Eygene Ryabinkin wrote:
Tue, Dec 04, 2007 at 05:25:35PM +0000, Alexey Dokuchaev wrote:
*ptr++ would choke since pointer arith on (void *) is undefined AFAIK.
I've been under impression that ++ on void * whould simply increase it
by one.
This behaviour is documented for GCC:
http://www.mcs.vuw.ac.nz/cgi-bin/info2www?(gcc)Pointer+Arith
Just for the record (gcc 4.2.1):
-----
$ gcc -o test -Wall -ansi -pedantic test.c
test.c: In function 'main':
test.c:9: warning: wrong type argument to increment
$ ./test
'2'
$ g++ -o test -Wall -ansi test.c
test.c: In function 'int main()':
test.c:9: error: ISO C++ forbids incrementing a pointer of type 'void*'
$ cat test.c
#include <stdio.h>
int
main(void)
{
char c[] = "123456789abcdef";
void *p = c;
p++;
printf("'%c'\n", *((char *)p));
return 0;
}
-----
It seems to me that ++ adds one to the void pointer because it is
demanded by C99 (ISO/IEC 9899:TC2, 6.2.5, requirement 26, page 36)
that 'char *' and 'void *' have the same representation and
alignment requirements. So, it seems to me that (p++) has implicit
conversion from 'void *' to 'char *' for 'void *p', at least it
can be interpreted in this way.
But some people say that void* arithmetics is GCC'ism. It worth to
note that the warning about void* arithmetics lived in GCC at least
since 1992: see
http://gcc.gnu.org/viewcvs/trunk/gcc/c-typeck.c?revision=364&view=markup
function 'pointer_int_sum'.
And the problem of 'void *' arithmetics had been touched in the
-current a while ago:
http://lists.freebsd.org/pipermail/freebsd-current/2003-July/006439.html
I am failing to find a place in the C standard where void arithmetics
is prohibited, but I can be blind. Anyone?
Arithmethic on void pointers is forbidden. The relevant parts of the
(C99) standard are:
?6.2.5 clause 19: "The void type comprises an empty set of values; it is
an incomplete type that cannot be completed."
?6.2.5 clause 1: "[...] Types are partitioned into object types [...],
function types [...], and incomplete types [...].
?6.5.6 clause 2: "For addition [...] one operand shall be a pointer to
an object type and the other shall have integer type. [...]"
(subtraction has an analogous statement, increment and decrement are
just addition/substraction by one)
So the conclusion is:
- void* is a pointer to an incomplete type.
- Incomplete types are not object types.
- Addition is only allowed on pointers to an object type.
Therefore arithmetic on void pointers is not allowed. Arithmetic on void
pointers is indeed a GCCism.
Christoph
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