Macros related to language levels

The following macros except C++ only __cplusplus, __STDC__ C++ only ends , and C only __STDC_VERSION__ C only ends are predefined to a value of 1 by a specific language level, represented by a suboption of the -std compiler option, or any invocation or pragma that implies that suboption.

If the suboption enabling the feature is not in effect, the macro is undefined. For descriptions of the features related to these macros, see the C and C++ language standards.

Table 1. Predefined macros for language features
Predefined macro name	Description	Predefined when the following language level or compiler option is in effect
__cplusplus	The numeric value that indicates the supported language standard as defined by that specific standard.	Always predefined. The format is `yyyymm`L. (For example, the format is 201103L for C++11.)
__STDC__	Indicates that the compiler conforms to the ANSI/ISO C standard.	Predefined to 1 if ANSI/ISO C standard conformance is in effect. Explicitly defined to 0.
__STDC_HOSTED__	Indicates that the implementation is a hosted implementation of the ANSI/ISO C standard. (That is, the hosted environment has all the facilities of the standard C available).	Predefined to 1 by default Predefined to 0 if -ffreestanding is specified.
__STDC_NO_ATOMICS__	Indicates that the implementation does not have the full support of the atomics feature.	-std=c11 \| -std=gnu11
__STDC_NO_THREADS__	Indicates that the implementation does not have the full support of the threads feature.	-std=c11 \| -std=gnu11
__STDC_VERSION__	Indicates the version of ANSI/ISO C standard which the compiler conforms to.	Always predefined. The format is `yyyymm`L. (For example, the format is 199901L for C99.)