[cfe-dev] RFC: Nullability qualifiers

[Douglas Gregor]

Sent from my iPhone

> On Mar 3, 2015, at 7:55 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> 
> 
> From: "Richard Smith" <richard at metafoo.co.uk>
> To: "Douglas Gregor" <dgregor at apple.com>
> Cc: "cfe-dev Developers" <cfe-dev at cs.uiuc.edu>
> Sent: Monday, March 2, 2015 5:34:18 PM
> Subject: Re: [cfe-dev] RFC: Nullability qualifiers
> 
>> On Mon, 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).
> A sanitizer for this feature would seem very useful, but this bullet point suggests that such a sanitizer would violate the model. Likewise, I don't see why we should rule out the option of optimizing on the basis of these qualifiers (under a -fstrict-nonnull flag or similar).
>  
>> 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.
> 
> On this last point, how do you want to define the interaction between these? Should we not consider the violation to be undefined behavior if these new qualifiers are present?

I think we should continue to say that the existing nonnull attributes require nonnull values (UB if a null gets in there). The type qualifiers will not. 

Moreover, we should have the existing nonnull/returns_nonnull attributes imply __nonnull, since we'll get more utility out of existing annotations with way. 

  - Doug
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