<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Another addendum: due to the conflict with glibc’s __nonnull, we’ll be renaming the __double_underscored keywords to _Big_underscored keywords, e.g.,<div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>__nonnull -> _Nonnull</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>__nullable -> _Nullable</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>__null_unspecified -> _Null_unspecified<br class=""><div class=""><br class=""></div><div class="">On Darwin, we’ll add predefines</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>#define __nonnull _Nonnull</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>#define __nullable _Nullable</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>#define __null_unspecified _Null_unspecified</div><div class=""><br class=""></div><div class="">to keep the existing headers working.</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>- Doug</div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Mar 2, 2015, at 1:22 PM, Douglas Gregor <<a href="mailto:dgregor@apple.com" class="">dgregor@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hello all,<div class=""><br class=""></div><div class="">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:</div><div class=""><br class=""></div><div class=""><div class=""><font face="Menlo" class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>char *strchr(const char *s, int c);</font></div></div><div class=""><br class=""></div><div class=""><div class="">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.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">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 <font face="Menlo" class="">strchr</font> [2]:</div></div><div class=""><br class=""></div><div class=""><div class=""><font face="Menlo" class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>__nullable char *strchr(__nonnull const char *s, int c);</font></div></div><div class=""><font face="Menlo" class=""><br class=""></font></div><div class="">With this, programmers and tools alike can better reason about the use of strchr with null pointers. </div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class=""><b class="">Goals</b></div><div class="">We have several specific goals that informed the design of this feature. </div><div class=""><br class=""></div><div class=""><ul class="MailOutline"><li class=""><b class="">Allow the intended nullability to be expressed on all pointers</b>: 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.</li><li class=""><b class="">Enable better tools support for detecting nullability problems:</b> 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.</li><li class=""><b class="">Support workflows where all interfaces provide nullability annotations:</b> 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.</li></ul></div><div class=""><div class=""><div class=""><ul class="MailOutline"><li class=""><b class="">Zero effect on ABI or code generation:</b> 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).</li></ul></div><div class=""><br class=""></div></div></div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><b class="">Why not __attribute__((nonnull))?</b></div><div class="">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.,</div><div class=""><pre class="smallexample"><span class="Apple-tab-span" style="white-space:pre"> </span>extern void *my_memcpy (void *dest, const void *src, size_t len) __attribute__((nonnull (1, 2)));</pre><div class="">or omits the list of indices to state that all pointer arguments cannot be null, e.g.,</div></div><div class=""><pre class="smallexample"><span class="Apple-tab-span"> </span>extern void *my_memcpy (void *dest, const void *src, size_t len) __attribute__((nonnull));</pre></div><div class="">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:</div><div class=""><pre class="smallexample"><span class="Apple-tab-span"> </span>extern void *my_memcpy (__attribute__((nonnull)) void *dest, __attribute__((nonnull)) const void *src, size_t len) __attribute__((returns_nonnull));</pre></div><div class="">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 <i class="">on the function</i> actually has an effect <i class="">on the function parameters</i> can get very, very confusing. Quick, which pointers are nullable vs. non-null in this example?</div><div class=""><br class=""></div><div class=""><font face="Menlo" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>__attribute__((nonnull)) void *my_realloc (void *ptr, size_t size);</font></div><div class=""><br class=""></div><div class="">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 <i class="">declaration</i> 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.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class=""><b class="">Nullability Qualifiers</b></div><div class="">We propose the addition of a new set of type qualifiers, spelled <font face="Menlo" class="">__nullable</font>, <font face="Menlo" class="">__nonnull</font>, and <font face="Menlo" class="">__null_unspecified</font>, to Clang. These are collectively known as <i class="">nullability qualifiers</i> and may be written anywhere any other type qualifier may be written (such as <font face="Menlo" class="">const</font>) on any type subject to the following restrictions:</div><div class=""><br class=""></div><div class=""><ul class="MailOutline"><li class="">Two nullability qualifiers shall not appear in the same set of qualifiers.</li><li class="">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.