1 //==--- AttrDocs.td - Attribute documentation ----------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===---------------------------------------------------------------------===//
10 def GlobalDocumentation {
12 -------------------------------------------------------------------
13 NOTE: This file is automatically generated by running clang-tblgen
14 -gen-attr-docs. Do not edit this file by hand!!
15 -------------------------------------------------------------------
26 This page lists the attributes currently supported by Clang.
30 def SectionDocs : Documentation {
31 let Category = DocCatVariable;
33 The ``section`` attribute allows you to specify a specific section a
34 global variable or function should be in after translation.
36 let Heading = "section (gnu::section, __declspec(allocate))";
39 def InitSegDocs : Documentation {
40 let Category = DocCatVariable;
42 The attribute applied by ``pragma init_seg()`` controls the section into
43 which global initialization function pointers are emitted. It is only
44 available with ``-fms-extensions``. Typically, this function pointer is
45 emitted into ``.CRT$XCU`` on Windows. The user can change the order of
46 initialization by using a different section name with the same
47 ``.CRT$XC`` prefix and a suffix that sorts lexicographically before or
48 after the standard ``.CRT$XCU`` sections. See the init_seg_
49 documentation on MSDN for more information.
51 .. _init_seg: http://msdn.microsoft.com/en-us/library/7977wcck(v=vs.110).aspx
55 def TLSModelDocs : Documentation {
56 let Category = DocCatVariable;
58 The ``tls_model`` attribute allows you to specify which thread-local storage
59 model to use. It accepts the following strings:
66 TLS models are mutually exclusive.
70 def ThreadDocs : Documentation {
71 let Category = DocCatVariable;
73 The ``__declspec(thread)`` attribute declares a variable with thread local
74 storage. It is available under the ``-fms-extensions`` flag for MSVC
75 compatibility. See the documentation for `__declspec(thread)`_ on MSDN.
77 .. _`__declspec(thread)`: http://msdn.microsoft.com/en-us/library/9w1sdazb.aspx
79 In Clang, ``__declspec(thread)`` is generally equivalent in functionality to the
80 GNU ``__thread`` keyword. The variable must not have a destructor and must have
81 a constant initializer, if any. The attribute only applies to variables
82 declared with static storage duration, such as globals, class static data
83 members, and static locals.
87 def CarriesDependencyDocs : Documentation {
88 let Category = DocCatFunction;
90 The ``carries_dependency`` attribute specifies dependency propagation into and
93 When specified on a function or Objective-C method, the ``carries_dependency``
94 attribute means that the return value carries a dependency out of the function,
95 so that the implementation need not constrain ordering upon return from that
96 function. Implementations of the function and its caller may choose to preserve
97 dependencies instead of emitting memory ordering instructions such as fences.
99 Note, this attribute does not change the meaning of the program, but may result
100 in generation of more efficient code.
104 def C11NoReturnDocs : Documentation {
105 let Category = DocCatFunction;
107 A function declared as ``_Noreturn`` shall not return to its caller. The
108 compiler will generate a diagnostic for a function declared as ``_Noreturn``
109 that appears to be capable of returning to its caller.
113 def CXX11NoReturnDocs : Documentation {
114 let Category = DocCatFunction;
116 A function declared as ``[[noreturn]]`` shall not return to its caller. The
117 compiler will generate a diagnostic for a function declared as ``[[noreturn]]``
118 that appears to be capable of returning to its caller.
122 def AssertCapabilityDocs : Documentation {
123 let Category = DocCatFunction;
124 let Heading = "assert_capability (assert_shared_capability, clang::assert_capability, clang::assert_shared_capability)";
126 Marks a function that dynamically tests whether a capability is held, and halts
127 the program if it is not held.
131 def AcquireCapabilityDocs : Documentation {
132 let Category = DocCatFunction;
133 let Heading = "acquire_capability (acquire_shared_capability, clang::acquire_capability, clang::acquire_shared_capability)";
135 Marks a function as acquiring a capability.
139 def TryAcquireCapabilityDocs : Documentation {
140 let Category = DocCatFunction;
141 let Heading = "try_acquire_capability (try_acquire_shared_capability, clang::try_acquire_capability, clang::try_acquire_shared_capability)";
143 Marks a function that attempts to acquire a capability. This function may fail to
144 actually acquire the capability; they accept a Boolean value determining
145 whether acquiring the capability means success (true), or failing to acquire
146 the capability means success (false).
150 def ReleaseCapabilityDocs : Documentation {
151 let Category = DocCatFunction;
152 let Heading = "release_capability (release_shared_capability, clang::release_capability, clang::release_shared_capability)";
154 Marks a function as releasing a capability.
158 def AssumeAlignedDocs : Documentation {
159 let Category = DocCatFunction;
161 Use ``__attribute__((assume_aligned(<alignment>[,<offset>]))`` on a function
162 declaration to specify that the return value of the function (which must be a
163 pointer type) has the specified offset, in bytes, from an address with the
164 specified alignment. The offset is taken to be zero if omitted.
168 // The returned pointer value has 32-byte alignment.
169 void *a() __attribute__((assume_aligned (32)));
171 // The returned pointer value is 4 bytes greater than an address having
172 // 32-byte alignment.
173 void *b() __attribute__((assume_aligned (32, 4)));
175 Note that this attribute provides information to the compiler regarding a
176 condition that the code already ensures is true. It does not cause the compiler
177 to enforce the provided alignment assumption.
181 def EnableIfDocs : Documentation {
182 let Category = DocCatFunction;
184 .. Note:: Some features of this attribute are experimental. The meaning of
\r
185 multiple enable_if attributes on a single declaration is subject to change in
\r
186 a future version of clang. Also, the ABI is not standardized and the name
\r
187 mangling may change in future versions. To avoid that, use asm labels.
189 The ``enable_if`` attribute can be placed on function declarations to control
190 which overload is selected based on the values of the function's arguments.
191 When combined with the ``overloadable`` attribute, this feature is also
197 int isdigit(int c) __attribute__((enable_if(c <= -1 || c > 255, "chosen when 'c' is out of range"))) __attribute__((unavailable("'c' must have the value of an unsigned char or EOF")));
202 isdigit(-10); // results in a compile-time error.
205 The enable_if attribute takes two arguments, the first is an expression written
206 in terms of the function parameters, the second is a string explaining why this
207 overload candidate could not be selected to be displayed in diagnostics. The
208 expression is part of the function signature for the purposes of determining
209 whether it is a redeclaration (following the rules used when determining
210 whether a C++ template specialization is ODR-equivalent), but is not part of
213 The enable_if expression is evaluated as if it were the body of a
214 bool-returning constexpr function declared with the arguments of the function
215 it is being applied to, then called with the parameters at the call site. If the
216 result is false or could not be determined through constant expression
217 evaluation, then this overload will not be chosen and the provided string may
218 be used in a diagnostic if the compile fails as a result.
