# C Support in Clang

Clang implements the following published and upcoming ISO C standards:

Language Standard | Flag | Available in Clang?
--- | --- | ---
C2y | `-std=c2y` | Partial
C23 | `-std=c23` | Partial
C17 | `-std=c17` | Partial
C11 | `-std=c11` | Partial
C99 | `-std=c99` | Clang 17
C89 | `-std=c89` | Yes

The implementation status for C11 and C23 are currently under investigation. Any proposal whose status in Clang is currently unknown will be marked in magenta.

The Clang community is continually striving to improve C standards compliance between releases by submitting and tracking C Defect Reports and implementing resolutions as they become available.

The LLVM bug tracker uses the "c", "c99", "c11", "c17", "c23", and "c2y" labels to track known bugs with Clang's language conformance.

## C2y implementation status

Clang has support for some of the features of the C standard following C23, informally referred to as C2y.

You can use Clang in C2y mode with the `-std=c2y` option (available in Clang 19 and later).

List of features and minimum Clang version with support

Language Feature | C2y Proposal | Available in Clang?
--- | --- | ---
Sequential hexdigits | N3192 | Yes
Generic selection expression with a type operand | N3260 | Clang 17
Round-trip rounding | N3232 | Yes
Accessing byte arrays | N3254 | Yes
Slay some earthly demons I | N3244 | Partial Clang does not document the implementation-defined behavior for decay of an array with the register storage class specifier. Clang does not diagnose an `extern inline` function with no definition in the TU. Clang accepts and rejects redeclarations with/without an alignment specifier, depending on the order of the declarations.
Support ++ and -- on complex values | N3259 | Yes
Usability of a byte-wise copy of va_list | N3262 | Yes
alignof of an incomplete array type | N3273 | Clang 3.5
Remove imaginary types | N3274 | Yes
Floating-point exceptions for macro replacements | N3286 | Yes
Obsolete implicitly octal literals and add delimited escape sequences | N3353 | Clang 21
'if' declarations, v2 | N3356 | No
Allowing stricter alignment for atomic types | N3312 | Yes
Slay Some Earthly Demons III | N3341 | Yes
Slay Some Earthly Demons IV | N3342 | Yes
Slay Some Earthly Demons VI | N3344 | Clang 20
Slay Some Earthly Demons VII | N3345 | Yes
Slay Some Earthly Demons VIII | N3346 | Yes
Introduce complex literals v. 2 | N3298 | Yes
Allow zero length operations on null pointers | N3322 | Yes
How do you add one to something? | N3323 | Yes
Give consistent wording for SNAN initialization v3 | N3364 | Yes
Case range expressions v3.1 | N3370 | Yes
_Countof operator
N3369 | Clang 21
N3469 | Clang 21
Named loops, v3 | N3355 | Clang 22
Slay Some Earthly Demons IX | N3347 | Yes
stdarg.h wording... v3 | N3363 | Clang 5
Preprocessor integer expressions, v. 2 | N3505 | Yes
Initialization of anonymous structures and unions | N3451 | Yes
Slay Some Earthly Demons X | N3409 | Clang 21
Slay Some Earthly Demons XI | N3410 | No
Slay Some Earthly Demons XII | N3411 | Clang 21
Slay Some Earthly Demons XIII | N3478 | Yes
Slay Some Earthly Demons XIV | N3418 | No
Slay Some Earthly Demons XVI | N3481 | Yes
Slay Some Earthly Demons XVII | N3482 | Yes
Clarify the specification of the width macros | N3496 | Clang 20
Complex operators | N3460 | Clang 12
Matching of Multi-Dimensional Arrays in Generic Selection Expressions | N3348 | No
The __COUNTER__ predefined macro | N3457 | Clang 22
Chasing Ghosts I: constant expressions v2 | N3558 | Unknown
Earthly Demon XV: Definition of Main | N3623 | Yes
static_assert without UB | N3525 | Yes
Allow calling static inline within extern inline | N3622 | Clang 22
Generic replacement (v. 2 of quasi-literals) | N3605 | Unknown
Member access of an incomplete object | N3532 | Yes

## C23 implementation status

Clang has support for some of the features of the C standard following C17, informally referred to as C23.

You can use Clang in C23 mode with the `-std=c23` option (available in Clang 18 and later) or with the `-std=c2x` option (available in Clang 9 and later).

