NOTES

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