Language Environment in conjunction with z/OS UNIX supports POSIX standards (POSIX 1003.1 and POSIX 1003.1c) and the XPG4 standard. The POSIX standard follows a program model which differs somewhat from the Language Environment program model. This section provides a helpful comparison of both models.

The following descriptions are intended to be a brief review for C users of the characteristics of POSIX program entities. For full definitions of these terms, refer to the ISO/IEC9945 for POSIX 1003.1 and POSIX 1003.1c. The XPG4 standard is described in detail in X/Open Specification Issue 4.