/usr/include/features.h 파일에는 컴파일 할 때 사용하는 -D 옵션과 긴밀한 관계가 있다.

 어떤 Define을 하느냐에 따라 각기 다른 내용의 컴파일과 링크가 이루어 진다.

 그 내용을 /usr/include/features.h 파일의 전문을 여기에 싣는다.

/* Copyright (C) 1991-1993,1995-2006,2007,2009 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, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#ifndef    _FEATURES_H
#define    _FEATURES_H    1

/* These are defined by the user (or the compiler)
   to specify the desired environment:

   __STRICT_ANSI__    ISO Standard C.
   _ISOC99_SOURCE    Extensions to ISO C89 from ISO C99.
   _POSIX_SOURCE    IEEE Std 1003.1.
   _POSIX_C_SOURCE    If ==1, like _POSIX_SOURCE; if >=2 add IEEE Std 1003.2;
            if >=199309L, add IEEE Std 1003.1b-1993;
            if >=199506L, add IEEE Std 1003.1c-1995;
            if >=200112L, all of IEEE 1003.1-2004
            if >=200809L, all of IEEE 1003.1-2008
   _XOPEN_SOURCE    Includes POSIX and XPG things.  Set to 500 if
            Single Unix conformance is wanted, to 600 for the
            sixth revision, to 700 for the seventh revision.
   _XOPEN_SOURCE_EXTENDED XPG things and X/Open Unix extensions.
   _LARGEFILE_SOURCE    Some more functions for correct standard I/O.
   _LARGEFILE64_SOURCE    Additional functionality from LFS for large files.
   _FILE_OFFSET_BITS=N    Select default filesystem interface.
   _BSD_SOURCE        ISO C, POSIX, and 4.3BSD things.
   _SVID_SOURCE        ISO C, POSIX, and SVID things.
   _ATFILE_SOURCE    Additional *at interfaces.
   _GNU_SOURCE        All of the above, plus GNU extensions.
   _REENTRANT        Select additionally reentrant object.
   _THREAD_SAFE        Same as _REENTRANT, often used by other systems.
   _FORTIFY_SOURCE    If set to numeric value > 0 additional security
            measures are defined, according to level.

   The `-ansi’ switch to the GNU C compiler defines __STRICT_ANSI__.
   If none of these are defined, the default is to have _SVID_SOURCE,
   _BSD_SOURCE, and _POSIX_SOURCE set to one and _POSIX_C_SOURCE set to
   200112L.  If more than one of these are defined, they accumulate.
   For example __STRICT_ANSI__, _POSIX_SOURCE and _POSIX_C_SOURCE
   together give you ISO C, 1003.1, and 1003.2, but nothing else.

   These are defined by this file and are used by the
   header files to decide what to declare or define:

   __USE_ISOC99        Define ISO C99 things.
   __USE_ISOC95        Define ISO C90 AMD1 (C95) things.
   __USE_POSIX        Define IEEE Std 1003.1 things.
   __USE_POSIX2        Define IEEE Std 1003.2 things.
   __USE_POSIX199309    Define IEEE Std 1003.1, and .1b things.
   __USE_POSIX199506    Define IEEE Std 1003.1, .1b, .1c and .1i things.
   __USE_XOPEN        Define XPG things.
   __USE_XOPEN_EXTENDED    Define X/Open Unix things.
   __USE_UNIX98        Define Single Unix V2 things.
   __USE_XOPEN2K        Define XPG6 things.
   __USE_XOPEN2K8       Define XPG7 things.
   __USE_LARGEFILE    Define correct standard I/O things.
   __USE_LARGEFILE64    Define LFS things with separate names.
   __USE_FILE_OFFSET64    Define 64bit interface as default.
   __USE_BSD        Define 4.3BSD things.
   __USE_SVID        Define SVID things.
   __USE_MISC        Define things common to BSD and System V Unix.
   __USE_ATFILE        Define *at interfaces and AT_* constants for them.
   __USE_GNU        Define GNU extensions.
   __USE_REENTRANT    Define reentrant/thread-safe *_r functions.
   __USE_FORTIFY_LEVEL    Additional security measures used, according to level.
   __FAVOR_BSD        Favor 4.3BSD things in cases of conflict.

