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

[Alejandro Colomar via Gcc]

Hi Paul,

On 12/13/22 20:22, Paul Koning 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.  So a 
warning about char* being passed in seems to be flat out wrong.

    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.

Look again:

        char *p, buf[5];

        p = buf + 6;

That's a pointer two-past-the-end; not one.

That's UB.

Cheers,

Alex

        paul

--
<http://www.alejandro-colomar.es/>

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