[cfe-dev] RFC: Nullability qualifiers

[b17 c0de]

Just to clarify...

__has_extension(nullability) always returning true is exactly what I would
expect from reading the clang documentation. I would also expect that
__has_extension(assume_nonnull) always returns true (which it currently
doesn't).

I am hoping someone can explain why GNU mode is required for
__has_feature(nullability)
and __has_feature(assume_nonnull) to return true.

On Sat, Jun 27, 2015 at 4:54 PM, Aaron Ballman <aaron at aaronballman.com>
wrote:

> On Sat, Jun 27, 2015 at 10:45 AM, b17 c0de <b17c0de at gmail.com> wrote:
> > __has_extension(nullability) returns true in non-GNU mode, on the other
> > hand,__has_extension(assume_nonnull) returns false in non-GNU mode. Are
> you
> > saying this difference is by design. If so, why?
>
> I answered a bit too early in the morning. ;-) I forgot that
> __has_extension inherits functionality from __has_feature. So
> assume_nonnull should be true with either __has_feature or
> __has_extension in GNU or ObjC mode, but is currently false in other
> modes. You are correct that nullability is a bit different, and I'm
> not certain why. __has_feature(nullability) will return true for GNU
> and ObjC mode. __has_extension(nullability) will always return true.
>
> I am not certain whether this is by design or is a bug, but perhaps
> Doug can explain. (I'm also a bit curious as to why GNU mode is
> required.)
>
> ~Aaron
>
> >
> > On Sat, Jun 27, 2015 at 4:35 PM, Aaron Ballman <aaron at aaronballman.com>
> > wrote:
> >>
> >> On Sat, Jun 27, 2015 at 8:06 AM, b17 c0de <b17c0de at gmail.com> wrote:
> >> > I figured out my issue. I was compiling with -std=c++14 and the
> >> > nullability
> >> > and assume_nonnull features are only enabled for ObjC and GNU mode.
> Why
> >> > are
> >> > these only supported in GNU mode? I thought GNU mode was only for
> >> > features
> >> > that contradict the standard. How does this feature contradict the
> >> > standard
> >> > given that the names are double and single underscore prefixed? I
> would
> >> > rather not have to compile my code in GNU mode just to enable
> >> > nullability. I
> >> > can check with __has_extension() but at least Apple headers seem to
> only
> >> > use
> >> > __has_feature so the checks there won't be enabled when not compiling
> in
> >> > GNU
> >> > mode. If the consensus is that __has_feature(nullability) should only
> be
> >> > enabled for GNU mode, would it make sense to have an f-group flag like
> >> > -fnullability to enable it for __has_feature when not compiling in GNU
> >> > mode?
> >> >
> >> > Also I found a bug in clang. __has_extension(assume_nonnull) doesn't
> >> > work
> >> > properly. It is missing from the StringCase at the end of the
> >> > HasExtension()
> >> > function in lib/Lex/PPMacroExpansion.cpp. I think it should be there.
> >>
> >> __has_feature(assume_nonnull) is the way to test for that feature
> >> (which is also GNU and Obj-C only).
> >>
> >> ~Aaron
> >>
> >> >
> >> > On Sat, Jun 27, 2015 at 12:44 AM, b17 c0de <b17c0de at gmail.com> wrote:
> >> >>
> >> >> Apple please implement  __has_feature(nullability) in clang for
> Xcode 7
> >> >> release. :-)
> >> >>
> >> >>
> >> >> On Sat, Jun 27, 2015 at 12:31 AM, Aaron Ballman
> >> >> <aaron at aaronballman.com>
> >> >> wrote:
> >> >>>
> >> >>> On Fri, Jun 26, 2015 at 6:29 PM, b17 c0de <b17c0de at gmail.com>
> wrote:
> >> >>> >  It also appears that the current versions of Apple clang (even
> the
> >> >>> > newest
> >> >>> > beta) don't even support __has_feature(nullability). I take it
> this
> >> >>> > has
> >> >>> > been
> >> >>> > fixed in trunk?
> >> >>>
> >> >>> Correct, trunk is likely also the only place that has _Nonnull and
> >> >>> friends, too. If you're developing on OS X and don't need cross
> >> >>> compiler support for your code base, I would stick with __nonnull
> >> >>> there and you'll be fine. If you need cross compiler support, you'll
> >> >>> likely have to piece it together with macros.
> >> >>>
> >> >>> ~Aaron
> >> >>>
> >> >>> >
> >> >>> > On Fri, Jun 26, 2015 at 11:53 PM, Aaron Ballman
> >> >>> > <aaron at aaronballman.com>
> >> >>> > wrote:
> >> >>> >>
> >> >>> >> On Fri, Jun 26, 2015 at 5:44 PM, b17 c0de <b17c0de at gmail.com>
> >> >>> >> wrote:
> >> >>> >> > OK. What would be the best way to detect if Apple clang
> supports
> >> >>> >> > _Nonnull or
> >> >>> >> > only __nonnull though.
