Sender: nate AT cartsys DOT com
Message-ID: <3587280D.E555624B@cartsys.com>
Date: Tue, 16 Jun 1998 19:21:01 -0700
From: Nate Eldredge <nate AT cartsys DOT com>
MIME-Version: 1.0
To: e96pd AT efd DOT lth DOT se
CC: djgpp AT delorie DOT com
Subject: Re: Help! Why is my program crashing?
References: <35867E83 DOT E01 AT efd DOT lth DOT se>
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Peter Danielsson wrote:
> 
> Why does my program crash all the time. It's a simple program, only a
> few lines:
> 
> int WIDTH=513;
> int *mapheight;
> 
> void initmapheight(int *n)//allocateing memory
> {
>  n=(int*)calloc((long)513*513,sizeof(int));
>  if(n==NULL)
>          exit(-1);
> }
> void reset(int *m,int W2)//set to zero. Here it crashes
> {
>  for(int a5=0;a5<W2;a5++)
>          for(int q=0;q<W2;q++)
>                 m[(long)a5*W2+q]=0;
> }
> void main(int argc, char *argv[])
> {
> initmapheight(mapheight);
> reset(mapheight,100);
> }

C uses pass-by-value.  Thus, when you assign the result of `calloc' to
`n', only the local copy is changed, and nothing happens to
`mapheight'.  To do it right, you must pass `mapheight's address:

void initmapheight(int **n)
{
  *n = (int *)calloc(whatever);
  if (!*n)
    exit(1);
}

int *mapheight;

...
initmapheight(&mapheight);

Note also that `main' must return `int'.
-- 

Nate Eldredge
nate AT cartsys DOT com