Feature-Test_Macros

   Feature-test macros provide a means for writing portable
   programs. They ensure that the C RTL symbolic names used
   by a program do not clash with the symbolic names supplied by the
   implementation.

   The C RTL header files are coded to support the use of
   a number of feature-test macros. When an application defines a
   feature-test macro, the C RTL header files supply the
   symbols and prototypes defined by that feature-test macro and
   nothing else. If a program does not define such a macro, the
   C RTL header files define symbols without restriction.

   The feature-test macros supported by the C RTL fall into
   the following broad categories for controlling the visibility of
   symbols in header files according to the following:

   o  Standards

   o  Multiple-version support

   o  Compatibility
 

Additional information available:

Standards_Macros                Selecting_a_Standard
Multiple-Version-Support_Macro  Compatibility_Modes
Curses_and_Socket_Compatibility_Macros          2_GB_File_Size_Macro
32-Bit_UID_and_GID_Macro_(Integrity_servers,_Alpha)_
Standard-Compliant_stat_Structure_(Integrity_servers,_Alpha)_
Using_Legacy__toupper_and__tolower_Behavior_(Integrity_servers,_Alpha)_
Using_Faster,_Inlined_Put_and_Get_Functions_(Integrity_servers,_Alpha)_
POSIX_Style_exit_(Integrity_servers,_Alpha)_