritePtr)malloc(sizeof(SPRITE));      // allocate memory for
PlayerOne
if (numPlayers==2)
   PlayerTwo=(SpritePtr)malloc(sizeof(SPRITE)); // allocate memory for PlayerTwo
else
   PlayerTwo=NULL;   // always initialize pointers, even if you are not
using them. It helps catch runtime errors

One thing you need to do if you malloc() memory is free() it when you are
done.  Thus, at the end of your game, you should

// release memory from the heap.
free(PlayerOne);
free(PlayerTwo);

(note: in order to use malloc(), you must call Init_Heap(). See the
_psstart.c file for information on doing this)

I hope this helped.  If you have any other code questions, please don't
hesitate to post examples. There are a lot of people here who can help.

(followups to scee.yaroze.programming.codewarrior)

-- 
PlayStation Development Tools Dude
Metrowerks