what is the suggested way to do void * arithmetic ?

Harti Brandt brandt at fokus.fraunhofer.de
Thu Jul 10 23:23:30 PDT 2003


On Thu, 10 Jul 2003, David Leimbach wrote:

DL> I always feel better when I convert void * to char * but that's probably
DL>because C++ doesn't allow pointer arithmetic on void *'s.  The argument
DL>being that you don't know the size of what's being pointed to with a void *
DL>and therefore can't know how far to seek the the pointer to get to the next
DL>valid address.
DL>
DL>I think C takes a more low-level approach and says "void * is just an address
DL>void * + 1 means the next valid address".

void pointer arithmetic is a sun and gcc specific extension and by no
means a standardized thing.

DL>
DL>Anyway... it just seems to help when porting code between C/C++ to use
DL>char *...

Yes.

harti

DL>Dave
DL>On Thursday, July 10, 2003, at 11:40AM, Luigi Rizzo <rizzo at icir.org> wrote:
DL>
DL>>On Thu, Jul 10, 2003 at 03:42:04AM -0700, Terry Lambert wrote:
DL>>> Luigi Rizzo wrote:
DL>>> > in several places in ipfw2.c i have to move pointers across
DL>>> > structures of variable length (lists of ipfw2 instructions
DL>>> > returned by the getsockopt()), and i use the following type of code:
DL>>> >
DL>>> >         void *next;
DL>>> >         foo *p;
DL>>> >         next = (void *)p + len;
DL>>> >         foo = (foo *)p + len;
DL>>            ^^^^^^^^^^^^^^
DL>>
DL>>sorry i meant   p = (void *)p + len;
DL>>
DL>>...
DL>>> I don't understand the second one.  The first one blows up because
DL>>> you aren't parenthesizing, e.g.:
DL>>>
DL>>> 	next = (void *)(p + len);
DL>>>
DL>>> The compiler is complaining because it doesn't know sizeof(*((void *)0))
DL>>
DL>>ok, it actually evaluates to 1 and i thought it was some standard, probably
DL>>it is not so i guess i have to cast to (char *) instead
DL>>
DL>>	thanks
DL>>	luigi
DL>>
DL>>> (pointer arithmatic is coerced to the type of the lvalue, in most
DL>>> cases of casts).
DL>>>
DL>>> Unless you are referencing them as array elements (in which case,
DL>>> packing becomes a problem for you, when referencing them as arrays
DL>>> of foo's, since you don't know how foo's are packed in an array),
DL>>> you should probably cast them to char for the arithmatic, and add
DL>>> them with byte counts.
DL>>>
DL>>> -- Terry
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-- 
harti brandt,
http://www.fokus.fraunhofer.de/research/cc/cats/employees/hartmut.brandt/private
brandt at fokus.fraunhofer.de, harti at freebsd.org


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