.C File Format – Versions

Summary

C89 and C90 are the same language with different section numbers. C17 is sometimes called C18, from its publication year. A compiler that conforms to any standard from C89 onwards defines __STDC__ as 1.

K&R C

Brian Kernighan and Dennis Ritchie’s book, first published in 1978, served as the informal definition of the language for over a decade. K&R C had:

• 28 keywords, listed below.
• long and unsigned integers, the standard input and output library, and /* */ comments.
• Compound assignment written += and so on, replacing the older =+ form because of ambiguities like i=-10.
• Implicit int, and no way to declare parameter types in a function declaration.
• Function definitions in the old style.

Its keywords were auto break case char continue default do double else entry extern float for goto if int long register return short sizeof static struct switch typedef union unsigned while. entry was reserved but never given a meaning.

Compilers soon added void functions, structure assignment, structures as function results, and enumerations. These features were not in the book.

C89 and C90

The first standard, developed by an ANSI committee.

• Function prototypes, with the old-style form kept and marked obsolescent.
• void and void *.
• const, volatile and signed.
• Enumerations, and assignment and passing of structures.
• long double.
• A standard library with defined headers.
• The preprocessor operators #, ## and defined, and the directives #elif, #error, #pragma and the null directive.
• Trigraphs and wide characters.
• The predefined macros __FILE__, __LINE__, __DATE__, __TIME__ and __STDC__.

C95

Amendment 1 to C90 was a small update, published in 1995.

• Digraphs, which are alternative spellings for some punctuators.
• The macro __STDC_VERSION__.
• The headers <iso646.h>, <wchar.h> and <wctype.h>.

C99

C99 was a large revision. Technical corrigenda followed in 2001, 2004 and 2007.

Language additions:

• // comments.
• _Bool, long long, and extended integer types.
• _Complex and _Imaginary types.
• Mixed declarations and statements, and declarations in for initialisers.
• Variable length arrays and flexible array members.
• Designated initialisers and compound literals.
• inline and restrict, and static and qualifiers in array parameter brackets.
• Hexadecimal floating constants, and universal character names.
• Variadic macros and the _Pragma operator.
• __func__, a trailing comma in enumerator lists, and an implicit return 0 at the end of main.
• Division that truncates toward zero, increased translation limits, and the standard floating-point pragmas.

New headers: <complex.h>, <fenv.h>, <inttypes.h>, <stdbool.h>, <stdint.h> and <tgmath.h>. New library features include snprintf, _Exit, va_copy, isblank, and printf length modifiers such as ll and z.

Removed: implicit int and implicit function declarations. C99 also deprecated gets.

C11

Language additions:

• _Thread_local and the _Atomic type, with a memory model.
• _Alignas and _Alignof.
• _Generic selection and _Noreturn.
• _Static_assert.
• Anonymous structures and unions.
• u and U character constants, and u8, u and U string literals.
• A finer description of evaluation order, through sequenced before.

New headers: <stdalign.h>, <stdatomic.h>, <stdnoreturn.h>, <threads.h> and <uchar.h>. Annex K, the bounds-checking interfaces, is also new.

C11 made complex types and variable length arrays optional, and removed gets.

C17

C17 is a defect-correction release. It adds no new language or library features, and only changes __STDC_VERSION__ and resolves defect reports. It also deprecates ATOMIC_VAR_INIT and calling realloc with a size of zero.

C23

C23 is a substantial revision, published as ISO/IEC 9899:2024. Its language additions include:

• The keywords bool, true, false, alignas, alignof, static_assert and thread_local, replacing older spellings.
• nullptr and the type nullptr_t.
• typeof and typeof_unqual, and auto with type inference.
• constexpr objects.
• Attributes, such as [[nodiscard]] and [[maybe_unused]].
• Bit-precise integers (_BitInt), and decimal floating types.
• Binary constants (0b1010), and digit separators (1'000).
• u8 character constants, and char8_t as the element type of u8 strings.
• Enumerations with a fixed underlying type.
• #embed, #elifdef, #elifndef, #warning, __has_include and __VA_OPT__.
• The empty initialiser = {}, unnamed parameters in definitions, and labels before declarations and closing braces.
• Single-argument static_assert, and variadic functions with no named parameter.
• Identifiers defined in terms of Unicode Standard Annex 31.

New headers: <stdbit.h> and <stdckdint.h>. Library additions include memset_explicit, strdup, strndup, memccpy, the b conversion for printf, and timespec_getres.

Removed:

• Old-style function declarations and definitions.
• Representations of signed integers other than two’s complement.
• Trigraphs.
• Mixed wide string literal concatenation.
• realloc with a size of zero, which is now undefined.
• static_assert and thread_local as macros, now keywords.

Deprecated: <stdnoreturn.h>, the _Noreturn specifier, asctime, ctime, __STDC_IEC_559__, and the older underscore keyword spellings.