   The macros `__GNU_LIBRARY__’, `__GLIBC__’, and `__GLIBC_MINOR__’ are
   defined by this file unconditionally.  `__GNU_LIBRARY__’ is provided
   only for compatibility.  All new code should use the other symbols
   to test for features.

   All macros listed above as possibly being defined by this file are
   explicitly undefined if they are not explicitly defined.
   Feature-test macros that are not defined by the user or compiler
   but are implied by the other feature-test macros defined (or by the
   lack of any definitions) are defined by the file.  */

/* Undefine everything, so we get a clean slate.  */
#undef    __USE_ISOC99
#undef    __USE_ISOC95
#undef    __USE_POSIX
#undef    __USE_POSIX2
#undef    __USE_POSIX199309
#undef    __USE_POSIX199506
#undef    __USE_XOPEN
#undef    __USE_XOPEN_EXTENDED
#undef    __USE_UNIX98
#undef    __USE_XOPEN2K
#undef    __USE_XOPEN2K8
#undef    __USE_LARGEFILE
#undef    __USE_LARGEFILE64
#undef    __USE_FILE_OFFSET64
#undef    __USE_BSD
#undef    __USE_SVID
#undef    __USE_MISC
#undef    __USE_ATFILE
#undef    __USE_GNU
#undef    __USE_REENTRANT
#undef    __USE_FORTIFY_LEVEL
#undef    __FAVOR_BSD
#undef    __KERNEL_STRICT_NAMES

/* Suppress kernel-name space pollution unless user expressedly asks
   for it.  */
#ifndef _LOOSE_KERNEL_NAMES
# define __KERNEL_STRICT_NAMES
#endif

/* Always use ISO C things.  */
#define    __USE_ANSI    1

/* Convenience macros to test the versions of glibc and gcc.
   Use them like this:
   #if __GNUC_PREREQ (2,8)
   … code requiring gcc 2.8 or later …
   #endif
   Note – they won’t work for gcc1 or glibc1, since the _MINOR macros
   were not defined then.  */
#if defined __GNUC__ && defined __GNUC_MINOR__
# define __GNUC_PREREQ(maj, min)
    ((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
#else
# define __GNUC_PREREQ(maj, min) 0
#endif

/* If _BSD_SOURCE was defined by the user, favor BSD over POSIX.  */
#if defined _BSD_SOURCE &&
    !(defined _POSIX_SOURCE || defined _POSIX_C_SOURCE ||
      defined _XOPEN_SOURCE || defined _XOPEN_SOURCE_EXTENDED ||
      defined _GNU_SOURCE || defined _SVID_SOURCE)
# define __FAVOR_BSD    1
#endif

/* If _GNU_SOURCE was defined by the user, turn on all the other features.  */
#ifdef _GNU_SOURCE
# undef  _ISOC99_SOURCE
# define _ISOC99_SOURCE    1
# undef  _POSIX_SOURCE
# define _POSIX_SOURCE    1
# undef  _POSIX_C_SOURCE
# define _POSIX_C_SOURCE    200809L
# undef  _XOPEN_SOURCE
# define _XOPEN_SOURCE    700
# undef  _XOPEN_SOURCE_EXTENDED
# define _XOPEN_SOURCE_EXTENDED    1
# undef     _LARGEFILE64_SOURCE
# define _LARGEFILE64_SOURCE    1
# undef  _BSD_SOURCE
# define _BSD_SOURCE    1
# undef  _SVID_SOURCE
# define _SVID_SOURCE    1
# undef  _ATFILE_SOURCE
# define _ATFILE_SOURCE    1
#endif

/* If nothing (other than _GNU_SOURCE) is defined,
   define _BSD_SOURCE and _SVID_SOURCE.  */
#if (!defined __STRICT_ANSI__ && !defined _ISOC99_SOURCE &&
     !defined _POSIX_SOURCE && !defined _POSIX_C_SOURCE &&
     !defined _XOPEN_SOURCE && !defined _XOPEN_SOURCE_EXTENDED &&
     !defined _BSD_SOURCE && !defined _SVID_SOURCE)
# define _BSD_SOURCE    1
# define _SVID_SOURCE    1
#endif