220 Because the enable_if expression is an unevaluated context, there are no global
221 state changes, nor the ability to pass information from the enable_if
222 expression to the function body. For example, suppose we want calls to
223 strnlen(strbuf, maxlen) to resolve to strnlen_chk(strbuf, maxlen, size of
224 strbuf) only if the size of strbuf can be determined:
228 __attribute__((always_inline))
229 static inline size_t strnlen(const char *s, size_t maxlen)
230 __attribute__((overloadable))
231 __attribute__((enable_if(__builtin_object_size(s, 0) != -1))),
232 "chosen when the buffer size is known but 'maxlen' is not")))
234 return strnlen_chk(s, maxlen, __builtin_object_size(s, 0));
237 Multiple enable_if attributes may be applied to a single declaration. In this
238 case, the enable_if expressions are evaluated from left to right in the
239 following manner. First, the candidates whose enable_if expressions evaluate to
240 false or cannot be evaluated are discarded. If the remaining candidates do not
241 share ODR-equivalent enable_if expressions, the overload resolution is
242 ambiguous. Otherwise, enable_if overload resolution continues with the next
243 enable_if attribute on the candidates that have not been discarded and have
244 remaining enable_if attributes. In this way, we pick the most specific
245 overload out of a number of viable overloads using enable_if.
249 void f() __attribute__((enable_if(true, ""))); // #1
250 void f() __attribute__((enable_if(true, ""))) __attribute__((enable_if(true, ""))); // #2
252 void g(int i, int j) __attribute__((enable_if(i, ""))); // #1
253 void g(int i, int j) __attribute__((enable_if(j, ""))) __attribute__((enable_if(true))); // #2
255 In this example, a call to f() is always resolved to #2, as the first enable_if
256 expression is ODR-equivalent for both declarations, but #1 does not have another
257 enable_if expression to continue evaluating, so the next round of evaluation has
258 only a single candidate. In a call to g(1, 1), the call is ambiguous even though
259 #2 has more enable_if attributes, because the first enable_if expressions are
262 Query for this feature with ``__has_attribute(enable_if)``.
266 def OverloadableDocs : Documentation {
267 let Category = DocCatFunction;
269 Clang provides support for C++ function overloading in C. Function overloading
270 in C is introduced using the ``overloadable`` attribute. For example, one
271 might provide several overloaded versions of a ``tgsin`` function that invokes
272 the appropriate standard function computing the sine of a value with ``float``,
273 ``double``, or ``long double`` precision:
278 float __attribute__((overloadable)) tgsin(float x) { return sinf(x); }
279 double __attribute__((overloadable)) tgsin(double x) { return sin(x); }
280 long double __attribute__((overloadable)) tgsin(long double x) { return sinl(x); }
282 Given these declarations, one can call ``tgsin`` with a ``float`` value to
283 receive a ``float`` result, with a ``double`` to receive a ``double`` result,
284 etc. Function overloading in C follows the rules of C++ function overloading
285 to pick the best overload given the call arguments, with a few C-specific
288 * Conversion from ``float`` or ``double`` to ``long double`` is ranked as a
289 floating-point promotion (per C99) rather than as a floating-point conversion
292 * A conversion from a pointer of type ``T*`` to a pointer of type ``U*`` is
293 considered a pointer conversion (with conversion rank) if ``T`` and ``U`` are
296 * A conversion from type ``T`` to a value of type ``U`` is permitted if ``T``
297 and ``U`` are compatible types. This conversion is given "conversion" rank.
299 The declaration of ``overloadable`` functions is restricted to function
300 declarations and definitions. Most importantly, if any function with a given
301 name is given the ``overloadable`` attribute, then all function declarations
302 and definitions with that name (and in that scope) must have the
303 ``overloadable`` attribute. This rule even applies to redeclarations of
304 functions whose original declaration had the ``overloadable`` attribute, e.g.,
308 int f(int) __attribute__((overloadable));
309 float f(float); // error: declaration of "f" must have the "overloadable" attribute
311 int g(int) __attribute__((overloadable));
312 int g(int) { } // error: redeclaration of "g" must also have the "overloadable" attribute
314 Functions marked ``overloadable`` must have prototypes. Therefore, the
315 following code is ill-formed:
319 int h() __attribute__((overloadable)); // error: h does not have a prototype
321 However, ``overloadable`` functions are allowed to use a ellipsis even if there
322 are no named parameters (as is permitted in C++). This feature is particularly
323 useful when combined with the ``unavailable`` attribute:
327 void honeypot(...) __attribute__((overloadable, unavailable)); // calling me is an error
329 Functions declared with the ``overloadable`` attribute have their names mangled
330 according to the same rules as C++ function names. For example, the three
331 ``tgsin`` functions in our motivating example get the mangled names
332 ``_Z5tgsinf``, ``_Z5tgsind``, and ``_Z5tgsine``, respectively. There are two
333 caveats to this use of name mangling:
335 * Future versions of Clang may change the name mangling of functions overloaded
336 in C, so you should not depend on an specific mangling. To be completely
337 safe, we strongly urge the use of ``static inline`` with ``overloadable``
340 * The ``overloadable`` attribute has almost no meaning when used in C++,
341 because names will already be mangled and functions are already overloadable.
342 However, when an ``overloadable`` function occurs within an ``extern "C"``
343 linkage specification, it's name *will* be mangled in the same way as it
346 Query for this feature with ``__has_extension(attribute_overloadable)``.
350 def ObjCMethodFamilyDocs : Documentation {
351 let Category = DocCatFunction;
353 Many methods in Objective-C have conventional meanings determined by their
354 selectors. It is sometimes useful to be able to mark a method as having a
355 particular conventional meaning despite not having the right selector, or as
356 not having the conventional meaning that its selector would suggest. For these
357 use cases, we provide an attribute to specifically describe the "method family"
358 that a method belongs to.
360 **Usage**: ``__attribute__((objc_method_family(X)))``, where ``X`` is one of
361 ``none``, ``alloc``, ``copy``, ``init``, ``mutableCopy``, or ``new``. This
362 attribute can only be placed at the end of a method declaration:
366 - (NSString *)initMyStringValue __attribute__((objc_method_family(none)));
368 Users who do not wish to change the conventional meaning of a method, and who
369 merely want to document its non-standard retain and release semantics, should
370 use the retaining behavior attributes (``ns_returns_retained``,
371 ``ns_returns_not_retained``, etc).