> >> >>> >>
> >> >>> >> I cannot speak for how Apple's Clang works in this regard, but
> >> >>> >> perhaps
> >> >>> >> Doug can.
> >> >>> >>
> >> >>> >> ~Aaron
> >> >>> >>
> >> >>> >> >
> >> >>> >> > On Fri, Jun 26, 2015 at 11:40 PM, Aaron Ballman
> >> >>> >> > <aaron at aaronballman.com>
> >> >>> >> > wrote:
> >> >>> >> >>
> >> >>> >> >> On Fri, Jun 26, 2015 at 5:36 PM, b17 c0de <b17c0de at gmail.com>
> >> >>> >> >> wrote:
> >> >>> >> >> > How can one detect if an Apple clang supports the new
> >> >>> >> >> > nullability
> >> >>> >> >> > attributes. I tried something like:
> >> >>> >> >> >
> >> >>> >> >> > #if __has_attribute(_Nonnull)
> >> >>> >> >> > #elif __has_attribute(__nonnull)
> >> >>> >> >> > #define _Nonnull __nonnull
> >> >>> >> >> > #else
> >> >>> >> >> > #define _Nonnull
> >> >>> >> >> > #endif
> >> >>> >> >> >
> >> >>> >> >> > But this didn't work. Why doesn't _Nonnull/__nonnull work
> with
> >> >>> >> >> > __has_attribute?
> >> >>> >> >>
> >> >>> >> >> __has_attribute is used to test for GNU-style attribute
> support
> >> >>> >> >> only.
> >> >>> >> >> To test for nullability, you should use:
> >> >>> >> >> __has_feature(nullability)
> >> >>> >> >>
> >> >>> >> >> ~Aaron
> >> >>> >> >>
> >> >>> >> >> >
> >> >>> >> >> > On Wed, Jun 24, 2015 at 10:39 PM, Douglas Gregor
> >> >>> >> >> > <dgregor at apple.com>
> >> >>> >> >> > wrote:
> >> >>> >> >> >>
> >> >>> >> >> >> Another addendum: due to the conflict with glibc’s
> __nonnull,
> >> >>> >> >> >> we’ll
> >> >>> >> >> >> be
> >> >>> >> >> >> renaming the __double_underscored keywords to
> >> >>> >> >> >> _Big_underscored
> >> >>> >> >> >> keywords,
> >> >>> >> >> >> e.g.,
> >> >>> >> >> >>
> >> >>> >> >> >> __nonnull -> _Nonnull
> >> >>> >> >> >> __nullable -> _Nullable
> >> >>> >> >> >> __null_unspecified -> _Null_unspecified
> >> >>> >> >> >>
> >> >>> >> >> >> On Darwin, we’ll add predefines
> >> >>> >> >> >>
> >> >>> >> >> >> #define __nonnull _Nonnull
> >> >>> >> >> >> #define __nullable _Nullable
> >> >>> >> >> >> #define __null_unspecified _Null_unspecified
> >> >>> >> >> >>
> >> >>> >> >> >> to keep the existing headers working.
> >> >>> >> >> >>
> >> >>> >> >> >> - Doug
> >> >>> >> >> >>
> >> >>> >> >> >> On Mar 2, 2015, at 1:22 PM, Douglas Gregor
> >> >>> >> >> >> <dgregor at apple.com>
> >> >>> >> >> >> wrote:
> >> >>> >> >> >>
> >> >>> >> >> >> Hello all,
> >> >>> >> >> >>
> >> >>> >> >> >> Null pointers are a significant source of problems in
> >> >>> >> >> >> applications.
> >> >>> >> >> >> Whether it’s SIGSEGV taking down a process or a foolhardy
> >> >>> >> >> >> attempt to
> >> >>> >> >> >> recover
> >> >>> >> >> >> from NullPointerException breaking invariants everywhere,
> >> >>> >> >> >> it’s a
> >> >>> >> >> >> problem
> >> >>> >> >> >> that’s bad enough for Tony Hoare to call the invention of
> the
> >> >>> >> >> >> null
> >> >>> >> >> >> reference
> >> >>> >> >> >> his billion dollar mistake [1]. It’s not the ability to
> >> >>> >> >> >> create a
> >> >>> >> >> >> null
> >> >>> >> >> >> pointer that is a problem—having a common sentinel value
> >> >>> >> >> >> meaning
> >> >>> >> >> >> “no
> >> >>> >> >> >> value”
> >> >>> >> >> >> is extremely useful—but that it’s very hard to determine
> >> >>> >> >> >> whether,
> >> >>> >> >> >> for a
> >> >>> >> >> >> particular pointer, one is expected to be able to use
> null. C
> >> >>> >> >> >> doesn’t
> >> >>> >> >> >> distinguish between “nullable” and “nonnull” pointers, so
> we
> >> >>> >> >> >> turn to
> >> >>> >> >> >> documentation and experimentation. Consider strchr from
> the C
> >> >>> >> >> >> standard
> >> >>> >> >> >> library:
> >> >>> >> >> >>
> >> >>> >> >> >> char *strchr(const char *s, int c);
> >> >>> >> >> >>
> >> >>> >> >> >> It is “obvious” to a programmer who knows the semantics of
> >> >>> >> >> >> strchr
> >> >>> >> >> >> that
> >> >>> >> >> >> it’s important to check for a returned null, because null
> is
> >> >>> >> >> >> used as
> >> >>> >> >> >> the
> >> >>> >> >> >> sentinel for “not found”. Of course, your tools don’t know
> >> >>> >> >> >> that,
> >> >>> >> >> >> so
> >> >>> >> >> >> they
> >> >>> >> >> >> cannot help when you completely forget to check for the
> null
> >> >>> >> >> >> case.
