cvs commit: src/sys/netinet/libalias alias_util.c

[Eygene Ryabinkin]

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?
-- 
Eygene