373 Query for this feature with ``__has_attribute(objc_method_family)``.
377 def NoDuplicateDocs : Documentation {
378 let Category = DocCatFunction;
380 The ``noduplicate`` attribute can be placed on function declarations to control
381 whether function calls to this function can be duplicated or not as a result of
382 optimizations. This is required for the implementation of functions with
383 certain special requirements, like the OpenCL "barrier" function, that might
384 need to be run concurrently by all the threads that are executing in lockstep
385 on the hardware. For example this attribute applied on the function
386 "nodupfunc" in the code below avoids that:
390 void nodupfunc() __attribute__((noduplicate));
391 // Setting it as a C++11 attribute is also valid
392 // void nodupfunc() [[clang::noduplicate]];
403 gets possibly modified by some optimizations into code similar to this:
415 where the call to "nodupfunc" is duplicated and sunk into the two branches
420 def NoSplitStackDocs : Documentation {
421 let Category = DocCatFunction;
423 The ``no_split_stack`` attribute disables the emission of the split stack
424 preamble for a particular function. It has no effect if ``-fsplit-stack``
429 def ObjCRequiresSuperDocs : Documentation {
430 let Category = DocCatFunction;
432 Some Objective-C classes allow a subclass to override a particular method in a
433 parent class but expect that the overriding method also calls the overridden
434 method in the parent class. For these cases, we provide an attribute to
435 designate that a method requires a "call to ``super``" in the overriding
436 method in the subclass.
438 **Usage**: ``__attribute__((objc_requires_super))``. This attribute can only
439 be placed at the end of a method declaration:
443 - (void)foo __attribute__((objc_requires_super));
445 This attribute can only be applied the method declarations within a class, and
446 not a protocol. Currently this attribute does not enforce any placement of
447 where the call occurs in the overriding method (such as in the case of
448 ``-dealloc`` where the call must appear at the end). It checks only that it
451 Note that on both OS X and iOS that the Foundation framework provides a
452 convenience macro ``NS_REQUIRES_SUPER`` that provides syntactic sugar for this
457 - (void)foo NS_REQUIRES_SUPER;
459 This macro is conditionally defined depending on the compiler's support for
460 this attribute. If the compiler does not support the attribute the macro
463 Operationally, when a method has this annotation the compiler will warn if the
464 implementation of an override in a subclass does not call super. For example:
468 warning: method possibly missing a [super AnnotMeth] call
469 - (void) AnnotMeth{};
474 def ObjCRuntimeNameDocs : Documentation {
475 let Category = DocCatFunction;
477 By default, the Objective-C interface or protocol identifier is used
478 in the metadata name for that object. The `objc_runtime_name`
479 attribute allows annotated interfaces or protocols to use the
480 specified string argument in the object's metadata name instead of the
483 **Usage**: ``__attribute__((objc_runtime_name("MyLocalName")))``. This attribute
484 can only be placed before an @protocol or @interface declaration:
488 __attribute__((objc_runtime_name("MyLocalName")))
495 def AvailabilityDocs : Documentation {
496 let Category = DocCatFunction;
498 The ``availability`` attribute can be placed on declarations to describe the
499 lifecycle of that declaration relative to operating system versions. Consider
500 the function declaration for a hypothetical function ``f``:
504 void f(void) __attribute__((availability(macosx,introduced=10.4,deprecated=10.6,obsoleted=10.7)));
506 The availability attribute states that ``f`` was introduced in Mac OS X 10.4,
507 deprecated in Mac OS X 10.6, and obsoleted in Mac OS X 10.7. This information
508 is used by Clang to determine when it is safe to use ``f``: for example, if
509 Clang is instructed to compile code for Mac OS X 10.5, a call to ``f()``
510 succeeds. If Clang is instructed to compile code for Mac OS X 10.6, the call
511 succeeds but Clang emits a warning specifying that the function is deprecated.
512 Finally, if Clang is instructed to compile code for Mac OS X 10.7, the call
513 fails because ``f()`` is no longer available.
515 The availability attribute is a comma-separated list starting with the
516 platform name and then including clauses specifying important milestones in the
517 declaration's lifetime (in any order) along with additional information. Those
520 introduced=\ *version*
521 The first version in which this declaration was introduced.
523 deprecated=\ *version*
524 The first version in which this declaration was deprecated, meaning that
525 users should migrate away from this API.
527 obsoleted=\ *version*
528 The first version in which this declaration was obsoleted, meaning that it
529 was removed completely and can no longer be used.
532 This declaration is never available on this platform.
534 message=\ *string-literal*
535 Additional message text that Clang will provide when emitting a warning or
536 error about use of a deprecated or obsoleted declaration. Useful to direct
537 users to replacement APIs.
539 Multiple availability attributes can be placed on a declaration, which may
540 correspond to different platforms. Only the availability attribute with the
541 platform corresponding to the target platform will be used; any others will be
542 ignored. If no availability attribute specifies availability for the current
543 target platform, the availability attributes are ignored. Supported platforms
547 Apple's iOS operating system. The minimum deployment target is specified by
548 the ``-mios-version-min=*version*`` or ``-miphoneos-version-min=*version*``
549 command-line arguments.
552 Apple's Mac OS X operating system. The minimum deployment target is
553 specified by the ``-mmacosx-version-min=*version*`` command-line argument.
555 A declaration can be used even when deploying back to a platform version prior
556 to when the declaration was introduced. When this happens, the declaration is
558 <https://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html>`_,
559 as if the ``weak_import`` attribute were added to the declaration. A
560 weakly-linked declaration may or may not be present a run-time, and a program
561 can determine whether the declaration is present by checking whether the
562 address of that declaration is non-NULL.
564 If there are multiple declarations of the same entity, the availability
565 attributes must either match on a per-platform basis or later
566 declarations must not have availability attributes for that
567 platform. For example:
571 void g(void) __attribute__((availability(macosx,introduced=10.4)));
572 void g(void) __attribute__((availability(macosx,introduced=10.4))); // okay, matches
573 void g(void) __attribute__((availability(ios,introduced=4.0))); // okay, adds a new platform
574 void g(void); // okay, inherits both macosx and ios availability from above.
575 void g(void) __attribute__((availability(macosx,introduced=10.5))); // error: mismatch
577 When one method overrides another, the overriding method can be more widely available than the overridden method, e.g.,:
582 - (id)method __attribute__((availability(macosx,introduced=10.4)));
583 - (id)method2 __attribute__((availability(macosx,introduced=10.4)));
587 - (id)method __attribute__((availability(macosx,introduced=10.3))); // okay: method moved into base class later
588 - (id)method __attribute__((availability(macosx,introduced=10.5))); // error: this method was available via the base class in 10.4
593 def FallthroughDocs : Documentation {
594 let Category = DocCatStmt;
596 The ``clang::fallthrough`` attribute is used along with the
597 ``-Wimplicit-fallthrough`` argument to annotate intentional fall-through
598 between switch labels. It can only be applied to a null statement placed at a
599 point of execution between any statement and the next switch label. It is
600 common to mark these places with a specific comment, but this attribute is
601 meant to replace comments with a more strict annotation, which can be checked
602 by the compiler. This attribute doesn't change semantics of the code and can
603 be used wherever an intended fall-through occurs. It is designed to mimic
604 control-flow statements like ``break;``, so it can be placed in most places
605 where ``break;`` can, but only if there are no statements on the execution path
606 between it and the next switch label.