> >> >>> >> >> >> Bugs
> >> >>> >> >> >> ensue.
> >> >>> >> >> >>
> >> >>> >> >> >> Can I pass a null string to strchr? The standard is unclear
> >> >>> >> >> >> [2],
> >> >>> >> >> >> and
> >> >>> >> >> >> my
> >> >>> >> >> >> platform’s implementation happily accepts a null parameter
> >> >>> >> >> >> and
> >> >>> >> >> >> returns
> >> >>> >> >> >> null,
> >> >>> >> >> >> so obviously I shouldn’t worry about it… until I port my
> >> >>> >> >> >> code,
> >> >>> >> >> >> or
> >> >>> >> >> >> the
> >> >>> >> >> >> underlying implementation changes because my expectations
> and
> >> >>> >> >> >> the
> >> >>> >> >> >> library
> >> >>> >> >> >> implementor’s expectations differ. Given the age of
> strchr, I
> >> >>> >> >> >> suspect
> >> >>> >> >> >> that
> >> >>> >> >> >> every implementation out there has an explicit, defensive
> >> >>> >> >> >> check
> >> >>> >> >> >> for
> >> >>> >> >> >> a
> >> >>> >> >> >> null
> >> >>> >> >> >> string, because it’s easier to add yet more defensive (and
> >> >>> >> >> >> generally
> >> >>> >> >> >> useless) null checks than it is to ask your clients to fix
> >> >>> >> >> >> their
> >> >>> >> >> >> code.
> >> >>> >> >> >> Scale
> >> >>> >> >> >> this up, and code bloat ensues, as well as wasted
> programmer
> >> >>> >> >> >> effort
> >> >>> >> >> >> that
> >> >>> >> >> >> obscures the places where checking for null really does
> >> >>> >> >> >> matter.
> >> >>> >> >> >>
> >> >>> >> >> >> In a recent version of Xcode, Apple introduced an extension
> >> >>> >> >> >> to
> >> >>> >> >> >> C/C++/Objective-C that expresses the nullability of
> pointers
> >> >>> >> >> >> in
> >> >>> >> >> >> the
> >> >>> >> >> >> type
> >> >>> >> >> >> system via new nullability qualifiers . Nullability
> >> >>> >> >> >> qualifiers
> >> >>> >> >> >> express
> >> >>> >> >> >> nullability as part of the declaration of strchr  [2]:
> >> >>> >> >> >>
> >> >>> >> >> >> __nullable char *strchr(__nonnull const char *s, int c);
> >> >>> >> >> >>
> >> >>> >> >> >> With this, programmers and tools alike can better reason
> >> >>> >> >> >> about
> >> >>> >> >> >> the
> >> >>> >> >> >> use
> >> >>> >> >> >> of
> >> >>> >> >> >> strchr with null pointers.
> >> >>> >> >> >>
> >> >>> >> >> >> We’d like to contribute the implementation (and there is a
> >> >>> >> >> >> patch
> >> >>> >> >> >> attached
> >> >>> >> >> >> at the end [3]), but since this is a nontrivial extension
> to
> >> >>> >> >> >> all
> >> >>> >> >> >> of
> >> >>> >> >> >> the
> >> >>> >> >> >> C
> >> >>> >> >> >> family of languages that Clang supports, we believe that it
> >> >>> >> >> >> needs to
> >> >>> >> >> >> be
> >> >>> >> >> >> discussed here first.
> >> >>> >> >> >>
> >> >>> >> >> >> Goals
> >> >>> >> >> >> We have several specific goals that informed the design of
> >> >>> >> >> >> this
> >> >>> >> >> >> feature.
> >> >>> >> >> >>
> >> >>> >> >> >> Allow the intended nullability to be expressed on all
> >> >>> >> >> >> pointers:
> >> >>> >> >> >> Pointers
> >> >>> >> >> >> are used throughout library interfaces, and the nullability
> >> >>> >> >> >> of
> >> >>> >> >> >> those
> >> >>> >> >> >> pointers is an important part of the API contract with
> users.
