manual/creature.texi

   1 @node Feature Test Macros
   2 @subsection Feature Test Macros
   3
   4 @cindex feature test macros
   5 The exact set of features available when you compile a source file
   6 is controlled by which @dfn{feature test macros} you define.
   7
   8 If you compile your programs using @samp{gcc -ansi}, you get only the
   9 @w{ISO C} library features, unless you explicitly request additional
  10 features by defining one or more of the feature macros.
  11 @xref{Invoking GCC,, GNU CC Command Options, gcc, The GNU CC Manual},
  12 for more information about GCC options.@refill
  13
  14 You should define these macros by using @samp{#define} preprocessor
  15 directives at the top of your source code files.  These directives
  16 @emph{must} come before any @code{#include} of a system header file.  It
  17 is best to make them the very first thing in the file, preceded only by
  18 comments.  You could also use the @samp{-D} option to GCC, but it's
  19 better if you make the source files indicate their own meaning in a
  20 self-contained way.
  21
  22 This system exists to allow the library to conform to multiple standards.
  23 Although the different standards are often described as supersets of each
  24 other, they are usually incompatible because larger standards require
  25 functions with names that smaller ones reserve to the user program.  This
  26 is not mere pedantry --- it has been a problem in practice.  For instance,
  27 some non-GNU programs define functions named @code{getline} that have
  28 nothing to do with this library's @code{getline}.  They would not be
  29 compilable if all features were enabled indiscriminately.
  30
  31 This should not be used to verify that a program conforms to a limited
  32 standard.  It is insufficient for this purpose, as it will not protect you
  33 from including header files outside the standard, or relying on semantics
  34 undefined within the standard.
  35
  36 @defvr Macro _POSIX_SOURCE
  37 @standards{POSIX.1, (none)}
  38 If you define this macro, then the functionality from the POSIX.1
  39 standard (IEEE Standard 1003.1) is available, as well as all of the
  40 @w{ISO C} facilities.
  41
  42 The state of @code{_POSIX_SOURCE} is irrelevant if you define the
  43 macro @code{_POSIX_C_SOURCE} to a positive integer.
  44 @end defvr
  45
  46 @defvr Macro _POSIX_C_SOURCE
  47 @standards{POSIX.2, (none)}
  48 Define this macro to a positive integer to control which POSIX
  49 functionality is made available.  The greater the value of this macro,
  50 the more functionality is made available.
  51
  52 If you define this macro to a value greater than or equal to @code{1},
  53 then the functionality from the 1990 edition of the POSIX.1 standard
  54 (IEEE Standard 1003.1-1990) is made available.
  55
  56 If you define this macro to a value greater than or equal to @code{2},
  57 then the functionality from the 1992 edition of the POSIX.2 standard
  58 (IEEE Standard 1003.2-1992) is made available.
  59
  60 If you define this macro to a value greater than or equal to @code{199309L},
  61 then the functionality from the 1993 edition of the POSIX.1b standard
  62 (IEEE Standard 1003.1b-1993) is made available.
  63
  64 If you define this macro to a value greater than or equal to
  65 @code{199506L}, then the functionality from the 1995 edition of the
  66 POSIX.1c standard (IEEE Standard 1003.1c-1995) is made available.
  67
  68 If you define this macro to a value greater than or equal to
  69 @code{200112L}, then the functionality from the 2001 edition of the
  70 POSIX standard (IEEE Standard 1003.1-2001) is made available.
  71
  72 If you define this macro to a value greater than or equal to
  73 @code{200809L}, then the functionality from the 2008 edition of the
  74 POSIX standard (IEEE Standard 1003.1-2008) is made available.
  75
  76 Greater values for @code{_POSIX_C_SOURCE} will enable future extensions.
  77 The POSIX standards process will define these values as necessary, and
  78 @theglibc{} should support them some time after they become standardized.
  79 The 1996 edition of POSIX.1 (ISO/IEC 9945-1: 1996) states that
  80 if you define @code{_POSIX_C_SOURCE} to a value greater than
  81 or equal to @code{199506L}, then the functionality from the 1996
  82 edition is made available.  In general, in @theglibc{}, bugfixes to
  83 the standards are included when specifying the base version; e.g.,
  84 POSIX.1-2004 will always be included with a value of @code{200112L}.
  85 @end defvr
  86
  87 @defvr Macro _XOPEN_SOURCE
  88 @defvrx Macro _XOPEN_SOURCE_EXTENDED
  89 @standards{X/Open, (none)}
  90 If you define this macro, functionality described in the X/Open
  91 Portability Guide is included.  This is a superset of the POSIX.1 and
  92 POSIX.2 functionality and in fact @code{_POSIX_SOURCE} and
  93 @code{_POSIX_C_SOURCE} are automatically defined.
