Predefined preprocessor names

When the compiler is called with cc, c11, c89 or CC, preprocessor macros and predicates are predefined, depending on the command and some options.

For a few macros the value is fixed. It cannot be changed, neither on the command line nor in the source (via #define or #undef). These macros are: __cplusplus, __STDC__, __STDC_VERSION__ and __SNI__STDCplusplus.

Predefined preprocessor macros (defines)

_BOOL	In the language modes C++ V3, C++ 2017 and C++ 2020 with the option -K bool, which is the default
__CGLOBALS_PRAGMA	Always set
__cplusplus	In all C++ language modes: == 1 in Cfront C++ mode == 2 in extended C++ V3 mode == 199612L in strict C++ V3 mode == 201703L in C++ 2017 mode == 202002L in C++ 2020 mode
c_plusplus	In all C++ language modes
__CFRONT_V3	In Cfront C++ mode
__EDG_NO_IMPLICIT_INCLUSION
	In the modes C++ V3, C++ 2017 and C++ 2020, if implicit inclusion has been disabled with the option -K no_implicit_include within the framework of template instantiation
__EXISTCGLOB	Always set
__HALF_TAG_LOOKUP	Always set
_IEEE	Option -K ieee_floats
LANGUAGE_C	Always set
_LANGUAGE_C	Always set
_LIBCPP_STD_VER	In mode C++ 2020: the compiler will set it to the value 17 In mode C++ 2017: the library headers will set it to the value 17
__LITERAL_ENCODING_ASCII
	Option  -K literal_encoding_ascii[_full]
__LITERAL_ENCODING_EBCDIC
	Option -K literal_encoding_{ebcdic[_full]|native}
_LONGLONG	Option -K longlong
__OLD_SPECIALIZATION_SYNTAX
	== 1 with the option  -K old_specialization
_OSD_POSIX	Always set
__OSD_POSIX	Always set
__SHORT_NAMES	Option –K c_names_short
__SIGNED_CHARS__	Option -K schar
__SMALL_VA_DCL	Always set
__SNI	In all C modes and in C++ V2 mode
__SNI_HOST_BS2000	Never set (reserved for compilation in BS2000 (SDF))
__SNI_HOST_BS2000_POSIX	Always set
__SNI_PRINTF_CHECK	Always set
__SNI__STDCplusplus	in all C++ language modes: == 0 in extended language modes (-X nostrict) == 1 strict language modes (-X strict)
__SNI_TARG_BS2000	Never set (reserved for compilation in BS2000 (SDF))
__SNI_TARG_BS2000_POSIX	Always set
__STDC__	Always set: == 0 in extended language modes (-X nostrict) == 1 in strict language modes (-X strict)
__STDC_HOSTED__	Always set
__STDC_NO_ATOMICS__	Always set
__STDC_NO_COMPLEX__	Always set
__STDC_NO_THREADS__	Always set
__STDCPP_DEFAULT_NEW_ALIGNMENT__
	Always set == 8U
__STDC_UTF_16__	Always set
__STDC_UTF_32__	Always set
__STDC_VERS_CRTE__	Undefined in K&R C mode == 199409L in language modes C89, C++ V2, C++ V3 == 201112L in language modes C11, C++ 2017 and C++ 2020
__STDC_VERSION__	Undefined in K&R C mode == 199409L in language mode C89 and in all C++ language modes == 201112L in language mode C11
_STRICT_STDC	in strict language modes (-X strict)
_WCHAR_T	Option -K wchar_t_keyword (default in language modes C++ V3, C++ 2017 and C++ 2020) If this option is not set (e.g. in C modes or in C++ V2 mode), _WCHAR_T is defined in various standard headers to issue a typedef for wchar_t.
_WCHAR_T_KEYWORD	Option -K wchar_t_keyword (default in language modes C++ V3, C++ 2017 and C++ 2020)
_XPG_IV	If called with c11 or c89

Predefined preprocessor predicates (#assert)

data_model(bit32)	Always set
cpu(7500)	With /390 code generation
machine(7500)	With /390 code generation
system(bs2000)	Always set