/* This is to enable the ISO C99 extension.  Also recognize the old macro
   which was used prior to the standard acceptance.  This macro will
   eventually go away and the features enabled by default once the ISO C99
   standard is widely adopted.  */
#if (defined _ISOC99_SOURCE || defined _ISOC9X_SOURCE
     || (defined __STDC_VERSION__ && __STDC_VERSION__ >= 199901L))
# define __USE_ISOC99    1
#endif

/* This is to enable the ISO C90 Amendment 1:1995 extension.  */
#if (defined _ISOC99_SOURCE || defined _ISOC9X_SOURCE
     || (defined __STDC_VERSION__ && __STDC_VERSION__ >= 199409L))
# define __USE_ISOC95    1
#endif

/* If none of the ANSI/POSIX macros are defined, use POSIX.1 and POSIX.2
   (and IEEE Std 1003.1b-1993 unless _XOPEN_SOURCE is defined).  */
#if ((!defined __STRICT_ANSI__ || (_XOPEN_SOURCE – 0) >= 500) &&
     !defined _POSIX_SOURCE && !defined _POSIX_C_SOURCE)
# define _POSIX_SOURCE    1
# if defined _XOPEN_SOURCE && (_XOPEN_SOURCE – 0) < 500
#  define _POSIX_C_SOURCE    2
# elif defined _XOPEN_SOURCE && (_XOPEN_SOURCE – 0) < 600
#  define _POSIX_C_SOURCE    199506L
# elif defined _XOPEN_SOURCE && (_XOPEN_SOURCE – 0) < 700
#  define _POSIX_C_SOURCE    200112L
# else
#  define _POSIX_C_SOURCE    200809L
# endif
# define __USE_POSIX_IMPLICITLY    1
#endif

#if defined _POSIX_SOURCE || _POSIX_C_SOURCE >= 1 || defined _XOPEN_SOURCE
# define __USE_POSIX    1
#endif

#if defined _POSIX_C_SOURCE && _POSIX_C_SOURCE >= 2 || defined _XOPEN_SOURCE
# define __USE_POSIX2    1
#endif

#if (_POSIX_C_SOURCE – 0) >= 199309L
# define __USE_POSIX199309    1
#endif

#if (_POSIX_C_SOURCE – 0) >= 199506L
# define __USE_POSIX199506    1
#endif

#if (_POSIX_C_SOURCE – 0) >= 200112L
# define __USE_XOPEN2K        1
# undef __USE_ISOC99
# define __USE_ISOC99        1
#endif

#if (_POSIX_C_SOURCE – 0) >= 200809L
# define __USE_XOPEN2K8        1
# undef  _ATFILE_SOURCE
# define _ATFILE_SOURCE    1
#endif

#ifdef    _XOPEN_SOURCE
# define __USE_XOPEN    1
# if (_XOPEN_SOURCE – 0) >= 500
#  define __USE_XOPEN_EXTENDED    1
#  define __USE_UNIX98    1
#  undef _LARGEFILE_SOURCE
#  define _LARGEFILE_SOURCE    1
#  if (_XOPEN_SOURCE – 0) >= 600
#   if (_XOPEN_SOURCE – 0) >= 700
#    define __USE_XOPEN2K8    1
#   endif
#   define __USE_XOPEN2K    1
#   undef __USE_ISOC99
#   define __USE_ISOC99        1
#  endif
# else
#  ifdef _XOPEN_SOURCE_EXTENDED
#   define __USE_XOPEN_EXTENDED    1
#  endif
# endif
#endif

#ifdef _LARGEFILE_SOURCE
# define __USE_LARGEFILE    1
#endif

#ifdef _LARGEFILE64_SOURCE
# define __USE_LARGEFILE64    1
#endif

#if defined _FILE_OFFSET_BITS && _FILE_OFFSET_BITS == 64
# define __USE_FILE_OFFSET64    1
#endif