  94
  95 As the unification of all Unices, functionality only available in
  96 BSD and SVID is also included.
  97
  98 If the macro @code{_XOPEN_SOURCE_EXTENDED} is also defined, even more
  99 functionality is available.  The extra functions will make all functions
 100 available which are necessary for the X/Open Unix brand.
 101
 102 If the macro @code{_XOPEN_SOURCE} has the value @math{500} this includes
 103 all functionality described so far plus some new definitions from the
 104 Single Unix Specification, @w{version 2}.  The value @math{600}
 105 (corresponding to the sixth revision) includes definitions from SUSv3,
 106 and using @math{700} (the seventh revision) includes definitions from
 107 SUSv4.
 108 @end defvr
 109
 110 @defvr Macro _LARGEFILE_SOURCE
 111 @standards{X/Open, (NONE)}
 112 If this macro is defined some extra functions are available which
 113 rectify a few shortcomings in all previous standards.  Specifically,
 114 the functions @code{fseeko} and @code{ftello} are available.  Without
 115 these functions the difference between the @w{ISO C} interface
 116 (@code{fseek}, @code{ftell}) and the low-level POSIX interface
 117 (@code{lseek}) would lead to problems.
 118
 119 This macro was introduced as part of the Large File Support extension (LFS).
 120 @end defvr
 121
 122 @defvr Macro _LARGEFILE64_SOURCE
 123 @standards{X/Open, (NONE)}
 124 If you define this macro an additional set of functions is made available
 125 which enables @w{32 bit} systems to use files of sizes beyond
 126 the usual limit of 2GB.  This interface is not available if the system
 127 does not support files that large.  On systems where the natural file
 128 size limit is greater than 2GB (i.e., on @w{64 bit} systems) the new
 129 functions are identical to the replaced functions.
 130
 131 The new functionality is made available by a new set of types and
 132 functions which replace the existing ones.  The names of these new objects
 133 contain @code{64} to indicate the intention, e.g., @code{off_t}
 134 vs. @code{off64_t} and @code{fseeko} vs. @code{fseeko64}.
 135
 136 This macro was introduced as part of the Large File Support extension
 137 (LFS).  It is a transition interface for the period when @w{64 bit}
 138 offsets are not generally used (see @code{_FILE_OFFSET_BITS}).
 139 @end defvr
 140
 141 @defvr Macro _FILE_OFFSET_BITS
 142 @standards{X/Open, (NONE)}
 143 This macro determines which file system interface shall be used, one
 144 replacing the other.  Whereas @code{_LARGEFILE64_SOURCE} makes the @w{64
 145 bit} interface available as an additional interface,
 146 @code{_FILE_OFFSET_BITS} allows the @w{64 bit} interface to
 147 replace the old interface.
 148
 149 If @code{_FILE_OFFSET_BITS} is undefined, or if it is defined to the
 150 value @code{32}, nothing changes.  The @w{32 bit} interface is used and
 151 types like @code{off_t} have a size of @w{32 bits} on @w{32 bit}
 152 systems.
 153
 154 If the macro is defined to the value @code{64}, the large file interface
 155 replaces the old interface.  I.e., the functions are not made available
 156 under different names (as they are with @code{_LARGEFILE64_SOURCE}).
 157 Instead the old function names now reference the new functions, e.g., a
 158 call to @code{fseeko} now indeed calls @code{fseeko64}.
 159
 160 This macro should only be selected if the system provides mechanisms for
 161 handling large files.  On @w{64 bit} systems this macro has no effect
 162 since the @code{*64} functions are identical to the normal functions.
 163
 164 This macro was introduced as part of the Large File Support extension
 165 (LFS).
 166 @end defvr
 167
 168 @defvr Macro _ISOC99_SOURCE
 169 @standards{GNU, (none)}
 170 If this macro is defined, features from ISO C99 are included.  Since
 171 these features are included by default, this macro is mostly relevant
 172 when the compiler uses an earlier language version.
