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X/Open Company, Ltd., originally the Open Group for Unix Systems,[1] was a consortium founded by several European UNIX systems manufacturers in 1984 to identify and promote open standards in the field of information technology. More specifically, the original aim was to define a single specification for operating systems derived from UNIX, to increase the interoperability of applications and reduce the cost of porting software. Its original members were Bull, ICL, Siemens, Olivetti, and Nixdorf—a group sometimes referred to as BISON. Philips and Ericsson joined soon afterwards, at which point the name X/Open was adopted.

The group published its specifications under the name X/Open Portability Guide (or XPG). Issue 1 covered basic operating system interfaces, and was published within a year of the group's formation. Issue 2 followed in 1987, and extended the coverage to include Internationalization, Terminal Interfaces, Inter-Process Communication, and the programming languages C, COBOL, FORTRAN, and Pascal, as well as data access interfaces for SQL and ISAM. In many cases these were profiles of existing international standards. XPG3 followed in 1988, its primary focus being convergence with the POSIX operating system specifications. This was probably the most widely used and influential deliverable of the X/Open organisation.

By 1990 the group had expanded to 21 members: in addition to the original five, Philips and Nokia from Europe; AT&T Corporation, Digital, Unisys, Hewlett-Packard, IBM, NCR, Sun Microsystems, Prime Computer, Apollo Computer from North America; Fujitsu, Hitachi, and NEC from Japan; plus the Open Software Foundation and Unix International.

X/Open managed the UNIX trademark from 1993 to 1996, when it merged with the Open Software Foundation to form The Open Group.

X/Open was also responsible for the XA protocol for heterogeneous distributed transaction processing, which was released in 1991.[2]


The X/Open Portability Guide is a standard for UNIX systems originally published by X/Open Company Ltd. Based on the AT&T System V Interface Definition,[3] it has a wider scope than POSIX, which is only concerned with direct operating system interfaces.

The Portability Guide specifies a Common Application Environment (CAE) intended to allow portability of applications across operating systems. The primary aim was compatibility between different vendors' implementations of UNIX, though some vendors also implemented the standards on non-UNIX platforms. The XPG3 and XPG4 standards, released in 1989 and 1992 respectively, define all aspects of the operating system, programming languages and protocols which compliant systems should have.

The last version of the XPG, the X/Open Portability Guide Issue 4 (also known as the Common Applications Environment Specification Issue 4 (CAE4)), was published in July 1992 by The Open Group. The Single UNIX Specification was based on the XPG4 standard.


The XPG4 specification includes these chapters:

  • System Interfaces and Headers (XSH), Issue 4, ISBN 1-872630-47-2, C202
  • Commands and Utilities (XCU), Issue 4, ISBN 1-872630-48-0, C203
  • System Interface Definitions (XBD), Issue 4, ISBN 1-872630-46-4, C204

See also[edit]


  1. ^ Kornel, Amiel (3 February 1986). "Unix advancing in drive toward European market acceptance". Computerworld. p. 51.
  2. ^ Kleppmann, Martin (April 2, 2017). Designing Data-Intensive Applications (1 ed.). O'Reilly Media. p. 361. ISBN 978-1449373320.
  3. ^ Libes, Don; Ressler, Sandy; Ressler, Sanford (1989). Life With UNIX: A Guide For Everyone. Prentice Hall. p. 74.

External links[edit]