> >> >>> >> >> >> It’s
> >> >>> >> >> >> too
> >> >>> >> >> >> simplistic to only allow function parameters to have
> >> >>> >> >> >> nullability,
> >> >>> >> >> >> for
> >> >>> >> >> >> example, because it’s also important information for data
> >> >>> >> >> >> members,
> >> >>> >> >> >> pointers-to-pointers (e.g., "a nonnull pointer to a
> nullable
> >> >>> >> >> >> pointer
> >> >>> >> >> >> to
> >> >>> >> >> >> an
> >> >>> >> >> >> integer”), arrays of pointers, etc.
> >> >>> >> >> >> Enable better tools support for detecting nullability
> >> >>> >> >> >> problems:
> >> >>> >> >> >> The
> >> >>> >> >> >> nullability annotations should be useful for tools
> >> >>> >> >> >> (especially
> >> >>> >> >> >> the
> >> >>> >> >> >> static
> >> >>> >> >> >> analyzer) that can reason about the use of null, to give
> >> >>> >> >> >> warnings
> >> >>> >> >> >> about
> >> >>> >> >> >> both
> >> >>> >> >> >> missed null checks (the result of strchr could be null…) as
> >> >>> >> >> >> well
> >> >>> >> >> >> as
> >> >>> >> >> >> for
> >> >>> >> >> >> unnecessarily-defensive code.
> >> >>> >> >> >> Support workflows where all interfaces provide nullability
> >> >>> >> >> >> annotations:
> >> >>> >> >> >> In
> >> >>> >> >> >> moving from a world where there are no nullability
> >> >>> >> >> >> annotations
> >> >>> >> >> >> to
> >> >>> >> >> >> one
> >> >>> >> >> >> where
> >> >>> >> >> >> we hope to see many such annotations, we’ve found it
> helpful
> >> >>> >> >> >> to
> >> >>> >> >> >> move
> >> >>> >> >> >> header-by-header, auditing a complete header to give it
> >> >>> >> >> >> nullability
> >> >>> >> >> >> qualifiers. Once one has done that, additions to the header
> >> >>> >> >> >> need
> >> >>> >> >> >> to
> >> >>> >> >> >> be
> >> >>> >> >> >> held
> >> >>> >> >> >> to the same standard, so we need a design that allows us to
> >> >>> >> >> >> warn
> >> >>> >> >> >> about
> >> >>> >> >> >> pointers that don’t provide nullability annotations for
> some
> >> >>> >> >> >> declarations in
> >> >>> >> >> >> a header that already has some nullability annotations.
> >> >>> >> >> >>
> >> >>> >> >> >> Zero effect on ABI or code generation: There are a huge
> >> >>> >> >> >> number
> >> >>> >> >> >> of
> >> >>> >> >> >> interfaces that could benefit from the use of nullability
> >> >>> >> >> >> qualifiers,
> >> >>> >> >> >> but we
> >> >>> >> >> >> won’t get widespread adoption if introducing the
> nullability
> >> >>> >> >> >> qualifiers
> >> >>> >> >> >> means breaking existing code, either in the ABI (say,
> because
> >> >>> >> >> >> nullability
> >> >>> >> >> >> qualifiers are mangled into the type) or at execution time
> >> >>> >> >> >> (e.g.,
> >> >>> >> >> >> because a
> >> >>> >> >> >> non-null pointer ends up being null along some error path
> and
> >> >>> >> >> >> causes
> >> >>> >> >> >> undefined behavior).
> >> >>> >> >> >>
> >> >>> >> >> >>
> >> >>> >> >> >>
> >> >>> >> >> >>
> >> >>> >> >> >> Why not __attribute__((nonnull))?
