From: Jason Green <news AT jgreen4 DOT fsnet DOT co DOT uk>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Pointers to stucts
Date: Mon, 29 May 2000 13:55:33 +0100
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Dogansmoobs <dogansmoobsNOdoSPAM AT hotmail DOT com DOT invalid> wrote:

> typedef struct bitmap
> {
>  int width;
>  int height;
>  char *dat;
> } bitmap;

> bitmap *bmp;

That only creates a pointer to a bitmap structure.  No memory is
allocated for the struct or for the data pointed to by dat.  In other
words, bmp has some random value and points to some random memory
location.  You need to allocate some memory for the strucure and then
make bmp point to it.  Likewise for the dat pointer.

> bmp = loadPcxFile("file.pcx");
> 
> In the loadPcxFile function, it always crashes on the line:
> bmp->width = width;
> 
> and also in any function that tries to access the passed pointer
> to a bitmap, I get a page fault.

This is consistent with you attempting to dereference a pointer
without first pointing it at some valid memory location.

I suggest to find a good book explaining the relationship between
pointers, arrays and memory; then try out a few simple examples to
understand this before continuing.  You will probably also want to
investigate the malloc() and free() functions.