x06agc, for multithreaded implementations, enables or disables the nesting of OpenMP parallel regions by setting an OpenMP Internal Control Variable (ICV) and any vendor library specific options where that is possible. See the Users' Note for your implementation for details of the scope of x06agc.

Nesting is disabled by default in OpenMP.

In serial implementations of the NAG Library x06agc has no effect. See the Chapter X06 for a discussion of the behaviour of these functions when called in serial.

4 References

The OpenMP API Specification for Parallel Programming

Chapman B, Jost G and van der Pas R (2008) Using OpenMP Portable Shared Memory Parallel Programming The MIT Press

5 Arguments

1: $nesting$ – Integer Input: On entry: if $nesting = 0$ , nesting of OpenMP parallel regions is disabled, otherwise it is enabled.

6 Error Indicators and Warnings

None.

7 Accuracy

Not applicable.

8 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.

x06agc is not threaded in any implementation.

9 Further Comments

None.

10 Example

In this example we presume a multithreaded implementation of the NAG Library. We call x06agc to enable the nesting of OpenMP parallel regions by setting an ICV. Inside a first, or ‘outer’, parallel region we call x06ahc to display the status of nesting, and expect to see it return the value

1

Each thread then spawns a second ‘inner’ parallel region. Within this region we have each thread increment the value total. As there are

5

threads in the outer region and

3

threads in each inner region we expect the final value of this variable to be

15

If you use a serial implementation of the NAG Library, regardless of whether the code is compiled with OpenMP or not, x06agc will have no effect and x06ahc will always return