612 // compile with -Wimplicit-fallthrough
615 case 33: // no warning: no statements between case labels
617 case 44: // warning: unannotated fall-through
619 [[clang::fallthrough]];
620 case 55: // no warning
627 [[clang::fallthrough]];
629 case 66: // no warning
631 [[clang::fallthrough]]; // warning: fallthrough annotation does not
632 // directly precede case label
634 case 77: // warning: unannotated fall-through
640 def ARMInterruptDocs : Documentation {
641 let Category = DocCatFunction;
643 Clang supports the GNU style ``__attribute__((interrupt("TYPE")))`` attribute on
644 ARM targets. This attribute may be attached to a function definition and
645 instructs the backend to generate appropriate function entry/exit code so that
646 it can be used directly as an interrupt service routine.
648 The parameter passed to the interrupt attribute is optional, but if
649 provided it must be a string literal with one of the following values: "IRQ",
650 "FIQ", "SWI", "ABORT", "UNDEF".
652 The semantics are as follows:
654 - If the function is AAPCS, Clang instructs the backend to realign the stack to
655 8 bytes on entry. This is a general requirement of the AAPCS at public
656 interfaces, but may not hold when an exception is taken. Doing this allows
657 other AAPCS functions to be called.
658 - If the CPU is M-class this is all that needs to be done since the architecture
659 itself is designed in such a way that functions obeying the normal AAPCS ABI
660 constraints are valid exception handlers.
661 - If the CPU is not M-class, the prologue and epilogue are modified to save all
662 non-banked registers that are used, so that upon return the user-mode state
663 will not be corrupted. Note that to avoid unnecessary overhead, only
664 general-purpose (integer) registers are saved in this way. If VFP operations
665 are needed, that state must be saved manually.
667 Specifically, interrupt kinds other than "FIQ" will save all core registers
668 except "lr" and "sp". "FIQ" interrupts will save r0-r7.
669 - If the CPU is not M-class, the return instruction is changed to one of the
670 canonical sequences permitted by the architecture for exception return. Where
671 possible the function itself will make the necessary "lr" adjustments so that
672 the "preferred return address" is selected.
674 Unfortunately the compiler is unable to make this guarantee for an "UNDEF"
675 handler, where the offset from "lr" to the preferred return address depends on
676 the execution state of the code which generated the exception. In this case
677 a sequence equivalent to "movs pc, lr" will be used.
681 def TargetDocs : Documentation {
682 let Category = DocCatFunction;
684 Clang supports the GNU style ``__attribute__((target("OPTIONS")))`` attribute.
685 This attribute may be attached to a function definition and instructs
686 the backend to use different code generation options than were passed on the
689 The current set of options correspond to the existing "subtarget features" for
690 the target with or without a "-mno-" in front corresponding to the absence
691 of the feature, as well as ``arch="CPU"`` which will change the default "CPU"
694 Example "subtarget features" from the x86 backend include: "mmx", "sse", "sse4.2",
695 "avx", "xop" and largely correspond to the machine specific options handled by
700 def DocCatAMDGPURegisterAttributes :
701 DocumentationCategory<"AMD GPU Register Attributes"> {
703 Clang supports attributes for controlling register usage on AMD GPU
704 targets. These attributes may be attached to a kernel function
705 definition and is an optimization hint to the backend for the maximum
706 number of registers to use. This is useful in cases where register
707 limited occupancy is known to be an important factor for the
708 performance for the kernel.
710 The semantics are as follows:
712 - The backend will attempt to limit the number of used registers to
713 the specified value, but the exact number used is not
714 guaranteed. The number used may be rounded up to satisfy the
715 allocation requirements or ABI constraints of the subtarget. For
716 example, on Southern Islands VGPRs may only be allocated in
717 increments of 4, so requesting a limit of 39 VGPRs will really
718 attempt to use up to 40. Requesting more registers than the
719 subtarget supports will truncate to the maximum allowed. The backend
720 may also use fewer registers than requested whenever possible.
722 - 0 implies the default no limit on register usage.
724 - Ignored on older VLIW subtargets which did not have separate scalar
725 and vector registers, R600 through Northern Islands.
731 def AMDGPUNumVGPRDocs : Documentation {
732 let Category = DocCatAMDGPURegisterAttributes;
735 ``__attribute__((amdgpu_num_vgpr(<num_registers>)))`` attribute on AMD
736 Southern Islands GPUs and later for controlling the number of vector
737 registers. A typical value would be between 4 and 256 in increments
742 def AMDGPUNumSGPRDocs : Documentation {
743 let Category = DocCatAMDGPURegisterAttributes;
747 ``__attribute__((amdgpu_num_sgpr(<num_registers>)))`` attribute on AMD
748 Southern Islands GPUs and later for controlling the number of scalar
749 registers. A typical value would be between 8 and 104 in increments of
752 Due to common instruction constraints, an additional 2-4 SGPRs are
753 typically required for internal use depending on features used. This
754 value is a hint for the total number of SGPRs to use, and not the
755 number of user SGPRs, so no special consideration needs to be given
760 def DocCatCallingConvs : DocumentationCategory<"Calling Conventions"> {
762 Clang supports several different calling conventions, depending on the target
763 platform and architecture. The calling convention used for a function determines
764 how parameters are passed, how results are returned to the caller, and other
765 low-level details of calling a function.
769 def PcsDocs : Documentation {
770 let Category = DocCatCallingConvs;
772 On ARM targets, this attribute can be used to select calling conventions
773 similar to ``stdcall`` on x86. Valid parameter values are "aapcs" and
778 def RegparmDocs : Documentation {
779 let Category = DocCatCallingConvs;
781 On 32-bit x86 targets, the regparm attribute causes the compiler to pass
782 the first three integer parameters in EAX, EDX, and ECX instead of on the
783 stack. This attribute has no effect on variadic functions, and all parameters
784 are passed via the stack as normal.
788 def SysVABIDocs : Documentation {
789 let Category = DocCatCallingConvs;
791 On Windows x86_64 targets, this attribute changes the calling convention of a
792 function to match the default convention used on Sys V targets such as Linux,
793 Mac, and BSD. This attribute has no effect on other targets.
797 def MSABIDocs : Documentation {
798 let Category = DocCatCallingConvs;
800 On non-Windows x86_64 targets, this attribute changes the calling convention of
801 a function to match the default convention used on Windows x86_64. This
802 attribute has no effect on Windows targets or non-x86_64 targets.
806 def StdCallDocs : Documentation {
807 let Category = DocCatCallingConvs;
809 On 32-bit x86 targets, this attribute changes the calling convention of a
810 function to clear parameters off of the stack on return. This convention does
811 not support variadic calls or unprototyped functions in C, and has no effect on
812 x86_64 targets. This calling convention is used widely by the Windows API and
813 COM applications. See the documentation for `__stdcall`_ on MSDN.
815 .. _`__stdcall`: http://msdn.microsoft.com/en-us/library/zxk0tw93.aspx
819 def FastCallDocs : Documentation {
820 let Category = DocCatCallingConvs;
822 On 32-bit x86 targets, this attribute changes the calling convention of a
823 function to use ECX and EDX as register parameters and clear parameters off of
824 the stack on return. This convention does not support variadic calls or
825 unprototyped functions in C, and has no effect on x86_64 targets. This calling
826 convention is supported primarily for compatibility with existing code. Users
827 seeking register parameters should use the ``regparm`` attribute, which does
828 not require callee-cleanup. See the documentation for `__fastcall`_ on MSDN.
