Standards.md

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Language Standards

Language standards requires the existence of pre-defined macros.

Name	Macro	Standard
C89	__STDC__	ANSI X3.159-1989
C90	__STDC__	ISO/IEC 9899:1990
C94	__STDC_VERSION__ = 199409L	ISO/IEC 9899-1:1994
C99	__STDC_VERSION__ = 199901L	ISO/IEC 9899:1999
C11	__STDC_VERSION__ = 201112L	ISO/IEC 9899:2011
C17	__STDC_VERSION__ = 201710L	ISO/IEC 9899:2018
C23	__STDC_VERSION__ = 202311L	ISO/IEC 9899:2024
C++98	__cplusplus = 199711L	ISO/IEC 14882:1998
C++11	__cplusplus = 201103L	ISO/IEC 14882:2011
C++14	__cplusplus = 201402L	ISO/IEC 14882:2014
C++17	__cplusplus = 201703L	ISO/IEC 14882:2017
C++20	__cplusplus = 202002L	ISO/IEC 14882:2020
C++23	__cplusplus = 202302L	ISO/IEC 14882:2024
C++/CLI	__cplusplus_cli = 200406L	ECMA-372
DSP-C		ISO/IEC JTC1/SC22 WG14/N854
EC++	__embedded_cplusplus	Embedded C++

Example: C Standards

#if defined(__STDC__)
# define PREDEF_STANDARD_C_1989
# if defined(__STDC_VERSION__)
# if (__STDC_VERSION__ >= 199409L)
# define PREDEF_STANDARD_C_1994
# endif
# if (__STDC_VERSION__ >= 199901L)
# define PREDEF_STANDARD_C_1999
# endif
# endif
#endif

Notice that not all compliant compilers provides the correct pre-defined macros. For example, Microsoft Visual C++ does not define __STDC__, or Sun Workshop 4.2 supports C94 without setting __STDC_VERSION__ to the proper value. Extra checks for such compilers must be added.

Notice that some compilers, such as the HP aC++, use the value 199707L to indicate the C++98 standard. This value was used in an earlier proposal of the C++98 standard.

Example: Pre-C89

In continuation of the above example, pre-C89 compilers do not recognize certain keywords. Let the preprocessor remove those keywords for those compilers.

#if !defined(PREDEF_STANDARD_C_1989) && !defined(__cplusplus)
# define const
# define volatile
#endif

Unix Standards

There are several related Unix standards, such as POSIX, X/Open, and LSB.

Unix standards require the existence macros in the <unistd.h> header file.

Name	Macro	Standard
POSIX.1-1988	_POSIX_VERSION = 198808L
POSIX.1-1990	_POSIX_VERSION = 199009L	ISO/IEC 9945-1:1990
POSIX.2	_POSIX2_C_VERSION = 199209L	ISO/IEC 9945-2:1993
POSIX.1b-1993	_POSIX_VERSION = 199309L	IEEE 1003.1b-1993
POSIX.1-1996	_POSIX_VERSION = 199506L	IEEE 1003.1-1996
POSIX.1-2001	_POSIX_VERSION = 200112L	IEEE 1003.1-2001
POSIX.1-2008	_POSIX_VERSION = 200809L	IEEE 1003.1-2008
XPG3	_XOPEN_VERSION = 3	X/Open Portability Guide 3 (1989)
XPG4	_XOPEN_VERSION = 4	X/Open Portability Guide 4 (1992)
SUS	_XOPEN_VERSION = 4 && _XOPEN_UNIX	X/Open Single UNIX Specification (UNIX95)
SUSv2	_XOPEN_VERSION = 500	X/Open Single UNIX Specification, Version 2 (UNIX98)
SUSv3	_XOPEN_VERSION = 600	Open Group Single UNIX Specification, Version 3 (UNIX03)
SUSv4	_XOPEN_VERSION = 700	Open Group Single UNIX Specification, Version 4
LSB	__LSB_VERSION__ = VR	Linux Standards Base V = Version R = Revision

Example: Unix Standards

The following examples assumes the definition of these macros.

#if defined(unix) || defined(__unix__) || defined(__unix)
# define PREDEF_PLATFORM_UNIX
#endif
#if defined(PREDEF_PLATFORM_UNIX)
# include <unistd.h>
# if defined(_XOPEN_VERSION)
# if (_XOPEN_VERSION >= 3)
# define PREDEF_STANDARD_XOPEN_1989
# endif
# if (_XOPEN_VERSION >= 4)
# define PREDEF_STANDARD_XOPEN_1992
# endif
# if (_XOPEN_VERSION >= 4) && defined(_XOPEN_UNIX)
# define PREDEF_STANDARD_XOPEN_1995
# endif
# if (_XOPEN_VERSION >= 500)
# define PREDEF_STANDARD_XOPEN_1998
# endif
# if (_XOPEN_VERSION >= 600)
# define PREDEF_STANDARD_XOPEN_2003
# endif
# if (_XOPEN_VERSION >= 700)
# define PREDEF_STANDARD_XOPEN_2008
# endif
# endif
#endif

Notice that not all compliant compilers provides the correct pre-defined macros. For example, IBM xlC supports Unix without setting any of the __unix__ macros. Extra checks for such compilers must be added.