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Re: [PATCH] c/67882 - improve -Warray-bounds for invalid offsetof
- From: Joseph Myers <joseph at codesourcery dot com>
- To: Segher Boessenkool <segher at kernel dot crashing dot org>
- Cc: Bernd Schmidt <bschmidt at redhat dot com>, Martin Sebor <msebor at gmail dot com>, "Gcc Patch List" <gcc-patches at gcc dot gnu dot org>
- Date: Tue, 10 Nov 2015 00:02:16 +0000
- Subject: Re: [PATCH] c/67882 - improve -Warray-bounds for invalid offsetof
- Authentication-results: sourceware.org; auth=none
- References: <56172C8C dot 2070202 at gmail dot com> <5620ED47 dot 2090009 at redhat dot com> <56215158 dot 5040404 at gmail dot com> <56263F80 dot 1090203 at t-online dot de> <56265E51 dot 4070009 at gmail dot com> <alpine dot DEB dot 2 dot 10 dot 1510201650350 dot 7944 at digraph dot polyomino dot org dot uk> <5626A5A0 dot 7040003 at redhat dot com> <alpine dot DEB dot 2 dot 10 dot 1510202200590 dot 9787 at digraph dot polyomino dot org dot uk> <20151107233804 dot GB9982 at gate dot crashing dot org>
On Sat, 7 Nov 2015, Segher Boessenkool wrote:
> > The last one is certainly invalid. The one before is arguably invalid as
> > well (in the unary '&' equivalent, &a5_7[5][0] which is equivalent to
> > a5_7[5] + 0, the questionable operation is implicit conversion of a5_7[5]
> > from array to pointer - an array expression gets converted to an
> > expression "that points to the initial element of the array object", but
> > there is no array object a5_7[5] here).
>
> C11, 6.5.2.1/3:
> Successive subscript operators designate an element of a
> multidimensional array object. If E is an n-dimensional array (n >= 2)
> with dimensions i x j x . . . x k, then E (used as other than an lvalue)
> is converted to a pointer to an (n - 1)-dimensional array with
> dimensions j x . . . x k. If the unary * operator is applied to this
> pointer explicitly, or implicitly as a result of subscripting, the
> result is the referenced (n - 1)-dimensional array, which itself is
> converted into a pointer if used as other than an lvalue. It follows
> from this that arrays are stored in row-major order (last subscript
> varies fastest).
>
> As far as I see, a5_7[5] here is never treated as an array, just as a
> pointer, and &a5_7[5][0] is valid.
As usual, based on taking the address, not offsetof where there's the open
question of whether the C standard actually requires support for anything
other than a single element name there:
a5_7[5] is an expression of array type. The only way for it to be treated
as a pointer is for it to be converted implicitly to pointer type. That
implicit conversion is what I think is problematic.
Only once the implicit conversion has taken place do the special rules
about &A[B] meaning A + B take effect. But since the problem I see is
with the conversion of A to a pointer, you still have undefined behavior.
The paragraph you quote seems to not to add anything to the semantics
defined elsewhere in the standard; it's purely descriptive of some
consequences of those semantics.
Whether we wish to be more permissive about some such cases (depending on
-Warray-bounds=N) is a pragmatic matter depending on the extent to which
they are used in practice.
--
Joseph S. Myers
joseph@codesourcery.com