830 .. _`__fastcall`: http://msdn.microsoft.com/en-us/library/6xa169sk.aspx
834 def ThisCallDocs : Documentation {
835 let Category = DocCatCallingConvs;
837 On 32-bit x86 targets, this attribute changes the calling convention of a
838 function to use ECX for the first parameter (typically the implicit ``this``
839 parameter of C++ methods) and clear parameters off of the stack on return. This
840 convention does not support variadic calls or unprototyped functions in C, and
841 has no effect on x86_64 targets. See the documentation for `__thiscall`_ on
844 .. _`__thiscall`: http://msdn.microsoft.com/en-us/library/ek8tkfbw.aspx
848 def VectorCallDocs : Documentation {
849 let Category = DocCatCallingConvs;
851 On 32-bit x86 *and* x86_64 targets, this attribute changes the calling
852 convention of a function to pass vector parameters in SSE registers.
854 On 32-bit x86 targets, this calling convention is similar to ``__fastcall``.
855 The first two integer parameters are passed in ECX and EDX. Subsequent integer
856 parameters are passed in memory, and callee clears the stack. On x86_64
857 targets, the callee does *not* clear the stack, and integer parameters are
858 passed in RCX, RDX, R8, and R9 as is done for the default Windows x64 calling
861 On both 32-bit x86 and x86_64 targets, vector and floating point arguments are
862 passed in XMM0-XMM5. Homogenous vector aggregates of up to four elements are
863 passed in sequential SSE registers if enough are available. If AVX is enabled,
864 256 bit vectors are passed in YMM0-YMM5. Any vector or aggregate type that
865 cannot be passed in registers for any reason is passed by reference, which
866 allows the caller to align the parameter memory.
868 See the documentation for `__vectorcall`_ on MSDN for more details.
870 .. _`__vectorcall`: http://msdn.microsoft.com/en-us/library/dn375768.aspx
874 def DocCatConsumed : DocumentationCategory<"Consumed Annotation Checking"> {
876 Clang supports additional attributes for checking basic resource management
877 properties, specifically for unique objects that have a single owning reference.
878 The following attributes are currently supported, although **the implementation
879 for these annotations is currently in development and are subject to change.**
883 def SetTypestateDocs : Documentation {
884 let Category = DocCatConsumed;
886 Annotate methods that transition an object into a new state with
887 ``__attribute__((set_typestate(new_state)))``. The new state must be
888 unconsumed, consumed, or unknown.
892 def CallableWhenDocs : Documentation {
893 let Category = DocCatConsumed;
895 Use ``__attribute__((callable_when(...)))`` to indicate what states a method
896 may be called in. Valid states are unconsumed, consumed, or unknown. Each
897 argument to this attribute must be a quoted string. E.g.:
899 ``__attribute__((callable_when("unconsumed", "unknown")))``
903 def TestTypestateDocs : Documentation {
904 let Category = DocCatConsumed;
906 Use ``__attribute__((test_typestate(tested_state)))`` to indicate that a method
907 returns true if the object is in the specified state..
911 def ParamTypestateDocs : Documentation {
912 let Category = DocCatConsumed;
914 This attribute specifies expectations about function parameters. Calls to an
915 function with annotated parameters will issue a warning if the corresponding
916 argument isn't in the expected state. The attribute is also used to set the
917 initial state of the parameter when analyzing the function's body.
921 def ReturnTypestateDocs : Documentation {
922 let Category = DocCatConsumed;
924 The ``return_typestate`` attribute can be applied to functions or parameters.
925 When applied to a function the attribute specifies the state of the returned
926 value. The function's body is checked to ensure that it always returns a value
927 in the specified state. On the caller side, values returned by the annotated
928 function are initialized to the given state.
930 When applied to a function parameter it modifies the state of an argument after
931 a call to the function returns. The function's body is checked to ensure that
932 the parameter is in the expected state before returning.
936 def ConsumableDocs : Documentation {
937 let Category = DocCatConsumed;
939 Each ``class`` that uses any of the typestate annotations must first be marked
940 using the ``consumable`` attribute. Failure to do so will result in a warning.
942 This attribute accepts a single parameter that must be one of the following:
943 ``unknown``, ``consumed``, or ``unconsumed``.
947 def NoSanitizeDocs : Documentation {
948 let Category = DocCatFunction;
950 Use the ``no_sanitize`` attribute on a function declaration to specify
951 that a particular instrumentation or set of instrumentations should not be
952 applied to that function. The attribute takes a list of string literals,
953 which have the same meaning as values accepted by the ``-fno-sanitize=``
954 flag. For example, ``__attribute__((no_sanitize("address", "thread")))``
955 specifies that AddressSanitizer and ThreadSanitizer should not be applied
958 See :ref:`Controlling Code Generation <controlling-code-generation>` for a
959 full list of supported sanitizer flags.
963 def NoSanitizeAddressDocs : Documentation {
964 let Category = DocCatFunction;
965 // This function has multiple distinct spellings, and so it requires a custom
966 // heading to be specified. The most common spelling is sufficient.
967 let Heading = "no_sanitize_address (no_address_safety_analysis, gnu::no_address_safety_analysis, gnu::no_sanitize_address)";
969 .. _langext-address_sanitizer:
971 Use ``__attribute__((no_sanitize_address))`` on a function declaration to
972 specify that address safety instrumentation (e.g. AddressSanitizer) should
973 not be applied to that function.
977 def NoSanitizeThreadDocs : Documentation {
978 let Category = DocCatFunction;
979 let Heading = "no_sanitize_thread";
981 .. _langext-thread_sanitizer:
983 Use ``__attribute__((no_sanitize_thread))`` on a function declaration to
984 specify that checks for data races on plain (non-atomic) memory accesses should
985 not be inserted by ThreadSanitizer. The function is still instrumented by the
986 tool to avoid false positives and provide meaningful stack traces.
990 def NoSanitizeMemoryDocs : Documentation {
991 let Category = DocCatFunction;
992 let Heading = "no_sanitize_memory";
994 .. _langext-memory_sanitizer:
996 Use ``__attribute__((no_sanitize_memory))`` on a function declaration to
997 specify that checks for uninitialized memory should not be inserted
998 (e.g. by MemorySanitizer). The function may still be instrumented by the tool
999 to avoid false positives in other places.