> >> >>> >> >> >> Clang already has an attribute to express nullability,
> >> >>> >> >> >> “nonnull”,
> >> >>> >> >> >> which
> >> >>> >> >> >> we
> >> >>> >> >> >> inherited from GCC [4]. The “nonnull” attribute can be
> placed
> >> >>> >> >> >> on
> >> >>> >> >> >> functions
> >> >>> >> >> >> to indicate which parameters cannot be null: one either
> >> >>> >> >> >> specifies
> >> >>> >> >> >> the
> >> >>> >> >> >> indices of the arguments that cannot be null, e.g.,
> >> >>> >> >> >>
> >> >>> >> >> >> extern void *my_memcpy (void *dest, const void *src, size_t
> >> >>> >> >> >> len)
> >> >>> >> >> >> __attribute__((nonnull (1, 2)));
> >> >>> >> >> >>
> >> >>> >> >> >> or omits the list of indices to state that all pointer
> >> >>> >> >> >> arguments
> >> >>> >> >> >> cannot
> >> >>> >> >> >> be
> >> >>> >> >> >> null, e.g.,
> >> >>> >> >> >>
> >> >>> >> >> >> extern void *my_memcpy (void *dest, const void *src, size_t
> >> >>> >> >> >> len)
> >> >>> >> >> >> __attribute__((nonnull));
> >> >>> >> >> >>
> >> >>> >> >> >> More recently, “nonnull”  has grown the ability to be
> applied
> >> >>> >> >> >> to
> >> >>> >> >> >> parameters, and one can use the companion attribute
> >> >>> >> >> >> returns_nonnull
> >> >>> >> >> >> to
> >> >>> >> >> >> state
> >> >>> >> >> >> that a function returns a non-null pointer:
> >> >>> >> >> >>
> >> >>> >> >> >> extern void *my_memcpy (__attribute__((nonnull)) void
> *dest,
> >> >>> >> >> >> __attribute__((nonnull)) const void *src, size_t len)
> >> >>> >> >> >> __attribute__((returns_nonnull));
> >> >>> >> >> >>
> >> >>> >> >> >> There are a number of problems here. First, there are
> >> >>> >> >> >> different
> >> >>> >> >> >> attributes
> >> >>> >> >> >> to express the same idea at different places in the
> grammar,
> >> >>> >> >> >> and
> >> >>> >> >> >> the
> >> >>> >> >> >> use of
> >> >>> >> >> >> the “nonnull” attribute on the function actually has an
> >> >>> >> >> >> effect
> >> >>> >> >> >> on
> >> >>> >> >> >> the
> >> >>> >> >> >> function parameters can get very, very confusing. Quick,
> >> >>> >> >> >> which
> >> >>> >> >> >> pointers
> >> >>> >> >> >> are
> >> >>> >> >> >> nullable vs. non-null in this example?
> >> >>> >> >> >>
> >> >>> >> >> >> __attribute__((nonnull)) void *my_realloc (void *ptr,
> size_t
> >> >>> >> >> >> size);
> >> >>> >> >> >>
> >> >>> >> >> >> According to that declaration, ptr is nonnull and the
> >> >>> >> >> >> function
> >> >>> >> >> >> returns
> >> >>> >> >> >> a
> >> >>> >> >> >> nullable pointer… but that’s the opposite of how it reads
> >> >>> >> >> >> (and
> >> >>> >> >> >> behaves,
> >> >>> >> >> >> if
> >> >>> >> >> >> this is anything like a realloc that cannot fail).
> Moreover,
> >> >>> >> >> >> because
> >> >>> >> >> >> these
> >> >>> >> >> >> two attributes are declaration attributes, not type
> >> >>> >> >> >> attributes,
> >> >>> >> >> >> you
> >> >>> >> >> >> cannot
> >> >>> >> >> >> express that nullability of the inner pointer in a
> >> >>> >> >> >> multi-level
> >> >>> >> >> >> pointer
> >> >>> >> >> >> or an
> >> >>> >> >> >> array of pointers, which makes these attributes verbose,
> >> >>> >> >> >> confusing,
> >> >>> >> >> >> and
> >> >>> >> >> >> not
> >> >>> >> >> >> sufficiently generally. These attributes fail the first of
> >> >>> >> >> >> our
> >> >>> >> >> >> goals.
> >> >>> >> >> >>
> >> >>> >> >> >> These attributes aren’t as useful as they could be for
> tools
> >> >>> >> >> >> support
> >> >>> >> >> >> (the
> >> >>> >> >> >> second and third goals), because they only express the
> >> >>> >> >> >> nonnull
> >> >>> >> >> >> case,
> >> >>> >> >> >> leaving
> >> >>> >> >> >> no way to distinguish between the unannotated case (nobody
> >> >>> >> >> >> has
> >> >>> >> >> >> documented
> >> >>> >> >> >> the nullability of some parameter) and the nullable case
> (we
> >> >>> >> >> >> know
> >> >>> >> >> >> the
> >> >>> >> >> >> pointer can be null). From a tooling perspective, this is a
> >> >>> >> >> >> killer:
> >> >>> >> >> >> the
> >> >>> >> >> >> static analyzer absolutely cannot warn that one has
> forgotten
> >> >>> >> >> >> to
> >> >>> >> >> >> check
> >> >>> >> >> >> for
> >> >>> >> >> >> null for every unannotated pointer, because the
> >> >>> >> >> >> false-positive
> >> >>> >> >> >> rate
> >> >>> >> >> >> would be
> >> >>> >> >> >> astronomical.
> >> >>> >> >> >>
> >> >>> >> >> >> Finally, we’ve recently started considering violations of
> the
> >> >>> >> >> >> __attribute__((nonnull)) contract to be undefined behavior,
> >> >>> >> >> >> which
> >> >>> >> >> >> fails
> >> >>> >> >> >> the
> >> >>> >> >> >> last of our goals. This is something we could debate
> further
> >> >>> >> >> >> if
> >> >>> >> >> >> it
> >> >>> >> >> >> were
> >> >>> >> >> >> the
> >> >>> >> >> >> only problem, but these declaration attributes fall all of
> >> >>> >> >> >> our
> >> >>> >> >> >> criteria, so
> >> >>> >> >> >> it’s not worth discussing.