List of features and minimum Clang version with support

Language Feature | C23 Proposal | Available in Clang?
--- | --- | ---
Evaluation formats | N2186 | Unknown
Harmonizing static_assert with C++ | N2665 | Clang 9
nodiscard attribute | N2267 | Clang 9
maybe_unused attribute | N2270 | Clang 9
TS 18661 Integration
N2314 | Unknown
N2341 | No
N2359 | No
N2546 | No
N2640 | Yes
N2755 | No
N2931 | No
N2754 | No
Preprocessor line numbers unspecified | N2322 | Partial The line number associated with a macro invocation is not the line number of the first character of the macro name in the invocation. Additionally, Clang may not associate the line number of a pp-directive with the first `#` token. As these are recommended practices and not normative requirements, Clang's behavior is still conforming.
deprecated attribute | N2334 | Clang 9
Attributes
N2335 | Clang 9
N2554 | Clang 9
Defining new types in offsetof | N2350 | Yes
fallthrough attribute | N2408 | Clang 9
Two's complement sign representation | N2412 | Clang 14
Adding the u8 character prefix | N2418 | Clang 15
Remove support for function definitions with identifier lists | N2432 | Clang 15
Annex F.8 update for implementation extensions and rounding | N2384 | Unknown
_Bool definitions for true and false | N2393 | Subsumed by N2935
[[nodiscard("should have a reason")]] | N2448 | Clang 10
Allowing unnamed parameters in function definitions | N2480 | Clang 11
Free positioning of labels inside compound statements | N2508 | Clang 18
Querying attribute support | N2553 | Clang 9
Binary literals | N2549 | Clang 9
Allow duplicate attributes | N2557 | Clang 13
Character encoding of diagnostic text | N2563 | Yes
What we think we reserve | N2572 | Partial
Remove mixed wide string literal concatenation | N2594 | Clang 9
Update to IEC 60559:2020 | N2600 | Unknown
Compatibility of Pointers to Arrays with Qualifiers | N2607 | Partial Much of the proposal is implemented, but Clang lacks pedantic diagnostics in C17 and earlier regarding use of incompatible pointer types as an extension. Further, Clang does not properly compute the correct result type for the `?:` operator when the result type should be a qualified array type.
Unclear type relationship between a format specifier and its argument | N2562 | Clang 16
Digit separators | N2626 | Clang 13
Missing +(x) in table | N2641 | Yes
Add support for preprocessing directives elifdef and elifndef | N2645 | Clang 13
[[maybe_unused]] for labels | N2662 | Clang 16
Zeros compare equal | N2670 | Yes
Negative values | N2671 | Yes
5.2.4.2.2 cleanup | N2672 | Yes
Towards Integer Safety | N2683 | Clang 18
Adding Fundamental Type for N-bit Integers
N2763 | Clang 15
N2775 | Clang 15
N2969 | Clang 15
N3035 | Clang 15
#warning directive | N2686 | Yes
Sterile characters | N2686 | Yes
Numerically equal | N2716 | Yes
char16_t & char32_t string literals shall be UTF-16 & UTF-32 | N2728 | Yes
IEC 60559 binding | N2749 | Unknown
__has_include for C | N2799 | Yes
Annex F overflow and underflow | N2747 | Yes
Remove UB from Incomplete Types in Function Parameters | N2770 | Yes
Variably-modified types | N2778 | Yes
Types do not have types | N2781 | Yes
5.2.4.2.2 cleanup (N2672 update) | N2806 | Yes
Allow 16-bit ptrdiff_t | N2808 | Yes
Proposal to update CFP freestanding requirements | N2823 | Unknown
Types and sizes | N2838 | Yes
Clarifying integer terms | N2837 | Yes
Clarification for max exponent macros | N2843 | Subsumed by N2882
Clarification about expression transformations | N2846 | Unknown
Contradiction about INFINITY macro | N2848 | Clang 19
Require exact-width integer type interfaces | N2872 | Yes
@, $, and ‘ in the source/execution character set | N2701 | Yes
The noreturn attribute | N2764 | Clang 15
*_HAS_SUBNORM==0 implies what? | N2797 | Yes
Disambiguate the storage class of some compound literals | N2819 | Clang 21
Add annotations for unreachable control flow v2 | N2826 | Clang 17
Unicode Sequences More Than 21 Bits are a Constraint Violation r0 | N2828 | Clang 3.6
Identifier Syntax using Unicode Standard Annex 31 | N2836 | Clang 15
No function declarators without prototypes | N2841 | Clang 15
Remove default argument promotions for _FloatN types | N2844 | No
Revised Suggestions of Change for Numerically Equal/Equivalent | N2847 | Yes
5.2.4.2.2 Cleanup, Again Again (N2806 update) | N2879 | Yes
char8_t: A type for UTF-8 characters and strings | N2653 | Clang 19
Clarification for max exponent macros-update | N2882 | Unknown
Consistent, Warningless, and Intuitive Initialization with {}
N2900 | Clang 17
N3011 | Clang 17
Not-so-magic: typeof
N2927 | Clang 16
N2930 | Clang 16
Revise spelling of keywords v7 | N2934 | Clang 17
Make false and true first-class language features v8 | N2935 | Clang 15
Properly define blocks as part of the grammar v3 | N2937 | Yes
Annex X (replacing Annex H) for IEC 60559 interchange | N2601 | No
Indeterminate Values and Trap Representations | N2861 | Yes
Remove ATOMIC_VAR_INIT v2 | N2886 | Clang 17
Require exact-width integer type interfaces v2 | N2888 | Yes
Wording Change for Variably-Modified Types | N2992 | Yes
Identifier syntax fixes | N2939 | Clang 15
Remove trigraphs??! | N2940 | Clang 18
Improved normal enumerations | N3029 | Clang 20
Relax requirements for va_start | N2975 | Clang 16
Enhanced enumerations | N3030 | Clang 20
Freestanding C and IEC 60559 conformance scope reduction | N2951 | Unknown
Unsequenced functions | N2956 | No
Comma ommission and deletion (__VA_OPT__) | N3033 | Clang 12
Underspecified object definitions | N3006 | Clang 19
Type inference for object declarations | N3007 | Clang 18
constexpr for object definitions | N3018 | Clang 19
Introduce storage class specifiers for compound literals | N3038 | No
Identifier primary expressions | N3034 | Yes
Introduce the nullptr constant | N3042 | Clang 17
Memory layout of unions | N2929 | Yes
Improved tag compatibility | N3037 | Clang 21
#embed | N3017 | Clang 19

