# NAG Library Routine Document

## 1Purpose

x10bc_a1w_f sets the callback computational mode in a configuration data structure for the NAG AD Library as created by a prior call to x10aa_a1w_f. The full set of callback computational modes currently available are: nagad_primal, nagad_dstate, nagad_dparam and nagad_dall.

## 2Specification

Fortran Interface
 Subroutine x10bc_a1w_f ( mode,
 Integer, Intent (In) :: mode Integer, Intent (Inout) :: ifail Type (c_ptr), Intent (Inout) :: ad_handle
extern "C" {
 void x10bc_a1w_f_ (void *&ad_handle, const Integer &mode, Integer &ifail)
}
 Declare PtrSafe Sub X10BCF Lib "libname" ( _ ad_handle As LongPtr, mode As Long, ifail As Long)

## 3Description

x10bc_a1w_f sets the callback computational mode within a supplied procedure argument to a NAG AD Library routine. The callback mode is stored in the configuration data object as created by x10aa_a1w_f via a handle, and that handle is passed as first argument to any procedure argument.
Generally there is no need to set the callback computational mode using x10bc_a1w_f; rather, the callback computational mode is supplied to a procedure argument via the computational data object (ad_handle) and this callback mode can be extracted using x10bd_a1w_f. The mode determines what form of calculation must be performed by the procedure argument.
Please refer to x10bd_a1w_f for details on how to obtain the callback computational mode for a given procedure argument and what computations to perform in each case.

### 3.1Life Cycle of the Handle

Each handle should pass four stages in its life: initialization; mode setting; problem solution using the NAG AD Library; and, destruction.
The initialization by x10aa_a1w_f and destruction by x10ab_a1w_f mark the beginning and the end of the life of the handle. During this time the handle must only be modified by NAG AD Library routines. Working with a handle which has not been properly initialized is potentially very dangerous as it may cause unpredictable behaviour.
After the handle has been initialized, two routines are provided to set or get the computational mode to be used in algorithmic differentiation. x10ac_a1w_f sets the computational mode and x10ad_a1w_f gets the computational mode.
The handle is then passed to the computational routines of the NAG AD Library. The computational mode can be changed, where appropriate, between calls to computational routines.
During a call to a NAG AD Library routine that contains a procedure argument, a call back to that procedure argument will contain the callback computational mode within the computational data object (ad_handle). This mode can be obtained by a call to x10bd_a1w_f. The computation to be performed within the supplied procedure argument will be based on the value obtained for this mode.
When all AD computation is completed, the handle must be destroyed by x10ab_a1w_f.

None.

## 5Arguments

1:     $\mathbf{ad_handle}$ – Type (c_ptr)Input/Output
On entry: a handle to the AD configuration data object, as created by x10aa_a1w_f.
On exit: holds a handle to the internal data structure where the computational AD configuration data, is defined. You must not change the handle other than via NAG AD Library calls until it is destroyed by x10ab_a1w_f.
2:     $\mathbf{mode}$ – IntegerInput
On entry: the callback computational mode within the current supplied procedure argument.
${\mathbf{mode}}=\mathit{nagad_primal}$
The current callback is expected to perform the primal calculation.
${\mathbf{mode}}=\mathit{nagad_dstate}$
The current callback is expected to return the derivatives of the primal calculation with respect to real-valued input arguments.
${\mathbf{mode}}=\mathit{nagad_dparam}$
The current callback is expected to return the derivatives of the primal calculation with respect to user-supplied real-valued inputs (e.g., in ruser).
${\mathbf{mode}}=\mathit{nagad_dall}$
The current callback is expected to return derivatives of the primal calculation with respect to both active input arguments and active supplied parameters (i.e., combining nagad_dstate and nagad_dparam).
Constraint: ${\mathbf{mode}}=\mathit{nagad_primal}$, $\mathit{nagad_dstate}$, $\mathit{nagad_dparam}$ or $\mathit{nagad_dall}$.
3:     $\mathbf{ifail}$ – IntegerInput/Output
On entry: ifail must be set to $0$, . If you are unfamiliar with this argument you should refer to Section 3.4 in How to Use the NAG Library and its Documentation for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value  is recommended. If the output of error messages is undesirable, then the value $1$ is recommended. Otherwise, if you are not familiar with this argument, the recommended value is $0$. When the value  is used it is essential to test the value of ifail on exit.
On exit: ${\mathbf{ifail}}={\mathbf{0}}$ unless the routine detects an error or a warning has been flagged (see Section 6).

## 6Error Indicators and Warnings

If on entry ${\mathbf{ifail}}=0$ or $-1$, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
${\mathbf{ifail}}=1$
On entry, ad_handle is not a valid handle for the AD computational data object. Either ad_handle has not been initialized or it has become corrupted.
${\mathbf{ifail}}=2$
On entry, ${\mathbf{mode}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{mode}}=\mathit{nagad_primal}$, $\mathit{nagad_dstate}$, $\mathit{nagad_dparam}$ or $\mathit{nagad_dall}$.
${\mathbf{ifail}}=-99$
See Section 3.9 in How to Use the NAG Library and its Documentation for further information.
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
See Section 3.8 in How to Use the NAG Library and its Documentation for further information.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.
See Section 3.7 in How to Use the NAG Library and its Documentation for further information.

Not applicable.

## 8Parallelism and Performance

x10bc_a1w_f is not threaded in any implementation.