Memory Allocation Problem

[Brian K. W. Hook]

In article <7156 at crash.cts.com> kevin at crash.cts.com (Kevin Hill) writes:
>
>  I am still having problems with the malloc use in allocating memory
>for a two dimensional array.  I thank everyone that has been attempting
>to help me with this.

I don't know exactly what system you are writing on, but I assume that it
is PC compatible.  Actually, some code looks like Mac, but either way it
is not very relevant.

Have you tried using a SINGLE pointer instead of two?  Or are you doing
something that I am not aware of.  For example, I wrote a windowing 
library that looks much like some of that code you are using (which actually
looks quite a lot like the MetaWINDOWS library).

typedef struct _window {
	int   width, height;
	BYTE *oldImage;
        ...
} WINDOW;

While it is more intuitive to declare an array of pointers for a rectangular
region I have found that the following works REAL well and adds some 
simplicity.

void createWin ( WINDOW *wnd /* Whatever else needs to be passed */ )
{
   ...
   wnd->oldImage=(BYTE *)(malloc(wnd->width*wnd->height)+1);  // I add 1
							      // just in case
   ...
}

This assumes that the WINDOW doesn't cast a shadow and that the area behind is
definable as a BYTE (8 bits, as far as I am concerned...a 256 color bit map)
for each discreate location behind the WINDOW (i.e. pixel).

Now, back to the two dimensional array.  How about indicing each pointer 
that you need using a little bit of pointer arithmetic.  The equation
should be:

BYTE *rowToFind=wnd->oldImage+(wnd->width*row+column);

This assumes that "row" and "column" is the starting pixel for the address you
wish to find.  Since you are looking for a pointer per row (thus simulating
a two dimensional array) "column" would be 0.  Then you would just grab
wnd->width amount of BYTEs to get that entire line.

Hope this helped,

Brian