## C17 implementation status

There are no major changes in this edition, only technical corrections and clarifications that are tracked by Defect Report.

You can use Clang in C17 mode with the `-std=c17` or `-std=c18` options (available in Clang 6 and later).

## C11 implementation status

Clang implements a significant portion of the ISO 9899:2011 (C11) standard, but the status of individual proposals is still under investigation.

You can use Clang in C11 mode with the `-std=c11` option (use `-std=c1x` in Clang 3.0 and earlier).

List of features and minimum Clang version with support

Language Feature | C11 Proposal | Available in Clang?
--- | --- | ---
A finer-grained specification for sequencing | N1252 | Unknown
Clarification of expressions | N1282 | Yes
Extending the lifetime of temporary objects (factored approach) | N1285 | Clang 21
Requiring signed char to have no padding bits | N1310 | Yes
Initializing static or external variables | N1311 | Yes
Conversion between pointers and floating types | N1316 | Yes
Adding TR 19769 to the C Standard Library | N1326 | Clang 3.3
Static assertions | N1330 | Yes
Parallel memory sequencing model proposal | N1349 | Unknown
_Bool bit-fields | N1356 | Yes
Technical corrigendum for C1X | N1359 | Yes
Benign typedef redefinition | N1360 | Clang 3.1
Thread-local storage | N1364 | Clang 3.3
Constant expressions | N1365 | Clang 16
Contractions and expression evaluation methods | N1367 | Unknown
Floating-point to int/_Bool conversions | N1391 | Yes
Wide function returns (alternate proposal) | N1396 | Yes* Clang conforms to this paper on all targets except 32-bit x86 without SSE2. However, Clang does not claim conformance to Annex F on any target and does not intend to ever conform to Annex F on that specific target, so no changes are needed to conform to this paper.
Alignment
N1397 | Clang 3.2
N1447 | Clang 3.2
Anonymous member-structures and unions (modulo "name lookup") | N1406 | Yes
Completeness of types | N1439 | Yes
Generic macro facility | N1441 | Yes
Dependency ordering for C memory model | N1444 | Unknown
Subsetting the standard | N1460 | Yes
Assumed types in F.9.2 | N1468 | Unknown
Supporting the 'noreturn' property in C1x | N1478 | Clang 3.3
Updates to C++ memory model based on formalization | N1480 | Unknown
Explicit initializers for atomics | N1482 | Clang 4
Atomics proposal (minus ternary op) | N1485 | Yes
UTF-8 string literals | N1488 | Clang 3.3
Optimizing away infinite loops | N1509 | Yes
Conditional normative status for Annex G | N1514 | Yes (1)
Creation of complex value | N1464 | Clang 12
Recommendations for extended identifier characters for C and C++ | N1518 | Clang 15
Atomic C1x/C++0x compatibility refinements (1st part only) | N1526 | Yes
Atomic bitfields implementation defined | N1530 | Yes
Small fix for the effect of alignment on struct/union type compatibility | N1532 | Yes
Clarification for wide evaluation | N1531 | Unknown

(1): Clang does not implement Annex G, so our conditional support conforms by not defining the `__STDC_IEC_559_COMPLEX__` macro.

