manual/creature.texi

   1 @node Feature Test Macros
   2 @subsection Feature Test Macros
   3 @include macros.texi
   4
   5 @cindex feature test macros
   6 The exact set of features available when you compile a source file
   7 is controlled by which @dfn{feature test macros} you define.
   8
   9 If you compile your programs using @samp{gcc -ansi}, you get only the
  10 @w{ISO C} library features, unless you explicitly request additional
  11 features by defining one or more of the feature macros.
  12 @xref{Invoking GCC,, GNU CC Command Options, gcc.info, The GNU CC Manual},
  13 for more information about GCC options.@refill
  14
  15 You should define these macros by using @samp{#define} preprocessor
  16 directives at the top of your source code files.  These directives
  17 @emph{must} come before any @code{#include} of a system header file.  It
  18 is best to make them the very first thing in the file, preceded only by
  19 comments.  You could also use the @samp{-D} option to GCC, but it's
  20 better if you make the source files indicate their own meaning in a
  21 self-contained way.
  22
  23 This system exists to allow the library to conform to multiple standards.
  24 Although the different standards are often described as supersets of each
  25 other, they are usually incompatible because larger standards require
  26 functions with names that smaller ones reserve to the user program.  This
  27 is not mere pedantry --- it has been a problem in practice.  For instance,
  28 some non-GNU programs define functions named @code{getline} that have
  29 nothing to do with this library's @code{getline}.  They would not be
  30 compilable if all features were enabled indiscriminately.
  31
  32 This should not be used to verify that a program conforms to a limited
  33 standard.  It is insufficient for this purpose, as it will not protect you
  34 from including header files outside the standard, or relying on semantics
  35 undefined within the standard.
  36
  37 @comment (none)
  38 @comment POSIX.1
  39 @defvr Macro _POSIX_SOURCE
  40 If you define this macro, then the functionality from the POSIX.1
  41 standard (IEEE Standard 1003.1) is available, as well as all of the
  42 @w{ISO C} facilities.
  43
  44 The state of @code{_POSIX_SOURCE} is irrelevant if you define the
  45 macro @code{_POSIX_C_SOURCE} to a positive integer.
  46 @end defvr
  47
  48 @comment (none)
  49 @comment POSIX.2
  50 @defvr Macro _POSIX_C_SOURCE
  51 Define this macro to a positive integer to control which POSIX
  52 functionality is made available.  The greater the value of this macro,
  53 the more functionality is made available.
  54
  55 If you define this macro to a value greater than or equal to @code{1},
  56 then the functionality from the 1990 edition of the POSIX.1 standard
  57 (IEEE Standard 1003.1-1990) is made available.
  58
  59 If you define this macro to a value greater than or equal to @code{2},
  60 then the functionality from the 1992 edition of the POSIX.2 standard
  61 (IEEE Standard 1003.2-1992) is made available.
  62
  63 If you define this macro to a value greater than or equal to @code{199309L},
  64 then the functionality from the 1993 edition of the POSIX.1b standard
  65 (IEEE Standard 1003.1b-1993) is made available.
  66
  67 Greater values for @code{_POSIX_C_SOURCE} will enable future extensions.
  68 The POSIX standards process will define these values as necessary, and
  69 @theglibc{} should support them some time after they become standardized.
  70 The 1996 edition of POSIX.1 (ISO/IEC 9945-1: 1996) states that
  71 if you define @code{_POSIX_C_SOURCE} to a value greater than
  72 or equal to @code{199506L}, then the functionality from the 1996
  73 edition is made available.
  74 @end defvr
  75
  76 @comment (none)
  77 @comment GNU
  78 @defvr Macro _BSD_SOURCE
  79 If you define this macro, functionality derived from 4.3 BSD Unix is
  80 included as well as the @w{ISO C}, POSIX.1, and POSIX.2 material.
