Edit

[EDIT] FILE: libc-header-start.h

/* Handle feature test macros at the start of a header.
   Copyright (C) 2016-2018 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

/* This header is internal to glibc and should not be included outside
   of glibc headers.  Headers including it must define
   __GLIBC_INTERNAL_STARTING_HEADER_IMPLEMENTATION first.  This header
   cannot have multiple include guards because ISO C feature test
   macros depend on the definition of the macro when an affected
   header is included, not when the first system header is
   included.  */

#ifndef __GLIBC_INTERNAL_STARTING_HEADER_IMPLEMENTATION
# error "Never include <bits/libc-header-start.h> directly."
#endif

#undef __GLIBC_INTERNAL_STARTING_HEADER_IMPLEMENTATION

#include <features.h>

/* ISO/IEC TR 24731-2:2010 defines the __STDC_WANT_LIB_EXT2__
   macro.  */
#undef __GLIBC_USE_LIB_EXT2
#if (defined __USE_GNU							\
     || (defined __STDC_WANT_LIB_EXT2__ && __STDC_WANT_LIB_EXT2__ > 0))
# define __GLIBC_USE_LIB_EXT2 1
#else
# define __GLIBC_USE_LIB_EXT2 0
#endif

/* ISO/IEC TS 18661-1:2014 defines the __STDC_WANT_IEC_60559_BFP_EXT__
   macro.  */
#undef __GLIBC_USE_IEC_60559_BFP_EXT
#if defined __USE_GNU || defined __STDC_WANT_IEC_60559_BFP_EXT__
# define __GLIBC_USE_IEC_60559_BFP_EXT 1
#else
# define __GLIBC_USE_IEC_60559_BFP_EXT 0
#endif

/* ISO/IEC TS 18661-4:2015 defines the
   __STDC_WANT_IEC_60559_FUNCS_EXT__ macro.  */
#undef __GLIBC_USE_IEC_60559_FUNCS_EXT
#if defined __USE_GNU || defined __STDC_WANT_IEC_60559_FUNCS_EXT__
# define __GLIBC_USE_IEC_60559_FUNCS_EXT 1
#else
# define __GLIBC_USE_IEC_60559_FUNCS_EXT 0
#endif

/* ISO/IEC TS 18661-3:2015 defines the
   __STDC_WANT_IEC_60559_TYPES_EXT__ macro.  */
#undef __GLIBC_USE_IEC_60559_TYPES_EXT
#if defined __USE_GNU || defined __STDC_WANT_IEC_60559_TYPES_EXT__
# define __GLIBC_USE_IEC_60559_TYPES_EXT 1
#else
# define __GLIBC_USE_IEC_60559_TYPES_EXT 0
#endif

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