> >> >>> >> >> >>
> >> >>> >> >> >> Nullability Qualifiers
> >> >>> >> >> >> We propose the addition of a new set of type qualifiers,
> >> >>> >> >> >> spelled
> >> >>> >> >> >> __nullable, __nonnull, and __null_unspecified, to Clang.
> >> >>> >> >> >> These
> >> >>> >> >> >> are
> >> >>> >> >> >> collectively known as nullability qualifiers and may be
> >> >>> >> >> >> written
> >> >>> >> >> >> anywhere any
> >> >>> >> >> >> other type qualifier may be written (such as const) on any
> >> >>> >> >> >> type
> >> >>> >> >> >> subject
> >> >>> >> >> >> to
> >> >>> >> >> >> the following restrictions:
> >> >>> >> >> >>
> >> >>> >> >> >> Two nullability qualifiers shall not appear in the same set
> >> >>> >> >> >> of
> >> >>> >> >> >> qualifiers.
> >> >>> >> >> >> A nullability qualifier shall qualify any pointer type,
> >> >>> >> >> >> including
> >> >>> >> >> >> pointers
> >> >>> >> >> >> to objects, pointers to functions, C++ pointers to members,
> >> >>> >> >> >> block
> >> >>> >> >> >> pointers,
> >> >>> >> >> >> and Objective-C object pointers.
> >> >>> >> >> >> A nullability qualifier in the declaration-specifiers
> applies
> >> >>> >> >> >> to
> >> >>> >> >> >> the
> >> >>> >> >> >> innermost pointer type of each declarator (e.g., __nonnull
> >> >>> >> >> >> int *
> >> >>> >> >> >> is
> >> >>> >> >> >> equivalent to int * __nonnull).
> >> >>> >> >> >> A nullability qualifier applied to a typedef of a
> >> >>> >> >> >> nullability-qualified
> >> >>> >> >> >> pointer type shall specify the same nullability as the
> >> >>> >> >> >> underlying
> >> >>> >> >> >> type
> >> >>> >> >> >> of
> >> >>> >> >> >> the typedef.
> >> >>> >> >> >>
> >> >>> >> >> >>
> >> >>> >> >> >> The meanings of the three nullability qualifiers are as
> >> >>> >> >> >> follows:
> >> >>> >> >> >>
> >> >>> >> >> >> __nullable: the pointer may store a null value at runtime
> (as
> >> >>> >> >> >> part
> >> >>> >> >> >> of
> >> >>> >> >> >> the
> >> >>> >> >> >> API contract)
> >> >>> >> >> >> __nonnull: the pointer should not store a null value at
> >> >>> >> >> >> runtime
> >> >>> >> >> >> (as
> >> >>> >> >> >> part
> >> >>> >> >> >> of the API contract). it is possible that the value can be
> >> >>> >> >> >> null,
> >> >>> >> >> >> e.g.,
> >> >>> >> >> >> in
> >> >>> >> >> >> erroneous historic uses of an API, and it is up to the
> >> >>> >> >> >> library
> >> >>> >> >> >> implementor
> >> >>> >> >> >> to decide to what degree she will accommodate such clients.
> >> >>> >> >> >> __null_unspecified: it is unclear whether the pointer can
> be
> >> >>> >> >> >> null or
> >> >>> >> >> >> not.
> >> >>> >> >> >> Use of this type qualifier is extremely rare in practice,
> but
> >> >>> >> >> >> it
> >> >>> >> >> >> fills
> >> >>> >> >> >> a
> >> >>> >> >> >> small but important niche when auditing a particular header
> >> >>> >> >> >> to
> >> >>> >> >> >> add
> >> >>> >> >> >> nullability qualifiers: sometimes the nullability contract
> >> >>> >> >> >> for a
> >> >>> >> >> >> few
> >> >>> >> >> >> APIs in
> >> >>> >> >> >> the header is unclear even when looking at the
> implementation
> >> >>> >> >> >> for
> >> >>> >> >> >> historical
> >> >>> >> >> >> reasons, and establishing the contract requires more
> >> >>> >> >> >> extensive
> >> >>> >> >> >> study.
> >> >>> >> >> >> In
> >> >>> >> >> >> such cases, it’s often best to mark that pointer as
> >> >>> >> >> >> __null_unspecified
> >> >>> >> >> >> (which will help silence the warning about unannotated
> >> >>> >> >> >> pointers
> >> >>> >> >> >> in a
> >> >>> >> >> >> header)
> >> >>> >> >> >> and move on, coming back to __null_unspecified pointers
> when
> >> >>> >> >> >> the
> >> >>> >> >> >> appropriate
> >> >>> >> >> >> graybeard has been summoned out of retirement [5].
