Note: this routine is made available for users of the NAG AD Library who are not using dco/c++.

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## 1Purpose

x10bc_AD_f sets the callback computational mode in a configuration data structure for the NAG AD Library as created by a prior call to x10aa_AD_f. The full set of callback computational modes currently available are: nagad_primal, nagad_dstate, nagad_dparam and nagad_dall.

## 2Specification

Fortran Interface
 Integer, Intent (In) :: mode Integer, Intent (Inout) :: ifail Type (c_ptr), Intent (Inout) :: ad_handle
To be consistent with the naming scheme for other routines in the AD Library, five routines are available with names formed by replacing AD in the above by one of p0w, a1w, t1w, a1t1w or t2w.
extern "C" {
}
To be consistent with the naming scheme for other functions in the AD Library, five functions are available with names formed by replacing AD in the above by one of p0w, a1w, t1w, a1t1w or t2w.

## 3Description

x10bc_AD_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_AD_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_AD_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_AD_f. The mode determines what form of calculation must be performed by the procedure argument.
Please refer to x10bd_AD_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_AD_f and destruction by x10ab_AD_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_AD_f sets the computational mode and x10ad_AD_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 configuration data object (ad_handle). This mode can be obtained by a call to x10bd_AD_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_AD_f.

None.

## 5Arguments

1: $\mathbf{ad_handle}$Pointer to AD Data Input/Output
On entry: a handle to the AD configuration data object, as created by x10aa_AD_f.
On exit: holds a handle to the internal data structure where the callback computational mode was changed according to mode. You must not change the handle other than via NAG AD Library calls until it is destroyed by x10ab_AD_f.
2: $\mathbf{mode}$Integer Input
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: $\mathbf{ifail}$Integer Input/Output
On entry: ifail must be set to $0$, $-1$ or $1$ to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of $0$ causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of $-1$ means that an error message is printed while a value of $1$ means that it is not.
If halting is not appropriate, the value $-1$ or $1$ is recommended. If message printing is undesirable, then the value $1$ is recommended. Otherwise, the value $0$ is recommended. When the value $-\mathbf{1}$ or $\mathbf{1}$ 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 7 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

Not applicable.