## 1Purpose

x06aac sets the number of threads requested for subsequent OpenMP parallel regions.

## 2Specification

 #include
 void x06aac (Integer num, NagError *fail)
The function may be called by the names: x06aac or nag_omp_set_num_threads.

## 3Description

x06aac, for multithreaded implementations, sets the number of threads to be requested for subsequent parallel regions to num. The first element of the list held by the OpenMP Internal Control Variable (ICV) used in determining the number of threads is set. See the Users' Note for your implementation for details of the scope of this function.
The number of threads used in parallel regions will be equal to, or less than, the value of the ICV. The actual number of threads used is dependent on several factors, such as the presence of a num_threads clause on the parallel directive or the number of threads already in use by the program. Please refer to Section 4 for a full description of how the number of threads is chosen for a particular parallel region.
In serial implementations of the NAG Library this function has no effect. See the X06 Chapter Introduction for a discussion of the behaviour of these functions when called in serial.

## 4References

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

## 5Arguments

1: $\mathbf{num}$Integer Input
On entry: the number of threads requested for subsequent OpenMP parallel regions.
Constraint: ${\mathbf{num}}\ge 1$.
2: $\mathbf{fail}$NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

## 6Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
NE_INT
On entry, ${\mathbf{num}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{num}}\ge 1$.
NE_INTERNAL_ERROR
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
See Section 7.5 in the Introduction to the NAG Library CL Interface for further information.
NE_NO_LICENCE
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface for further information.

Not applicable.

## 8Parallelism and Performance

x06aac is not threaded in any implementation.

None.

## 10Example

In this example we presume a multithreaded implementation of the NAG Library. We set the OpenMP Internal Control Variable used in determining the number of threads to $5$ with a call to x06aac and retrieve it again with x06acc.
We then, using x06abc, display the number of threads in use both outside, and inside, the OpenMP parallel region.
We expect to see x06abc returning $1$ outside of the parallel region, as the current team of threads there will consist of a single thread, and $5$ from within it.
If you use a serial implementation of the NAG Library, regardless of whether the code is compiled with OpenMP or not, calling x06aac has no effect and x06abc and x06acc will always return $1$. The appropriate results file will be included with the distribution material for your implementation.

### 10.1Program Text

Program Text (x06aace.c)

None.

### 10.3Program Results

Program Results (x06aace.r)