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## 1Purpose

x10ad_AD_f gets the computational mode from a configuration data structure for the NAG AD Library as created by a prior call to x10aa_AD_f and where the mode has been set using a prior call to x10ac_AD_f. The full set of computational modes currently available are: nagad_algorithmic, nagad_symbolic and nagad_symbolic_expert. See mode. This routine must be called after a call to x10aa_AD_f (to create a handle to the configuration data object).

## 2Specification

Fortran Interface
 Integer, Intent (Inout) :: ifail Integer, Intent (Out) :: mode Type (c_ptr), Intent (In) :: 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.
namespace nag {
}
}
This function is applicable whichever mode of AD is being used.

## 3Description

x10ad_AD_f gets the computational mode of differentiation from the configuration data object as created by x10aa_AD_f and where the computational mode has been set by a call to x10ac_AD_f (see mode). This handle may then be passed to any computational routine in the NAG AD Library as listed in the Introduction to the NAG AD Library.

### 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. If, for example, a callback needs to know the computational mode in order to provide differentials appropriately, x10ad_AD_f will return the current computational mode as stored in the ad_handle.
The handle is then passed to the computational routines of the NAG AD Library.
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
On entry: contains a handle to the AD configuration data object in which the computational mode is defined.
2: $\mathbf{mode}$Integer Output
On exit: the computational mode as stored in the AD configuration data object with handle ad_handle. The returned value will be one of $\mathit{nagad_algorithmic}$, $\mathit{nagad_symbolic}$ or $\mathit{nagad_symbolic_expert}$.
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.
${\mathbf{ifail}}=2$
Unexpected value of mode set on exit, ${\mathbf{mode}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{mode}}=\mathit{nagad_algorithmic}$, $\mathit{nagad_symbolic}$ or $\mathit{nagad_symbolic_expert}$.
${\mathbf{ifail}}=-99$
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.