Comparing OpenMP MPI Programming Linux Threads Pthread LIbrary: In 1997, a group of vendors came together under the aegis of hardware manufacturer, Silicon Graphics, to formulate a new threading interface. Their common problem was that the primary operating systems of the time all imposed drastically different ways of programming for threads. UNIX employed Pthreads, Sun used Solaris threads, Windows used its own API, and Linux used Linux threads (until its subsequent adoption of Pthreads).

---

What is OpenMP?

In 1997, a group of vendors came together under the aegis of hardware manufacturer, Silicon Graphics, to formulate a new threading interface. Their common problem was that the primary operating systems of the time all imposed drastically different ways of programming for threads. UNIX employed Pthreads, Sun used Solaris threads, Windows used its own API, and Linux used Linux threads (until its subsequent adoption of Pthreads). The committee wanted to design an API that would enable a codebase to run without changes equally well on Windows and UNIX/Linux. In 1998, it delivered the first API specification of what was called OpenMP (In those days, the term ‘open’ was associated with the concept of support from multiple vendors—as in open systems—rather than with today’s implication of open source.)

The OpenMP specification consists of APIs, a set of pragmas, and several settings for OpenMP-specific environment variables. As further revisions have been made to the standard, it has become clear that one of OpenMP’s most useful feature is its set of pragmas. By judicious use of these pragmas, a single-threaded program can be made multithreaded without recourse to APIs or environment variables. With the recent release of OpenMP 2.0, the OpenMP Architecture Review Board (ARB), which is the official name for the committee that formulates the OpenMP specification, made clear its preference that developers use the pragmas, rather than the APIs. Let’s examine this approach in greater depth, starting with a recap of what pragmas are.