C pointer manipulation (was Re: C question for you old guys ;-))
C pointer manipulation (was Re: C question for you old guys ;-))
- Subject: C pointer manipulation (was Re: C question for you old guys ;-))
- From: "Alastair J.Houghton" <email@hidden>
- Date: Mon, 23 Jun 2003 00:05:27 +0100
On Sunday, June 22, 2003, at 11:24 pm, Martin Hdcker wrote:
Hi there,
to get back on topic of programming, could anybody of you tell me why
this bit of code produces two different results?
freeSpacePointer adress = 0x00040000; // Really a ((unsigned char *)
0x00040000)
long tryToGet = 1024; // 0x400
// return freeSpacePointer + tryToGet; /* Doesn't work... Why? */
// returns 0x42000
// versus
return (freeSpacePointer)((long)adress + tryToGet);
// returns 0x40400 -- what I wanted to get
The reason is probably that freeSpacePointer is defined as a pointer to
a type with a size of eight bytes (although as you haven't included the
typedef, it's hard to tell); if you write
type *ptr;
then do
ptr + number
where number evaluates to an integer of some sort (like in your
example), your C compiler will actually compute
ptr + number * sizeof type
This behaviour is useful in many cases, but it can be a pain if you
just want to do some quick address manipulation, and it sometimes
(perhaps even regularly?) surprises newbies.
Regards,
Alastair.
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