</li><li class="">A nullability qualifier in the declaration-specifiers applies to the innermost pointer type of each declarator (e.g., <font face="Menlo" class="">__nonnull int *</font> is equivalent to <font face="Menlo" class="">int * __nonnull</font>).</li><li class="">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.</li></ul></div><div class=""><b class=""><br class=""></b></div><div class="">The meanings of the three nullability qualifiers are as follows:</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span><font face="Menlo" class="">__nullable</font>: the pointer may store a null value at runtime (as part of the API contract)</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span><font face="Menlo" class="">__nonnull</font>: 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.</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span><font face="Menlo" class="">__null_unspecified</font>: 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 <i class="">even when looking at the implementation</i> 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].</div><div class=""><b class=""><br class=""></b></div><div class=""><b class="">Assumes-nonnull Regions</b></div><div class="">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:</div><div class=""><br class=""></div><div class=""><font face="Menlo" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>#pragma clang assume_nonnull begin</font></div><div class=""><font face="Menlo" class=""> __nullable char *strchr(const char *s, int c); // s is inferred to be __nonnull</font></div><div class=""><font face="Menlo" class=""><span class="Apple-tab-span" style="white-space:pre"> </span></font><span style="font-family: Menlo;" class="">void *my_realloc (__nullable void *ptr, size_t size); // my_realloc is inferred to return __nonnull</span></div><div class=""><span class="Apple-tab-span" style="font-family: Menlo; white-space: pre;"> </span><span style="font-family: Menlo;" class="">#pragma clang assume_nonnull end</span></div><div class=""><br class=""></div><div class="">We infer __nonnull within an assumes_nonnull region when:</div><div class=""><ul class="MailOutline"><li class="">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.</li><li class="">The pointer is a single-level pointer, e.g., <font face="Menlo" class="">int*</font> but not <font face="Menlo" class="">int**</font>, 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.</li></ul></div><div class=""><br class=""></div><div class="">Note that no #include may occur within an assumes_nonnull region, and assumes_nonnull regions cannot cross header boundaries.</div><div class=""><br class=""></div><div class=""><div class=""><b class="">Type System Impact</b></div><div class="">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. </div><div class=""><br class=""></div><div class="">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 <i class="">nullability qualifiers are not part of the canonical type</i> 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.</div><div class=""><br class=""></div><div class="">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.</div></div><div class=""><br class=""></div><div class="">Looking forward to our discussion.</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>- Doug (with Jordan Rose and Anna Zaks)</div><div class=""><br class=""></div><div class="">[1] <a href="https://urldefense.proofpoint.com/v2/url?u=http-3A__en.wikipedia.org_wiki_Tony-5FHoare-23Apologies-5Fand-5Fretractions&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=CnzuN65ENJ1H9py9XLiRvC_UQz6u3oG6GUNn7_wosSM&m=Xis_wMl9_nMGSZN546Z9ftQfCQpyudmMk0QwlxmJyGE&s=zZxNDBFvw8bNcgYI_mkOY-AFzM8yEeizNlMdaGQcNMk&e=" class="">http://en.wikipedia.org/wiki/Tony_Hoare#Apologies_and_retractions</a></div><div class="">[2] The standard description of strchr seems to imply that the parameter cannot be null</div><div class="">[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:</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>(a) __nonnull/__nullable/__null_unspecified type specifiers</div><div class=""><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>(b) nonnull/nullable/null_unspecified syntactic sugar for Objective-C</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>(c) Warning about inconsistent application of nullability specifiers within a given header</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>(d) assume_nonnnull begin/end pragmas</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>(e) Objective-C null_resettable property attribute</div></div><div class="">[4] <a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__gcc.gnu.org_onlinedocs_gcc_Function-2DAttributes.html&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=CnzuN65ENJ1H9py9XLiRvC_UQz6u3oG6GUNn7_wosSM&m=Xis_wMl9_nMGSZN546Z9ftQfCQpyudmMk0QwlxmJyGE&s=8WynBe2ZnYN1l5Kk3kTfWmwE0M1cZZy3L7bhX0swM9Y&e=" class="">https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html</a> (search for “nonnull”)</div><div class="">[5] No graybeards were harmed in the making of this feature.</div><div class="">[6] Template instantiation is the notable exception here, because it always canonicalizes types.</div><div class=""><br class=""></div><div class=""></div></div><span id="cid:4BE4FEE9-31F3-48FD-B7A8-A4C24C7624CD@apple.com"><nullability.patch></span><meta http-equiv="Content-Type" content="text/html charset=us-ascii" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""></div></div>_______________________________________________<br class="">cfe-dev mailing list<br class=""><a href="mailto:cfe-dev@cs.uiuc.edu" class="">cfe-dev@cs.uiuc.edu</a><br class="">http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev<br class=""></div></blockquote></div><br class=""></div></div></body></html>