In addition to the directive set described, OpenMP provides a callable runtime library (RTL) and accompanying environment variables. The RTL includes query functions, runtime functions and lock functions. The runtime functions allow an application to specify the mode in which to run. An application developer may wish to maximize throughput performance of the system, rather than time to completion. In such cases, the developer may tell the system to dynamically set the number of processes used to execute parallel regions. This can have a dramatic effect on the throughput performance of a system with only a minimal impact on the time to completion for a program.
The runtime functions also allow a developer to specify when to enable nested parallelism. Enabling nested parallelism allows the system act accordingly when it encounters a nested PARALLEL construct. On the other hand, by disabling nested parallelism a developer can write a parallel library that will perform in an easily predictable fashion whether encountered dynamically from within or outside a parallel region.
OpenMP also provides a conditional compilation facility both through CPP and with a built in sentinel. This allows calls to the runtime library to be protected as compiler directives, so OpenMP code can be compiled on non-OpenMP systems without linking in a stub library or using some other awkward workaround.
OpenMP provides standard environment variables to accompany the RTL functions where it makes sense. This again is important to application developers who, in addition to creating portable applications also need a portable runtime environment. A standard set of environment variables greatly simplifies the start up scripts for portable applications.