  81
  82 Some of the features derived from 4.3 BSD Unix conflict with the
  83 corresponding features specified by the POSIX.1 standard.  If this
  84 macro is defined, the 4.3 BSD definitions take precedence over the
  85 POSIX definitions.
  86
  87 Due to the nature of some of the conflicts between 4.3 BSD and POSIX.1,
  88 you need to use a special @dfn{BSD compatibility library} when linking
  89 programs compiled for BSD compatibility.  This is because some functions
  90 must be defined in two different ways, one of them in the normal C
  91 library, and one of them in the compatibility library.  If your program
  92 defines @code{_BSD_SOURCE}, you must give the option @samp{-lbsd-compat}
  93 to the compiler or linker when linking the program, to tell it to find
  94 functions in this special compatibility library before looking for them in
  95 the normal C library.
  96 @pindex -lbsd-compat
  97 @pindex bsd-compat
  98 @cindex BSD compatibility library.
  99 @end defvr
 100
 101 @comment (none)
 102 @comment GNU
 103 @defvr Macro _SVID_SOURCE
 104 If you define this macro, functionality derived from SVID is
 105 included as well as the @w{ISO C}, POSIX.1, POSIX.2, and X/Open material.
 106 @end defvr
 107
 108 @comment (none)
 109 @comment X/Open
 110 @defvr Macro _XOPEN_SOURCE
 111 @comment (none)
 112 @comment X/Open
 113 @defvrx Macro _XOPEN_SOURCE_EXTENDED
 114 If you define this macro, functionality described in the X/Open
 115 Portability Guide is included.  This is a superset of the POSIX.1 and
 116 POSIX.2 functionality and in fact @code{_POSIX_SOURCE} and
 117 @code{_POSIX_C_SOURCE} are automatically defined.
 118
 119 As the unification of all Unices, functionality only available in
 120 BSD and SVID is also included.
 121
 122 If the macro @code{_XOPEN_SOURCE_EXTENDED} is also defined, even more
 123 functionality is available.  The extra functions will make all functions
 124 available which are necessary for the X/Open Unix brand.
 125
 126 If the macro @code{_XOPEN_SOURCE} has the value @math{500} this includes
 127 all functionality described so far plus some new definitions from the
 128 Single Unix Specification, @w{version 2}.
 129 @end defvr
 130
 131 @comment (NONE)
 132 @comment X/Open
 133 @defvr Macro _LARGEFILE_SOURCE
 134 If this macro is defined some extra functions are available which
 135 rectify a few shortcomings in all previous standards.  Specifically,
 136 the functions @code{fseeko} and @code{ftello} are available.  Without
 137 these functions the difference between the @w{ISO C} interface
 138 (@code{fseek}, @code{ftell}) and the low-level POSIX interface
 139 (@code{lseek}) would lead to problems.
 140
 141 This macro was introduced as part of the Large File Support extension (LFS).
 142 @end defvr
 143
 144 @comment (NONE)
 145 @comment X/Open
 146 @defvr Macro _LARGEFILE64_SOURCE
 147 If you define this macro an additional set of functions is made available
 148 which enables @w{32 bit} systems to use files of sizes beyond
 149 the usual limit of 2GB.  This interface is not available if the system
 150 does not support files that large.  On systems where the natural file
 151 size limit is greater than 2GB (i.e., on @w{64 bit} systems) the new
 152 functions are identical to the replaced functions.
 153
 154 The new functionality is made available by a new set of types and
 155 functions which replace the existing ones.  The names of these new objects
 156 contain @code{64} to indicate the intention, e.g., @code{off_t}
 157 vs. @code{off64_t} and @code{fseeko} vs. @code{fseeko64}.
 158
 159 This macro was introduced as part of the Large File Support extension
 160 (LFS).  It is a transition interface for the period when @w{64 bit}
 161 offsets are not generally used (see @code{_FILE_OFFSET_BITS}).
 162 @end defvr
 163
 164 @comment (NONE)
 165 @comment X/Open
 166 @defvr Macro _FILE_OFFSET_BITS
 167 This macro determines which file system interface shall be used, one
 168 replacing the other.  Whereas @code{_LARGEFILE64_SOURCE} makes the @w{64
 169 bit} interface available as an additional interface,
 170 @code{_FILE_OFFSET_BITS} allows the @w{64 bit} interface to
 171 replace the old interface.
