From: horst DOT kraemer AT snafu DOT de (Horst Kraemer)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: help: void pointers
Date: Sat, 07 Aug 1999 10:44:15 GMT
Organization: [Posted via] Interactive Networx
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

On Sat, 07 Aug 1999 07:04:36 GMT, "YUBS" <jdebever1 AT home DOT com> wrote:

> Hi, I'm having some trouble using void pointers. I'm writing a function that
> will allocate enough memory for any of my types by using a void pointer as a
> parameter in the function. I.e.
 
> void pcx_init (void *image, char type)
 
> So say I have the following type:
 
 
> typdef struct pcx_image_typ
> {
>     char *buffer;
>     int width, height;
> } pcx_image, *pcx_image_ptr

 
> I want to be able to pass a pointer to a variable of type pcx_image to
> pcx_init and have it allocate width*height memory for buffer. The problem
> I'm getting is that DJGPP keeps complaining that I'm "dereferencing "void *"
> pointer" and that "request for memeber "buffer" in something not a structure
> or a union".
> 
> here's my exact code:
> 
> void pcx_init(void *image, char type)
> {
>     switch (type)
>         {
>             case 's': {
>                             if (!(((pcx_image_ptr)(image->buffer)) = (char
> * )malloc (image->width* image->height)))
>                          }
>         }
> }

Of course. image is a pointer to _void_ and therefore image->buffer
etc. is meaningless.

If I understand you correctly pcx_init has a void* parameter because
it shall be used for different structure types.

In this case you'd have to cast 'index' to the appropriate type every
time _before_ you applying the -> operator to it. It's not

	(pcx_image_ptr)(image->buffer)

which is casting the illegal expression (image->buffer) but

	((pcx_image_ptr)image)->buffer

which is casting 'image' and then dereferences it.


Better readable would be a simple

  case 's': 
  {
     pcx_image_ptr p = image;
     if ( (p->buffer=malloc(p->width * p->height))==NULL ) {/**/}
     ...
     break;
  }

using a local helper variable instead of the verbose and terrible


((pcx_image_ptr)index)->buffer =
    malloc( ((pcx_image_ptr)index)->width * 
            ((pcx_image_ptr)index)->height
          );


Regards
Horst