The X/Open Portability Guide which is published by the
X/Open Company, is a more general standard than
POSIX for
Unix-like systems. X/Open owns the Unix trademark, and the Portability Guide specifies the requirements for systems which are intended to be a Unix system, or Common Application Environment (CAE). The XPG3 and XPG4 standards, released on 1989 and 1992 respectively, define all aspects of the operating system, programming languages and protocols which compliant systems should have. The
Single UNIX Specification was based on the XPG4 standard.
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