On Tue, 6 Dec 2005, Chris Shoemaker wrote:
> > A pointer to a char is the same size as a pointer to an int and is the
> > same size as a pointer to void.
>
> This is not generally true for typed null pointers. Implementations
> are free to use different representations for null pointers of
> different types. This has in fact been done. Using the null-pointer
> of the wrong type is not portable. This is why David was right about
> (void*) being better than (char*) on the sentinel.
Pointers are generally the same size on byte-addressable machines like
i386 and ppc. You run into trouble with old architectures and embedded
RISC.
<old-timer>
My favorite old architecture was the DEC-20. It had 36-bit words. C types:
char 9 bits
short 18 bits
int 36 bits
long 72 bits
float 36 bits
double 72 bits
char * 36 bits
short * 36 bits
int * 18 bits
long * 18 bits
int ** 36 bits
char ** 36 bits
...
Most pointers were word pointers. Char and short pointers used a special
subfield pointer extension that had its own load/store/inc/dec instructions.
I find it useful to ask myself whether the code would run properly on that
system when considering a portablity question. Not that a DEC-20 would
be all that useful today. 2^18 * 36 bits = 2Meg * 9-bits byte addressable RAM.
The system I used in college had 256K words = 1M * 9-bit RAM. If you
were lucky enough to be allowed to use the console, it had a memory mapped
graphics display, and the extended pointer format could also address individual
pixels! The display was featured in my favorite "Flatland" fiction:
_The_Planiverse_
http://www.amazon.com/gp/product/0387989161/
</old-timer>
More relevant today are graphics processors (which I guess is a type of
embedded), which often have varying pointer sizes, including pointers
to bits and nybbles, to optimize bit pixel operations.
--
Stuart D. Gathman <[EMAIL PROTECTED]>
Business Management Systems Inc. Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flamis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.
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