макросы тестирования функций (feature test macros)
Стандарты (Conforming to)
POSIX.1 specifies _POSIX_C_SOURCE, _POSIX_SOURCE, and
_XOPEN_SOURCE.
_XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1), but
is not present in SUSv2 and later. _FILE_OFFSET_BITS is not
specified by any standard, but is employed on some other
implementations.
_BSD_SOURCE, _SVID_SOURCE, _DEFAULT_SOURCE, _ATFILE_SOURCE,
_GNU_SOURCE, _FORTIFY_SOURCE, _REENTRANT, and _THREAD_SAFE are
specific to Linux (glibc).
