[Gllug] Pointer arithmetic with void *

[Alain Williams]

On Thu, Aug 15, 2002 at 06:17:17PM +0100, Tethys wrote:
> 
> >> what does the above
> >> give you over just reading the appropriate amount of data directly
> >> from the mmap()ed region in C (either via a direct cast to a suitable
> >> data type, or via a memcpy)?
> >
> >Bloat?
> 
> Which is exactly why I was asking. If there are any benefits in using
> it, then I'd like to know. Otherwise, I'll avoid the bloat, and stick
> with C...

Take care with something like:

	void*		p = mmap(....)
	struct foo*	fp;
	int		i;

	p = (void*)((char*)p + some_number);
	fp = (struct foo*)p;
	i = fp->some_member;

The point is that types (like int) usually need certain alignments (eg must
be a multiple of 4). If some_number is not a multiple of 4, you could get a
bus error. To avoid this you should go:

	struct	foo	f;

	....

	memcpy((void*)&f, p, sizeof(f));
	i = f.some_member;

If you work on Intel then you won't in fact get a bus error, but it takes more
memory cycles to load non aligned data. Some chips are not so forgiving - don't
write CPU specific code, you never know when your code gets being run on something
else.

-- 
Alain Williams

#include <std_disclaimer.h>

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