On Tue, 13 Dec 2022, 19:23 Paul Koning via Gcc, <gcc@gcc.gnu.org> wrote:
>
>
>
> > On Dec 13, 2022, at 2:08 PM, Alejandro Colomar via Gcc <gcc@gcc.gnu.org>
> > wrote:
> >
> > Hi!
> >
> > For the following program:
> >
> >
> > $ cat buf.c
> > #include <stdio.h>
> >
> > int main(void)
> > {
> > char *p, buf[5];
> >
> > p = buf + 6;
> > printf("%p\n", p);
> > }
> >
> >
> > There are no warnings in gcc, as I would expect:
> >
> > $ gcc -Wall -Wextra buf.c -O0
> >
> > Clang does warn, however:
> >
> > $ clang -Weverything -Wall -Wextra buf.c -O0
> > buf.c:8:17: warning: format specifies type 'void *' but the argument has
> > type 'char *' [-Wformat-pedantic]
> > printf("%p\n", p);
> > ~~ ^
> > %s
> > buf.c:7:6: warning: the pointer incremented by 6 refers past the end of
> > the array (that contains 5 elements) [-Warray-bounds-pointer-arithmetic]
> > p = buf + 6;
> > ^ ~
>
> I thought void * is a generic pointer that accepts any pointer argument.
Yes, but printf doesn't take a void* argument.
>
> So a warning about char* being passed in seems to be flat out wrong.
I wouldn't say flat out wrong. It's wrong because the C standard
guarantees that void* and char* have the same representation and
alignment requirements, so no conversion is needed to pass a char*
through a varargs ellipsis where a void* is expected. But if it was
int* or struct X* then the warning would be correct (but not a problem
in practice on common arches, which is why Clang only enabels it with
-Wformat-pedantic).
>
> > buf.c:5:2: note: array 'buf' declared here
> > char *p, buf[5];
> > ^
> > 2 warnings generated.
>
> That was discussed just days ago: C says that a pointer one past the end of
> the array is legal. So here too it looks like Clang is wrong and GCC is
> right.
No, clang is totally right here.
