XPG (The X/Open Portability Guide)
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The X/Open Portability Guide, published by the X/Open Company, Ltd.,
is a more general standard than POSIX. X/Open owns the Unix copyright
and the XPG specifies the requirements for systems which are intended
to be a Unix system.
The GNU C library complies to the X/Open Portability Guide, Issue
4.2, with all extensions common to XSI (X/Open System Interface)
compliant systems and also all X/Open UNIX extensions.
The additions on top of POSIX are mainly derived from functionality
available in System V and BSD systems. Some of the really bad mistakes
in System V systems were corrected, though. Since fulfilling the XPG
standard with the Unix extensions is a precondition for getting the
Unix brand chances are good that the functionality is available on
commercial systems.