# NAG Library Routine Document

## 1Purpose

x10ba_a1w_f creates a data object that can be used to pass data between procedure arguments (callbacks) of NAG AD Library computational routines and companion callbacks.

## 2Specification

Fortran Interface
 Subroutine x10ba_a1w_f (
 Integer, Intent (Inout) :: ifail Type (c_ptr), Intent (Out) :: cb_handle
extern "C" {
 void x10ba_a1w_f_ (void *&cb_handle, Integer &ifail)
}
 Declare PtrSafe Sub X10BAF Lib "libname" ( _ cb_handle As LongPtr, ifail As Long)

## 3Description

If a NAG AD Library routine contains a procedure argument (callback) which performs an operation on active variables, then that operation must also be differentiated. When the algorithmic computational mode is being used and the operation involves standard arithmetic operators and intrinsic functions then the differentiation is performed automatically. Similarly, even when in symbolic mode, algorithmic differentiation will be performed automatically for simple operators and functions used in supplied callbacks.
However, there are circumstances in which the algorithmic differentiation of a callback operation needs to be circumvented. Such a case would be where the symbolic differential of the operation is readily available and to be preferred to one calculated algorithmically. Another case is where the operation involves calls to, for example, NAG Library routines for which there is not yet a NAG AD Library equivalent routine. In such cases, algorithmic differentiation is circumvented by pushing the operations into a fixed interface companion callback; there can be several such companion callbacks. These companion callbacks have a single argument: a handle to a callback data object. x10ba_a1w_f creates this callback data object.
A callback data object created by x10ba_a1w_f is used to: pass data to the companion callback; set primal calculation values or derivatives; and, pass the location of the companion callback so that it may be called internally during adjoint evaluations.
Within a user-supplied procedure argument, the companion callbacks perform different tasks as requested: primal evaluation of output arguments; differentiation of output arguments with respect to real-valued input arguments; and, differentiation with respect to any user-supplied values (i.e., in the ruser array in c05ay_a1w_f).
Where algorithmic differentiation is to be circumvented with a supplied procedure argument, x10ba_a1w_f should be called to create a callback data object inside the supplied procedure. x10bd_a1w_f will return which mode of operation should be performed. A separate companion callback can be created for each mode of operation or the mode can be passed to a single companion callback to handle all three (potentially) cases.
Input data should be written to the callback data object using one of x10be_a1w_f, x10bf_a1w_f or x10bj_a1w_f depending on input data type. The companion callback will extract data from the callback data object, perform the required operations, and update the global data object with primal values or derivative increments.

None.

## 5Arguments

1:     $\mathbf{cb_handle}$ – Type (c_ptr)Output
On exit: will contain a handle to the callback data object.
2:     $\mathbf{ifail}$ – 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:
${\mathbf{ifail}}=-99$
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

x10ba_a1w_f is not threaded in any implementation.

None.

## 10Example

The following example shows the procedure for calling a NAG AD Library routine (s01ba_a1w_f). x10aa_a1w_f is called to initialize the ad_handle; x10ac_a1w_f and x10ad_a1w_f are called to set and get the computational mode from the handle; and x10ab_a1w_f is called to destroy the handle.
 Language Source File Data Results Fortan x10ba_a1w_fe.f90 None x10ba_a1w_fe.r