1003 def DocCatTypeSafety : DocumentationCategory<"Type Safety Checking"> {
1005 Clang supports additional attributes to enable checking type safety properties
1006 that can't be enforced by the C type system. Use cases include:
1008 * MPI library implementations, where these attributes enable checking that
1009 the buffer type matches the passed ``MPI_Datatype``;
1010 * for HDF5 library there is a similar use case to MPI;
1011 * checking types of variadic functions' arguments for functions like
1012 ``fcntl()`` and ``ioctl()``.
1014 You can detect support for these attributes with ``__has_attribute()``. For
1019 #if defined(__has_attribute)
1020 # if __has_attribute(argument_with_type_tag) && \
1021 __has_attribute(pointer_with_type_tag) && \
1022 __has_attribute(type_tag_for_datatype)
1023 # define ATTR_MPI_PWT(buffer_idx, type_idx) __attribute__((pointer_with_type_tag(mpi,buffer_idx,type_idx)))
1024 /* ... other macros ... */
1028 #if !defined(ATTR_MPI_PWT)
1029 # define ATTR_MPI_PWT(buffer_idx, type_idx)
1032 int MPI_Send(void *buf, int count, MPI_Datatype datatype /*, other args omitted */)
1037 def ArgumentWithTypeTagDocs : Documentation {
1038 let Category = DocCatTypeSafety;
1039 let Heading = "argument_with_type_tag";
1041 Use ``__attribute__((argument_with_type_tag(arg_kind, arg_idx,
1042 type_tag_idx)))`` on a function declaration to specify that the function
1043 accepts a type tag that determines the type of some other argument.
1044 ``arg_kind`` is an identifier that should be used when annotating all
1045 applicable type tags.
1047 This attribute is primarily useful for checking arguments of variadic functions
1048 (``pointer_with_type_tag`` can be used in most non-variadic cases).
1054 int fcntl(int fd, int cmd, ...)
1055 __attribute__(( argument_with_type_tag(fcntl,3,2) ));
1059 def PointerWithTypeTagDocs : Documentation {
1060 let Category = DocCatTypeSafety;
1061 let Heading = "pointer_with_type_tag";
1063 Use ``__attribute__((pointer_with_type_tag(ptr_kind, ptr_idx, type_tag_idx)))``
1064 on a function declaration to specify that the function accepts a type tag that
1065 determines the pointee type of some other pointer argument.
1071 int MPI_Send(void *buf, int count, MPI_Datatype datatype /*, other args omitted */)
1072 __attribute__(( pointer_with_type_tag(mpi,1,3) ));
1076 def TypeTagForDatatypeDocs : Documentation {
1077 let Category = DocCatTypeSafety;
1079 Clang supports annotating type tags of two forms.
1081 * **Type tag that is an expression containing a reference to some declared
1082 identifier.** Use ``__attribute__((type_tag_for_datatype(kind, type)))`` on a
1083 declaration with that identifier:
1087 extern struct mpi_datatype mpi_datatype_int
1088 __attribute__(( type_tag_for_datatype(mpi,int) ));
1089 #define MPI_INT ((MPI_Datatype) &mpi_datatype_int)
1091 * **Type tag that is an integral literal.** Introduce a ``static const``
1092 variable with a corresponding initializer value and attach
1093 ``__attribute__((type_tag_for_datatype(kind, type)))`` on that declaration,
1098 #define MPI_INT ((MPI_Datatype) 42)
1099 static const MPI_Datatype mpi_datatype_int
1100 __attribute__(( type_tag_for_datatype(mpi,int) )) = 42
1102 The attribute also accepts an optional third argument that determines how the
1103 expression is compared to the type tag. There are two supported flags:
1105 * ``layout_compatible`` will cause types to be compared according to
1106 layout-compatibility rules (C++11 [class.mem] p 17, 18). This is
1107 implemented to support annotating types like ``MPI_DOUBLE_INT``.
1114 struct internal_mpi_double_int { double d; int i; };
1115 extern struct mpi_datatype mpi_datatype_double_int
1116 __attribute__(( type_tag_for_datatype(mpi, struct internal_mpi_double_int, layout_compatible) ));
1118 #define MPI_DOUBLE_INT ((MPI_Datatype) &mpi_datatype_double_int)
1121 struct my_pair { double a; int b; };
1122 struct my_pair *buffer;
1123 MPI_Send(buffer, 1, MPI_DOUBLE_INT /*, ... */); // no warning
1125 struct my_int_pair { int a; int b; }
1126 struct my_int_pair *buffer2;
1127 MPI_Send(buffer2, 1, MPI_DOUBLE_INT /*, ... */); // warning: actual buffer element
1128 // type 'struct my_int_pair'
1129 // doesn't match specified MPI_Datatype
1131 * ``must_be_null`` specifies that the expression should be a null pointer
1132 constant, for example:
1137 extern struct mpi_datatype mpi_datatype_null
1138 __attribute__(( type_tag_for_datatype(mpi, void, must_be_null) ));
1140 #define MPI_DATATYPE_NULL ((MPI_Datatype) &mpi_datatype_null)
1143 MPI_Send(buffer, 1, MPI_DATATYPE_NULL /*, ... */); // warning: MPI_DATATYPE_NULL
1144 // was specified but buffer
1145 // is not a null pointer
1149 def FlattenDocs : Documentation {
1150 let Category = DocCatFunction;
1152 The ``flatten`` attribute causes calls within the attributed function to
1153 be inlined unless it is impossible to do so, for example if the body of the
1154 callee is unavailable or if the callee has the ``noinline`` attribute.
1158 def FormatDocs : Documentation {
1159 let Category = DocCatFunction;
1162 Clang supports the ``format`` attribute, which indicates that the function
1163 accepts a ``printf`` or ``scanf``-like format string and corresponding
1164 arguments or a ``va_list`` that contains these arguments.
1166 Please see `GCC documentation about format attribute
1167 <http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_ to find details
1168 about attribute syntax.
1170 Clang implements two kinds of checks with this attribute.
1172 #. Clang checks that the function with the ``format`` attribute is called with
1173 a format string that uses format specifiers that are allowed, and that
1174 arguments match the format string. This is the ``-Wformat`` warning, it is
1177 #. Clang checks that the format string argument is a literal string. This is
1178 the ``-Wformat-nonliteral`` warning, it is off by default.
1180 Clang implements this mostly the same way as GCC, but there is a difference
1181 for functions that accept a ``va_list`` argument (for example, ``vprintf``).
1182 GCC does not emit ``-Wformat-nonliteral`` warning for calls to such
1183 functions. Clang does not warn if the format string comes from a function
1184 parameter, where the function is annotated with a compatible attribute,
1185 otherwise it warns. For example:
1189 __attribute__((__format__ (__scanf__, 1, 3)))
1190 void foo(const char* s, char *buf, ...) {
1194 vprintf(s, ap); // warning: format string is not a string literal
1197 In this case we warn because ``s`` contains a format string for a
1198 ``scanf``-like function, but it is passed to a ``printf``-like function.
1200 If the attribute is removed, clang still warns, because the format string is
1201 not a string literal.