## C99 implementation status

Clang implements all of the ISO 9899:1999 (C99) standard.

Note, the list of C99 features comes from the C99 committee draft. Not all C99 documents are publicly available, so the documents referenced in this section may be inaccurate, unknown, or not linked.

You can use Clang in C99 mode with the `-std=c99` option.

List of features and minimum Clang version with support

Language Feature | C99 Proposal | Available in Clang?
--- | --- | ---
restricted character set support via digraphs and <iso646.h> | Unknown | Yes
more precise aliasing rules via effective type | Unknown | Yes
restricted pointers | N448 | Partial Clang's support for `restrict` is fully conforming but considered only partially implemented. Clang implements all of the constraints required for `restrict` support, but LLVM only supports `restrict` optimization semantics for restricted pointers used as function parameters; it does not fully support the semantics for restrict on local variables or data members.
variable length arrays | N683 | Yes
flexible array members | Unknown | Yes
static and type qualifiers in parameter array declarators | Unknown | Yes
complex and imaginary support in <complex.h> | N693 | Partial Clang supports `_Complex` type specifiers but does not support `_Imaginary` type specifiers. Support for `_Imaginary` is optional in C99 and Clang does not claim conformance to Annex G. `_Complex` support requires an underlying support library such as compiler-rt to provide functions like `__divsc3`. Compiler-rt isn't linked in automatically in MSVC style environments.
type-generic math macros in <tgmath.h> | N693 | Yes
the long long int type | N601 | Yes
increase minimum translation limits | N590 | Clang 3.2
additional floating-point characteristics in <float.h> | Unknown | Clang 16
remove implicit int
N635 | Yes
N692 | Yes
N722 | Yes
reliable integer division | N617 | Yes
universal character names (\u and \U) | Unknown | Yes
extended identifiers | N717 | Clang 17
hexadecimal floating-point constants | N308 | Yes
compound literals | N716 | Yes
designated initializers | N494 | Yes
// comments | N644 | Yes
extended integer types and library functions in <inttypes.h> and <stdint.h> | Unknown | Yes
remove implicit function declaration | N636 | Yes
preprocessor arithmetic done in intmax_t/uintmax_t | N736 | Yes
mixed declarations and code new block scopes for selection and iteration statements | N740 | Yes
integer constant type rules | N629 | Yes
integer promotion rules | N725 | Yes
macros with a variable number of arguments | N707 | Yes
IEC 60559 support | Unknown | Partial Clang supports much of the language requirements for Annex F, but full conformance is only possible to determine when considering the compiler's language support, the C runtime library's math library support, and the target system's floating-point environment support. Clang does not currently raise an "invalid" floating-point exception on certain conversions, does not raise floating-point exceptions for arithmetic constant expressions, and other corner cases. Note, Clang does not define `__STDC_IEC_559__` because the compiler does not fully conform. However, some C standard library implementations ( glibc, musl will define the macro regardless of compiler support unless the compiler defines `__GCC_IEC_559`, which Clang does not currently define. Additionally, Clang intentionally will not conform to Annex F on 32-bit x86 without SSE2 due to the behavior of floating-point operations in x87.
trailing comma allowed in enum declaration | Unknown | Yes
inline functions | N741 | Yes
boolean type in <stdbool.h> | N815 | Yes
idempotent type qualifiers | N505 | Yes
empty macro arguments | N570 | Yes
additional predefined macro names | Unknown | Yes
_Pragma preprocessing operator | N634 | Yes
standard pragmas
N631 | Yes
N696 | Yes
__func__ predefined identifier | N611 | Yes
va_copy macro | N671 | Yes
remove deprecation of aliased array parameters | Unknown | Yes
conversion of array to pointer not limited to lvalues | N835 | Yes
relaxed constraints on aggregate and union initialization | N782 | Clang 3.4
relaxed restrictions on portable header names | N772 | Yes
return without an expression not permitted in function that returns a value | Unknown | Yes

## C89 implementation status

Clang implements all of the ISO 9899:1990 (C89) standard.

You can use Clang in C89 mode with the `-std=c89` or `-std=c90` options.