Re: [BUG] missing warning for pointer arithmetic out of bounds

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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.