Re: [PATCH v3] Document that the 'access' and 'nonnull' attributes are independent

[David Malcolm <dmalcolm@redhat.com>]

On Fri, 2022-03-25 at 14:38 -0600, Martin Sebor wrote:
> On 3/25/22 12:45, David Malcolm wrote:
> > On Wed, 2022-03-23 at 17:52 +0100, Sebastian Huber wrote:
> > > On 23/03/2022 17:31, Martin Sebor via Gcc-patches wrote:
> > > > 
> > > > The concern is that the constraints implied by atttributes
> > > > access
> > > > and
> > > > nonnull are independent of each other.  I would suggest to
> > > > document
> > > > that without talking about dereferencing because that's not
> > > > implied
> > > > by either of them.  E.g., something like this (feel free to
> > > > tweak
> > > > it
> > > > as you see fit):
> > > > 
> > > >     Note that the @code{access} attribute doesn't imply the
> > > > same
> > > >     constraint as attribute @code{nonnull} (@pxref{Attribute
> > > > nonnull}).
> > > >     The latter attribute should be used to annotate arguments
> > > > that
> > > > must
> > > >     never be null, regardless of the value of the size
> > > > argument.
> > > 
> > > I would not give an advice on using the nonnull attribute here.
> > > This
> > > attribute could have pretty dangerous effects in the function
> > > definition
> > > (removal of null pointer checks).
> > > 
> > 
> > That's a fair point.
> > 
> > Here's a v3 of the patch, which tones down the advice, and mentions
> > that
> > there are caveats when directing the reader to the "nonnull"
> > attribute.
> > 
> > How does this look?
> 
> This version looks good to me.

Thanks.

I tested this, and have gone ahead and pushed this to trunk
(as r12-8007-g0b5723d74f3a73).

Dave


> 
> Thanks
> Martin
> 
> > 
> > gcc/ChangeLog:
> >         * doc/extend.texi (Common Function Attributes): Document
> > that
> >         'access' does not imply 'nonnull'.
> > 
> > Signed-off-by: David Malcolm <dmalcolm@redhat.com>
> > ---
> >   gcc/doc/extend.texi | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> > 
> > diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> > index a4a25e86928..539dad7001d 100644
> > --- a/gcc/doc/extend.texi
> > +++ b/gcc/doc/extend.texi
> > @@ -2652,6 +2652,14 @@ The mode is intended to be used as a means
> > to help validate the expected
> >   object size, for example in functions that call
> > @code{__builtin_object_size}.
> >   @xref{Object Size Checking}.
> >   
> > +Note that the @code{access} attribute merely specifies how an
> > object
> > +referenced by the pointer argument can be accessed; it does not
> > imply that
> > +an access @strong{will} happen.  Also, the @code{access} attribute
> > does not
> > +imply the attribute @code{nonnull}; it may be appropriate to add
> > both attributes
> > +at the declaration of a function that unconditionally manipulates
> > a buffer via
> > +a pointer argument.  See the @code{nonnull} attribute for more
> > information and
> > +caveats.
> > +
> >   @item alias ("@var{target}")
> >   @cindex @code{alias} function attribute
> >   The @code{alias} attribute causes the declaration to be emitted
> > as an alias
>