> >> >>> >> >> >>
> >> >>> >> >> >> Assumes-nonnull Regions
> >> >>> >> >> >> We’ve found that it's fairly common for the majority of
> >> >>> >> >> >> pointers
> >> >>> >> >> >> within
> >> >>> >> >> >> a
> >> >>> >> >> >> particular header to be __nonnull. Therefore, we’ve
> >> >>> >> >> >> introduced
> >> >>> >> >> >> assumes-nonnull regions that assume that certain
> unannotated
> >> >>> >> >> >> pointers
> >> >>> >> >> >> implicitly get the __nonnull nullability qualifiers.
> >> >>> >> >> >> Assumes-nonnull
> >> >>> >> >> >> regions
> >> >>> >> >> >> are marked by pragmas:
> >> >>> >> >> >>
> >> >>> >> >> >> #pragma clang assume_nonnull begin
> >> >>> >> >> >>         __nullable char *strchr(const char *s, int c); // s
> >> >>> >> >> >> is
> >> >>> >> >> >> inferred
> >> >>> >> >> >> to
> >> >>> >> >> >> be __nonnull
> >> >>> >> >> >> void *my_realloc (__nullable void *ptr, size_t size); //
> >> >>> >> >> >> my_realloc
> >> >>> >> >> >> is
> >> >>> >> >> >> inferred to return __nonnull
> >> >>> >> >> >> #pragma clang assume_nonnull end
> >> >>> >> >> >>
> >> >>> >> >> >> We infer __nonnull within an assumes_nonnull region when:
> >> >>> >> >> >>
> >> >>> >> >> >> The pointer is a non-typedef declaration, such as a
> function
> >> >>> >> >> >> parameter,
> >> >>> >> >> >> variable, or data member, or the result type of a function.
> >> >>> >> >> >> It’s
> >> >>> >> >> >> very
> >> >>> >> >> >> rare
> >> >>> >> >> >> for one to warn typedefs to specify nullability
> information;
> >> >>> >> >> >> rather,
> >> >>> >> >> >> it’s
> >> >>> >> >> >> usually the user of the typedef that needs to specify
> >> >>> >> >> >> nullability.
> >> >>> >> >> >> The pointer is a single-level pointer, e.g., int* but not
> >> >>> >> >> >> int**,
> >> >>> >> >> >> because
> >> >>> >> >> >> we’ve found that programmers can get confused about the
> >> >>> >> >> >> nullability
> >> >>> >> >> >> of
> >> >>> >> >> >> multi-level pointers (is it a __nullable pointer to
> __nonnull
> >> >>> >> >> >> pointers,
> >> >>> >> >> >> or
> >> >>> >> >> >> the other way around?) and inferring nullability for any of
> >> >>> >> >> >> the
> >> >>> >> >> >> pointers in
> >> >>> >> >> >> a multi-level pointer compounds the situation.
> >> >>> >> >> >>
> >> >>> >> >> >>
> >> >>> >> >> >> Note that no #include may occur within an assumes_nonnull
> >> >>> >> >> >> region,
> >> >>> >> >> >> and
> >> >>> >> >> >> assumes_nonnull regions cannot cross header boundaries.
> >> >>> >> >> >>
> >> >>> >> >> >> Type System Impact
> >> >>> >> >> >> Nullability qualifiers are mapped to type attributes within
> >> >>> >> >> >> the
> >> >>> >> >> >> Clang
> >> >>> >> >> >> type
> >> >>> >> >> >> system, but a nullability-qualified pointer type is not
> >> >>> >> >> >> semantically
> >> >>> >> >> >> distinct from its unqualified pointer type. Therefore, one
> >> >>> >> >> >> may
> >> >>> >> >> >> freely
> >> >>> >> >> >> convert between nullability-qualified and
> >> >>> >> >> >> non-nullability-qualified
> >> >>> >> >> >> pointers, or between nullability-qualified pointers with
> >> >>> >> >> >> different
> >> >>> >> >> >> nullability qualifiers. One cannot overload on nullability
> >> >>> >> >> >> qualifiers,
> >> >>> >> >> >> write
> >> >>> >> >> >> C++ class template partial specializations that identify
> >> >>> >> >> >> nullability
> >> >>> >> >> >> qualifiers, or inspect nullability via type traits in any
> >> >>> >> >> >> way.
> >> >>> >> >> >>
> >> >>> >> >> >> Said more strongly, removing nullability qualifiers from a
> >> >>> >> >> >> well-formed
> >> >>> >> >> >> program will not change its behavior in any way, nor will
> the
> >> >>> >> >> >> semantics
> >> >>> >> >> >> of a
> >> >>> >> >> >> program change when any set of (well-formed) nullability
> >> >>> >> >> >> qualifiers
> >> >>> >> >> >> are
> >> >>> >> >> >> added to it. Operationally, this means that nullability
> >> >>> >> >> >> qualifiers
> >> >>> >> >> >> are
> >> >>> >> >> >> not
> >> >>> >> >> >> part of the canonical type in Clang’s type system, and that
> >> >>> >> >> >> any
> >> >>> >> >> >> warnings we
> >> >>> >> >> >> produce based on nullability information will necessarily
> be
> >> >>> >> >> >> dependent
> >> >>> >> >> >> on
> >> >>> >> >> >> Clang’s ability to retain type sugar during semantic
> >> >>> >> >> >> analysis.