 172
 173 If @code{_FILE_OFFSET_BITS} is undefined, or if it is defined to the
 174 value @code{32}, nothing changes.  The @w{32 bit} interface is used and
 175 types like @code{off_t} have a size of @w{32 bits} on @w{32 bit}
 176 systems.
 177
 178 If the macro is defined to the value @code{64}, the large file interface
 179 replaces the old interface.  I.e., the functions are not made available
 180 under different names (as they are with @code{_LARGEFILE64_SOURCE}).
 181 Instead the old function names now reference the new functions, e.g., a
 182 call to @code{fseeko} now indeed calls @code{fseeko64}.
 183
 184 This macro should only be selected if the system provides mechanisms for
 185 handling large files.  On @w{64 bit} systems this macro has no effect
 186 since the @code{*64} functions are identical to the normal functions.
 187
 188 This macro was introduced as part of the Large File Support extension
 189 (LFS).
 190 @end defvr
 191
 192 @comment (none)
 193 @comment GNU
 194 @defvr Macro _ISOC99_SOURCE
 195 Until the revised @w{ISO C} standard is widely adopted the new features
 196 are not automatically enabled.  @Theglibc{} nevertheless has a complete
 197 implementation of the new standard and to enable the new features the
 198 macro @code{_ISOC99_SOURCE} should be defined.
 199 @end defvr
 200
 201 @comment (none)
 202 @comment GNU
 203 @defvr Macro _GNU_SOURCE
 204 If you define this macro, everything is included: @w{ISO C89}, @w{ISO
 205 C99}, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions.  In
 206 the cases where POSIX.1 conflicts with BSD, the POSIX definitions take
 207 precedence.
 208
 209 If you want to get the full effect of @code{_GNU_SOURCE} but make the
 210 BSD definitions take precedence over the POSIX definitions, use this
 211 sequence of definitions:
 212
 213 @smallexample
 214 #define _GNU_SOURCE
 215 #define _BSD_SOURCE
 216 #define _SVID_SOURCE
 217 @end smallexample
 218
 219 Note that if you do this, you must link your program with the BSD
 220 compatibility library by passing the @samp{-lbsd-compat} option to the
 221 compiler or linker.  @strong{NB:} If you forget to do this, you may
 222 get very strange errors at run time.
 223 @end defvr
 224
 225 @comment (none)
 226 @comment GNU
 227 @defvr Macro _REENTRANT
 228 @defvrx Macro _THREAD_SAFE
 229 If you define one of these macros, reentrant versions of several functions get
 230 declared.  Some of the functions are specified in POSIX.1c but many others
 231 are only available on a few other systems or are unique to @theglibc{}.
 232 The problem is the delay in the standardization of the thread safe C library
 233 interface.
 234
 235 Unlike on some other systems, no special version of the C library must be
 236 used for linking.  There is only one version but while compiling this
 237 it must have been specified to compile as thread safe.
 238 @end defvr
 239
 240 We recommend you use @code{_GNU_SOURCE} in new programs.  If you don't
 241 specify the @samp{-ansi} option to GCC and don't define any of these
 242 macros explicitly, the effect is the same as defining
 243 @code{_POSIX_C_SOURCE} to 2 and @code{_POSIX_SOURCE},
 244 @code{_SVID_SOURCE}, and @code{_BSD_SOURCE} to 1.
 245
 246 When you define a feature test macro to request a larger class of features,
 247 it is harmless to define in addition a feature test macro for a subset of
 248 those features.  For example, if you define @code{_POSIX_C_SOURCE}, then
 249 defining @code{_POSIX_SOURCE} as well has no effect.  Likewise, if you
 250 define @code{_GNU_SOURCE}, then defining either @code{_POSIX_SOURCE} or
 251 @code{_POSIX_C_SOURCE} or @code{_SVID_SOURCE} as well has no effect.
 252
 253 Note, however, that the features of @code{_BSD_SOURCE} are not a subset of
 254 any of the other feature test macros supported.  This is because it defines
 255 BSD features that take precedence over the POSIX features that are
 256 requested by the other macros.  For this reason, defining
 257 @code{_BSD_SOURCE} in addition to the other feature test macros does have
 258 an effect: it causes the BSD features to take priority over the conflicting
 259 POSIX features.