1207 __attribute__((__format__ (__printf__, 1, 3)))
1208 void foo(const char* s, char *buf, ...) {
1212 vprintf(s, ap); // warning
1215 In this case Clang does not warn because the format string ``s`` and
1216 the corresponding arguments are annotated. If the arguments are
1217 incorrect, the caller of ``foo`` will receive a warning.
1221 def AlignValueDocs : Documentation {
1222 let Category = DocCatType;
1224 The align_value attribute can be added to the typedef of a pointer type or the
1225 declaration of a variable of pointer or reference type. It specifies that the
1226 pointer will point to, or the reference will bind to, only objects with at
1227 least the provided alignment. This alignment value must be some positive power
1232 typedef double * aligned_double_ptr __attribute__((align_value(64)));
1233 void foo(double & x __attribute__((align_value(128)),
1234 aligned_double_ptr y) { ... }
1236 If the pointer value does not have the specified alignment at runtime, the
1237 behavior of the program is undefined.
1241 def FlagEnumDocs : Documentation {
1242 let Category = DocCatType;
1244 This attribute can be added to an enumerator to signal to the compiler that it
1245 is intended to be used as a flag type. This will cause the compiler to assume
1246 that the range of the type includes all of the values that you can get by
1247 manipulating bits of the enumerator when issuing warnings.
1251 def MSInheritanceDocs : Documentation {
1252 let Category = DocCatType;
1253 let Heading = "__single_inhertiance, __multiple_inheritance, __virtual_inheritance";
1255 This collection of keywords is enabled under ``-fms-extensions`` and controls
1256 the pointer-to-member representation used on ``*-*-win32`` targets.
1258 The ``*-*-win32`` targets utilize a pointer-to-member representation which
1259 varies in size and alignment depending on the definition of the underlying
1262 However, this is problematic when a forward declaration is only available and
1263 no definition has been made yet. In such cases, Clang is forced to utilize the
1264 most general representation that is available to it.
1266 These keywords make it possible to use a pointer-to-member representation other
1267 than the most general one regardless of whether or not the definition will ever
1268 be present in the current translation unit.
1270 This family of keywords belong between the ``class-key`` and ``class-name``:
1274 struct __single_inheritance S;
1278 This keyword can be applied to class templates but only has an effect when used
1279 on full specializations:
1283 template <typename T, typename U> struct __single_inheritance A; // warning: inheritance model ignored on primary template
1284 template <typename T> struct __multiple_inheritance A<T, T>; // warning: inheritance model ignored on partial specialization
1285 template <> struct __single_inheritance A<int, float>;
1287 Note that choosing an inheritance model less general than strictly necessary is
1292 struct __multiple_inheritance S; // error: inheritance model does not match definition
1298 def MSNoVTableDocs : Documentation {
1299 let Category = DocCatType;
1301 This attribute can be added to a class declaration or definition to signal to
1302 the compiler that constructors and destructors will not reference the virtual
1307 def OptnoneDocs : Documentation {
1308 let Category = DocCatFunction;
1310 The ``optnone`` attribute suppresses essentially all optimizations
1311 on a function or method, regardless of the optimization level applied to
1312 the compilation unit as a whole. This is particularly useful when you
1313 need to debug a particular function, but it is infeasible to build the
1314 entire application without optimization. Avoiding optimization on the
1315 specified function can improve the quality of the debugging information
1318 This attribute is incompatible with the ``always_inline`` and ``minsize``
1323 def LoopHintDocs : Documentation {
1324 let Category = DocCatStmt;
1325 let Heading = "#pragma clang loop";
1327 The ``#pragma clang loop`` directive allows loop optimization hints to be
1328 specified for the subsequent loop. The directive allows vectorization,
1329 interleaving, and unrolling to be enabled or disabled. Vector width as well
1330 as interleave and unrolling count can be manually specified. See
1331 `language extensions
1332 <http://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations>`_
1337 def UnrollHintDocs : Documentation {
1338 let Category = DocCatStmt;
1339 let Heading = "#pragma unroll, #pragma nounroll";
1341 Loop unrolling optimization hints can be specified with ``#pragma unroll`` and
1342 ``#pragma nounroll``. The pragma is placed immediately before a for, while,
1343 do-while, or c++11 range-based for loop.
1345 Specifying ``#pragma unroll`` without a parameter directs the loop unroller to
1346 attempt to fully unroll the loop if the trip count is known at compile time:
1355 Specifying the optional parameter, ``#pragma unroll _value_``, directs the
1356 unroller to unroll the loop ``_value_`` times. The parameter may optionally be
1357 enclosed in parentheses:
1371 Specifying ``#pragma nounroll`` indicates that the loop should not be unrolled:
1380 ``#pragma unroll`` and ``#pragma unroll _value_`` have identical semantics to
1381 ``#pragma clang loop unroll(full)`` and
1382 ``#pragma clang loop unroll_count(_value_)`` respectively. ``#pragma nounroll``
1383 is equivalent to ``#pragma clang loop unroll(disable)``. See
1384 `language extensions
1385 <http://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations>`_
1386 for further details including limitations of the unroll hints.
1390 def DocOpenCLAddressSpaces : DocumentationCategory<"OpenCL Address Spaces"> {
1392 The address space qualifier may be used to specify the region of memory that is
1393 used to allocate the object. OpenCL supports the following address spaces:
1394 __generic(generic), __global(global), __local(local), __private(private),
1395 __constant(constant).
1399 __constant int c = ...;
1401 __generic int* foo(global int* g) {
1408 More details can be found in the OpenCL C language Spec v2.0, Section 6.5.
1412 def OpenCLAddressSpaceGenericDocs : Documentation {
1413 let Category = DocOpenCLAddressSpaces;
1414 let Heading = "__generic(generic)";
1416 The generic address space attribute is only available with OpenCL v2.0 and later.
1417 It can be used with pointer types. Variables in global and local scope and
1418 function parameters in non-kernel functions can have the generic address space
1419 type attribute. It is intended to be a placeholder for any other address space
1420 except for '__constant' in OpenCL code which can be used with multiple address
1425 def OpenCLAddressSpaceConstantDocs : Documentation {
1426 let Category = DocOpenCLAddressSpaces;
1427 let Heading = "__constant(constant)";
1429 The constant address space attribute signals that an object is located in
1430 a constant (non-modifiable) memory region. It is available to all work items.
1431 Any type can be annotated with the constant address space attribute. Objects
1432 with the constant address space qualifier can be declared in any scope and must
1433 have an initializer.
1437 def OpenCLAddressSpaceGlobalDocs : Documentation {
1438 let Category = DocOpenCLAddressSpaces;
1439 let Heading = "__global(global)";
1441 The global address space attribute specifies that an object is allocated in
1442 global memory, which is accessible by all work items. The content stored in this
1443 memory area persists between kernel executions. Pointer types to the global
1444 address space are allowed as function parameters or local variables. Starting
1445 with OpenCL v2.0, the global address space can be used with global (program
1446 scope) variables and static local variable as well.