> >> >>> >> >> >>
> >> >>> >> >> >> While it’s somewhat exceptional for us to introduce new
> type
> >> >>> >> >> >> qualifiers
> >> >>> >> >> >> that don’t produce semantically distinct types, we feel
> that
> >> >>> >> >> >> this is
> >> >>> >> >> >> the
> >> >>> >> >> >> only plausible design and implementation strategy for this
> >> >>> >> >> >> feature:
> >> >>> >> >> >> pushing
> >> >>> >> >> >> nullability qualifiers into the type system semantically
> >> >>> >> >> >> would
> >> >>> >> >> >> cause
> >> >>> >> >> >> significant changes to the language (e.g., overloading,
> >> >>> >> >> >> partial
> >> >>> >> >> >> specialization) and break ABI (due to name mangling) that
> >> >>> >> >> >> would
> >> >>> >> >> >> drastically
> >> >>> >> >> >> reduce the number of potential users, and we feel that
> >> >>> >> >> >> Clang’s
> >> >>> >> >> >> support
> >> >>> >> >> >> for
> >> >>> >> >> >> maintaining type sugar throughout semantic analysis is
> >> >>> >> >> >> generally
> >> >>> >> >> >> good
> >> >>> >> >> >> enough
> >> >>> >> >> >> [6] to get the benefits of nullability annotations in our
> >> >>> >> >> >> tools.
> >> >>> >> >> >>
> >> >>> >> >> >> Looking forward to our discussion.
> >> >>> >> >> >>
> >> >>> >> >> >> - Doug (with Jordan Rose and Anna Zaks)
> >> >>> >> >> >>
> >> >>> >> >> >> [1]
> >> >>> >> >> >>
> >> >>> >> >> >>
> >> >>> >> >> >>
> http://en.wikipedia.org/wiki/Tony_Hoare#Apologies_and_retractions
> >> >>> >> >> >> [2] The standard description of strchr seems to imply that
> >> >>> >> >> >> the
> >> >>> >> >> >> parameter
> >> >>> >> >> >> cannot be null
> >> >>> >> >> >> [3] The patch is complete, but should be reviewed on
> >> >>> >> >> >> cfe-commits
> >> >>> >> >> >> rather
> >> >>> >> >> >> than here. There are also several logic parts to this
> >> >>> >> >> >> monolithic
> >> >>> >> >> >> patch:
> >> >>> >> >> >> (a) __nonnull/__nullable/__null_unspecified type specifiers
> >> >>> >> >> >> (b) nonnull/nullable/null_unspecified syntactic sugar for
> >> >>> >> >> >> Objective-C
> >> >>> >> >> >> (c) Warning about inconsistent application of nullability
> >> >>> >> >> >> specifiers
> >> >>> >> >> >> within a given header
> >> >>> >> >> >> (d) assume_nonnnull begin/end pragmas
> >> >>> >> >> >> (e) Objective-C null_resettable property attribute
> >> >>> >> >> >> [4]
> >> >>> >> >> >>
> https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
> >> >>> >> >> >> (search
> >> >>> >> >> >> for “nonnull”)
> >> >>> >> >> >> [5] No graybeards were harmed in the making of this
> feature.
> >> >>> >> >> >> [6] Template instantiation is the notable exception here,
> >> >>> >> >> >> because it
> >> >>> >> >> >> always canonicalizes types.
> >> >>> >> >> >>
> >> >>> >> >> >> <nullability.patch>
> >> >>> >> >> >> _______________________________________________
> >> >>> >> >> >> cfe-dev mailing list
> >> >>> >> >> >> cfe-dev at cs.uiuc.edu
> >> >>> >> >> >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
> >> >>> >> >> >>
> >> >>> >> >> >>
> >> >>> >> >> >>
> >> >>> >> >> >> _______________________________________________
> >> >>> >> >> >> cfe-dev mailing list
> >> >>> >> >> >> cfe-dev at cs.uiuc.edu
> >> >>> >> >> >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
> >> >>> >> >> >>
> >> >>> >> >> >
> >> >>> >> >> >
> >> >>> >> >> > _______________________________________________
> >> >>> >> >> > cfe-dev mailing list
> >> >>> >> >> > cfe-dev at cs.uiuc.edu
> >> >>> >> >> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
> >> >>> >> >> >
> >> >>> >> >
> >> >>> >> >
> >> >>> >
> >> >>> >
> >> >>
> >> >>
> >> >
> >> >
> >> > _______________________________________________
> >> > cfe-dev mailing list
> >> > cfe-dev at cs.uiuc.edu
> >> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
> >> >
> >
> >
>
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