 173 @end defvr
 174
 175 @defvr Macro _ISOC11_SOURCE
 176 @standards{C11, (none)}
 177 If this macro is defined, ISO C11 extensions to ISO C99 are included.
 178 @end defvr
 179
 180 @defvr Macro __STDC_WANT_LIB_EXT2__
 181 @standards{ISO, (none)}
 182 If you define this macro to the value @code{1}, features from ISO/IEC
 183 TR 24731-2:2010 (Dynamic Allocation Functions) are enabled.  Only some
 184 of the features from this TR are supported by @theglibc{}.
 185 @end defvr
 186
 187 @defvr Macro __STDC_WANT_IEC_60559_BFP_EXT__
 188 @standards{ISO, (none)}
 189 If you define this macro, features from ISO/IEC TS 18661-1:2014
 190 (Floating-point extensions for C: Binary floating-point arithmetic)
 191 are enabled.  Only some of the features from this TS are supported by
 192 @theglibc{}.
 193 @end defvr
 194
 195 @defvr Macro __STDC_WANT_IEC_60559_FUNCS_EXT__
 196 @standards{ISO, (none)}
 197 If you define this macro, features from ISO/IEC TS 18661-4:2015
 198 (Floating-point extensions for C: Supplementary functions) are
 199 enabled.  Only some of the features from this TS are supported by
 200 @theglibc{}.
 201 @end defvr
 202
 203 @defvr Macro __STDC_WANT_IEC_60559_TYPES_EXT__
 204 @standards{ISO, (none)}
 205 If you define this macro, features from ISO/IEC TS 18661-3:2015
 206 (Floating-point extensions for C: Interchange and extended types) are
 207 enabled.  Only some of the features from this TS are supported by
 208 @theglibc{}.
 209 @end defvr
 210
 211 @defvr Macro _GNU_SOURCE
 212 @standards{GNU, (none)}
 213 If you define this macro, everything is included: @w{ISO C89}, @w{ISO
 214 C99}, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions.  In
 215 the cases where POSIX.1 conflicts with BSD, the POSIX definitions take
 216 precedence.
 217 @end defvr
 218
 219 @defvr Macro _DEFAULT_SOURCE
 220 @standards{GNU, (none)}
 221 If you define this macro, most features are included apart from
 222 X/Open, LFS and GNU extensions: the effect is to enable features from
 223 the 2008 edition of POSIX, as well as certain BSD and SVID features
 224 without a separate feature test macro to control them.
 225
 226 Be aware that compiler options also affect included features:
 227
 228 @itemize
 229 @item
 230 If you use a strict conformance option, features beyond those from the
 231 compiler's language version will be disabled, though feature test
 232 macros may be used to enable them.
 233
 234 @item
 235 Features enabled by compiler options are not overridden by feature
 236 test macros.
 237 @end itemize
 238 @end defvr
 239
 240 @defvr Macro _ATFILE_SOURCE
 241 @standards{GNU, (none)}
 242 If this macro is defined, additional @code{*at} interfaces are
 243 included.
 244 @end defvr
 245
 246 @defvr Macro _FORTIFY_SOURCE
 247 @standards{GNU, (none)}
 248 If this macro is defined to @math{1}, security hardening is added to
 249 various library functions.  If defined to @math{2}, even stricter
 250 checks are applied.
 251 @end defvr
 252
 253 @defvr Macro _REENTRANT
 254 @defvrx Macro _THREAD_SAFE
 255 @standards{Obsolete, (none)}
 256 These macros are obsolete.  They have the same effect as defining
 257 @code{_POSIX_C_SOURCE} with the value @code{199506L}.
 258
 259 Some very old C libraries required one of these macros to be defined
 260 for basic functionality (e.g.@: @code{getchar}) to be thread-safe.
 261 @end defvr
 262
 263 We recommend you use @code{_GNU_SOURCE} in new programs.  If you don't
 264 specify the @samp{-ansi} option to GCC, or other conformance options
 265 such as @option{-std=c99}, and don't define any of these macros
 266 explicitly, the effect is the same as defining @code{_DEFAULT_SOURCE}
 267 to 1.
 268
 269 When you define a feature test macro to request a larger class of features,
 270 it is harmless to define in addition a feature test macro for a subset of
 271 those features.  For example, if you define @code{_POSIX_C_SOURCE}, then
 272 defining @code{_POSIX_SOURCE} as well has no effect.  Likewise, if you
 273 define @code{_GNU_SOURCE}, then defining either @code{_POSIX_SOURCE} or
 274 @code{_POSIX_C_SOURCE} as well has no effect.