1450 def OpenCLAddressSpaceLocalDocs : Documentation {
1451 let Category = DocOpenCLAddressSpaces;
1452 let Heading = "__local(local)";
1454 The local address space specifies that an object is allocated in the local (work
1455 group) memory area, which is accessible to all work items in the same work
1456 group. The content stored in this memory region is not accessible after
1457 the kernel execution ends. In a kernel function scope, any variable can be in
1458 the local address space. In other scopes, only pointer types to the local address
1459 space are allowed. Local address space variables cannot have an initializer.
1463 def OpenCLAddressSpacePrivateDocs : Documentation {
1464 let Category = DocOpenCLAddressSpaces;
1465 let Heading = "__private(private)";
1467 The private address space specifies that an object is allocated in the private
1468 (work item) memory. Other work items cannot access the same memory area and its
1469 content is destroyed after work item execution ends. Local variables can be
1470 declared in the private address space. Function arguments are always in the
1471 private address space. Kernel function arguments of a pointer or an array type
1472 cannot point to the private address space.
1476 def NullabilityDocs : DocumentationCategory<"Nullability Attributes"> {
1478 Whether a particular pointer may be "null" is an important concern when working with pointers in the C family of languages. The various nullability attributes indicate whether a particular pointer can be null or not, which makes APIs more expressive and can help static analysis tools identify bugs involving null pointers. Clang supports several kinds of nullability attributes: the ``nonnull`` and ``returns_nonnull`` attributes indicate which function or method parameters and result types can never be null, while nullability type qualifiers indicate which pointer types can be null (``_Nullable``) or cannot be null (``_Nonnull``).
1480 The nullability (type) qualifiers express whether a value of a given pointer type can be null (the ``_Nullable`` qualifier), doesn't have a defined meaning for null (the ``_Nonnull`` qualifier), or for which the purpose of null is unclear (the ``_Null_unspecified`` qualifier). Because nullability qualifiers are expressed within the type system, they are more general than the ``nonnull`` and ``returns_nonnull`` attributes, allowing one to express (for example) a nullable pointer to an array of nonnull pointers. Nullability qualifiers are written to the right of the pointer to which they apply. For example:
1484 // No meaningful result when 'ptr' is null (here, it happens to be undefined behavior).
1485 int fetch(int * _Nonnull ptr) { return *ptr; }
1487 // 'ptr' may be null.
1488 int fetch_or_zero(int * _Nullable ptr) {
1489 return ptr ? *ptr : 0;
1492 // A nullable pointer to non-null pointers to const characters.
1493 const char *join_strings(const char * _Nonnull * _Nullable strings, unsigned n);
1495 In Objective-C, there is an alternate spelling for the nullability qualifiers that can be used in Objective-C methods and properties using context-sensitive, non-underscored keywords. For example:
1497 .. code-block:: objective-c
1499 @interface NSView : NSResponder
1500 - (nullable NSView *)ancestorSharedWithView:(nonnull NSView *)aView;
1501 @property (assign, nullable) NSView *superview;
1502 @property (readonly, nonnull) NSArray *subviews;
1507 def TypeNonNullDocs : Documentation {
1508 let Category = NullabilityDocs;
1509 let Heading = "_Nonnull";
1511 The ``_Nonnull`` nullability qualifier indicates that null is not a meaningful value for a value of the ``_Nonnull`` pointer type. For example, given a declaration such as:
1515 int fetch(int * _Nonnull ptr);
1517 a caller of ``fetch`` should not provide a null value, and the compiler will produce a warning if it sees a literal null value passed to ``fetch``. Note that, unlike the declaration attribute ``nonnull``, the presence of ``_Nonnull`` does not imply that passing null is undefined behavior: ``fetch`` is free to consider null undefined behavior or (perhaps for backward-compatibility reasons) defensively handle null.
1521 def TypeNullableDocs : Documentation {
1522 let Category = NullabilityDocs;
1523 let Heading = "_Nullable";
1525 The ``_Nullable`` nullability qualifier indicates that a value of the ``_Nullable`` pointer type can be null. For example, given:
1529 int fetch_or_zero(int * _Nullable ptr);
1531 a caller of ``fetch_or_zero`` can provide null.
1535 def TypeNullUnspecifiedDocs : Documentation {
1536 let Category = NullabilityDocs;
1537 let Heading = "_Null_unspecified";
1539 The ``_Null_unspecified`` nullability qualifier indicates that neither the ``_Nonnull`` nor ``_Nullable`` qualifiers make sense for a particular pointer type. It is used primarily to indicate that the role of null with specific pointers in a nullability-annotated header is unclear, e.g., due to overly-complex implementations or historical factors with a long-lived API.
1543 def NonNullDocs : Documentation {
1544 let Category = NullabilityDocs;
1545 let Heading = "nonnull";
1547 The ``nonnull`` attribute indicates that some function parameters must not be null, and can be used in several different ways. It's original usage (`from GCC <https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes>`_) is as a function (or Objective-C method) attribute that specifies which parameters of the function are nonnull in a comma-separated list. For example:
1551 extern void * my_memcpy (void *dest, const void *src, size_t len)
1552 __attribute__((nonnull (1, 2)));
1554 Here, the ``nonnull`` attribute indicates that parameters 1 and 2
1555 cannot have a null value. Omitting the parenthesized list of parameter indices means that all parameters of pointer type cannot be null:
1559 extern void * my_memcpy (void *dest, const void *src, size_t len)
1560 __attribute__((nonnull));
1562 Clang also allows the ``nonnull`` attribute to be placed directly on a function (or Objective-C method) parameter, eliminating the need to specify the parameter index ahead of type. For example:
1566 extern void * my_memcpy (void *dest __attribute__((nonnull)),
1567 const void *src __attribute__((nonnull)), size_t len);
1569 Note that the ``nonnull`` attribute indicates that passing null to a non-null parameter is undefined behavior, which the optimizer may take advantage of to, e.g., remove null checks. The ``_Nonnull`` type qualifier indicates that a pointer cannot be null in a more general manner (because it is part of the type system) and does not imply undefined behavior, making it more widely applicable.
1573 def ReturnsNonNullDocs : Documentation {
1574 let Category = NullabilityDocs;
1575 let Heading = "returns_nonnull";
1577 The ``returns_nonnull`` attribute indicates that a particular function (or Objective-C method) always returns a non-null pointer. For example, a particular system ``malloc`` might be defined to terminate a process when memory is not available rather than returning a null pointer:
1581 extern void * malloc (size_t size) __attribute__((returns_nonnull));
1583 The ``returns_nonnull`` attribute implies that returning a null pointer is undefined behavior, which the optimizer may take advantage of. The ``_Nonnull`` type qualifier indicates that a pointer cannot be null in a more general manner (because it is part of the type system) and does not imply undefined behavior, making it more widely applicable