#if defined _BSD_SOURCE || defined _SVID_SOURCE
# define __USE_MISC    1
#endif

#ifdef    _BSD_SOURCE
# define __USE_BSD    1
#endif

#ifdef    _SVID_SOURCE
# define __USE_SVID    1
#endif

#ifdef    _ATFILE_SOURCE
# define __USE_ATFILE    1
#endif

#ifdef    _GNU_SOURCE
# define __USE_GNU    1
#endif

#if defined _REENTRANT || defined _THREAD_SAFE
# define __USE_REENTRANT    1
#endif

#if defined _FORTIFY_SOURCE && _FORTIFY_SOURCE > 0
    && __GNUC_PREREQ (4, 1) && defined __OPTIMIZE__ && __OPTIMIZE__ > 0
# if _FORTIFY_SOURCE > 1
#  define __USE_FORTIFY_LEVEL 2
# else
#  define __USE_FORTIFY_LEVEL 1
# endif
#else
# define __USE_FORTIFY_LEVEL 0
#endif

/* Define __STDC_IEC_559__ and other similar macros.  */
#include <bits/predefs.h>

/* wchar_t uses ISO 10646-1 (2nd ed., published 2000-09-15) / Unicode 3.1.  */
#define __STDC_ISO_10646__        200009L

/* This macro indicates that the installed library is the GNU C Library.
   For historic reasons the value now is 6 and this will stay from now
   on.  The use of this variable is deprecated.  Use __GLIBC__ and
   __GLIBC_MINOR__ now (see below) when you want to test for a specific
   GNU C library version and use the values in <gnu/lib-names.h> to get
   the sonames of the shared libraries.  */
#undef  __GNU_LIBRARY__
#define __GNU_LIBRARY__ 6

/* Major and minor version number of the GNU C library package.  Use
   these macros to test for features in specific releases.  */
#define    __GLIBC__    2
#define    __GLIBC_MINOR__    11

#define __GLIBC_PREREQ(maj, min)
    ((__GLIBC__ << 16) + __GLIBC_MINOR__ >= ((maj) << 16) + (min))

/* Decide whether a compiler supports the long long datatypes.  */
#if defined __GNUC__
    || (defined __PGI && defined __i386__ )
    || (defined __INTEL_COMPILER && (defined __i386__ || defined __ia64__))
    || (defined __STDC_VERSION__ && __STDC_VERSION__ >= 199901L)
# define __GLIBC_HAVE_LONG_LONG    1
#endif

/* This is here only because every header file already includes this one.  */
#ifndef __ASSEMBLER__
# ifndef _SYS_CDEFS_H
#  include <sys/cdefs.h>
# endif

/* If we don’t have __REDIRECT, prototypes will be missing if
   __USE_FILE_OFFSET64 but not __USE_LARGEFILE[64]. */
# if defined __USE_FILE_OFFSET64 && !defined __REDIRECT
#  define __USE_LARGEFILE    1
#  define __USE_LARGEFILE64    1
# endif

#endif    /* !ASSEMBLER */

/* Decide whether we can define ‘extern inline’ functions in headers.  */
#if __GNUC_PREREQ (2, 7) && defined __OPTIMIZE__
    && !defined __OPTIMIZE_SIZE__ && !defined __NO_INLINE__
    && defined __extern_inline
# define __USE_EXTERN_INLINES    1
#endif

/* There are some functions that must be declared ‘extern inline’ even with
   -Os when building LIBC, or they’ll end up undefined.  */
#if __GNUC_PREREQ (2, 7) && defined __OPTIMIZE__
    && (defined _LIBC || !defined __OPTIMIZE_SIZE__) && !defined __NO_INLINE__
    && defined __extern_inline
# define __USE_EXTERN_INLINES_IN_LIBC    1
#endif

/* This is here only because every header file already includes this one.
   Get the definitions of all the appropriate `__stub_FUNCTION’ symbols.
   <gnu/stubs.h> contains `#define __stub_FUNCTION’ when FUNCTION is a stub
   that will always return failure (and set errno to ENOSYS).  */
#include <gnu/stubs.h>

#endif    /* features.h  */