C standards

Names specified here
Name	Description	Notes				Source	Availability
`__STDC__`	Indicator of conforming implementation	L		M		Predefined	C89	C90	C95	C99	C11
`__STDC_VERSION__`	Standard version identifier	L		M		Predefined			C95	C99	C11

Name	Standard	Number of headers	Number of names	Expansion of `__STDC_VERSION__`
C89	ANSI X3.159-1989	15	323	undefined
C90	ISO/IEC 9899-1990	15	323	undefined
C95	ISO/IEC 9899-1995	18	416	`199409L`
C99	ISO/IEC 9899-1999	24	1058	`199901L`
C11	ISO/IEC 9899-2011	29	1308	`201112L`

__STDC__ expands to 1 on all conforming implementations of C. From C95, __STDC_VERSION__ expands to an integer constant of the form yyyymmL, indicating the year yyyy and month mm in which the standard was published. 201112L indicates C11, 199901L indicates C99, and 199409L indicates C95. Its absence indicates C89 or C90. Whatever the value, if you need to detect a minimum version, compare numerically. For example:

#if __STDC_VERSION__ > 199409L
/* C95 or later detected */
#elif __STDC__
/* C90 detected */
#else
/* Substandard implementation detected */
#endif