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OpenStep is an open object-oriented API specification, the result of a 1993 collaboration between NeXT Computer, and Sun Microsystems. OpenStep was based on the API of the NeXTSTEP operating system. The first draft was published by NeXT in summer 1994.

OpenStep contrasts with the earlier NeXTSTEP primarily in four ways:

  • OpenStep includes only the upper-level libraries and services (like Display PostScript), whereas NeXTSTEP referred to both these libraries and the operating system as well
  • removal of any code depending entirely on the Mach kernel, so that OpenStep could be run on top of any reasonably powerful operating system
  • a significant amount of effort was put into making the system "endian-free", an issue NeXT had already faced during a port of NeXTSTEP to the Intel platform
  • low-level objects such as strings were represented with C datatypes in NeXTSTEP, in OpenStep a number of new classes (NSString, NSNumber, etc.) were introduced to support endian-conversion as well as provide added functionality. This had ripple-effects throughout the API, mostly for the better. This code was collected in a new library called Foundation.

Sun never seemed terribly interested in the product, likely a result of NIH. In fact it's somewhat unclear why they were ever interested, although it appears it was an attempt to "get in" on the object world before Microsoft released its Cairo OS (which never happened). Nevertheless they started a port to Solaris some time in 1994, and released it in 1996. When Sun started work on Java, Solaris OpenStep was never seen again.

NeXT completed an implementation of OpenStep on their existing Mach-based OS and called it OPENSTEP. It was, for all intents, NeXTSTEP 4.0. It became their primary OS from 1995 on, and was used mainly on the Intel platform. In addition to being a complete OpenStep implementation, the system was delivered with a set of NeXTSTEP libraries for backward compatibility. This was an easy thing to do in OpenStep due to library versioning.

NeXT also delivered an implementation running on top of Windows NT 4.0 called OPENSTEP Enterprise (often abbreviated OSE). This allowed their existing customer base to continue using their tools and applications, but running them inside the Windows system which many of them were in the process of switching to. Never a clean match from the UI perspective, OSE nevertheless managed to work fairly well and allowed OpenStep to exist for perhaps another year.

The standardization on OpenStep also allowed for the creation of several new library packages that were delivered on the platform, including PDO (Portable Distributed Objects) which allowed for CORBA-like remote object invocation with almost zero code, and EOF (Enterprise Objects Framework), a tremendously powerful (for the time) object-relational mapping product. These tools became very popular in the enterprise market, notably in the financial sector where OpenStep caused something of a minor revolution.

OPENSTEP and OSE had two revisions (and one major one that was never released) before NeXT was purchased by Apple in 1997. This would end the commercial development of OpenStep for other platforms; GNUstep is an ongoing open source development.

At Apple, OPENSTEP/NeXTSTEP became the basis for their new operating system, Mac OS X, and OpenStep (the development libraries and tools) became Cocoa. As Mac OS X, OPENSTEP managed to become the most used Unix in the world.

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