# NAG Library Routine Document

## 1Purpose

x10cf_a1w_f reads a real value from a callback data object created by x10ba_a1w_f.

## 2Specification

Fortran Interface
 Subroutine x10cf_a1w_f ( x,
 Integer, Intent (Inout) :: ifail Real (Kind=nag_wp), Intent (Out) :: x Type (c_ptr), Intent (Inout) :: cb_handle
extern "C" {
 void x10cf_a1w_f_ (void *&cb_handle, double &x, Integer &ifail)
}
 Declare PtrSafe Sub X10CFF Lib "libname" ( _ cb_handle As LongPtr, x As Double, ifail As Long)

## 3Description

For routines where it is necessary to perform callback operations via one or more companion callbacks, data must be passed between the procedure argument and any such companion callback via a callback data object as created by x10ba_a1w_f. x10cf_a1w_f can be used to extract a real value from the callback data object. In this way real values can be communicated between a procedure argument and its companion callbacks.
Note that writing to and reading from a callback data object must be performed in strict order without omission. Reading a data item from the object is equivalent to removing that item from a data stack; reading is therefore not passive, but updates the callback data object.

None.

## 5Arguments

1:     $\mathbf{cb_handle}$ – Type (c_ptr)Input/Output
On entry: will contain a handle to the callback data object.
On exit: will contain a handle to the updated callback data object, with data item removed from data stack.
2:     $\mathbf{x}$ – Real (Kind=nag_wp)Output
On exit: a real value, $x$, to be extracted from the callback data object.
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, cb_handle does not define a valid handle to a NAG AD callback data object. It has not been initialized or has been corrupted.
${\mathbf{ifail}}=-99$
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

x10cf_a1w_f is not threaded in any implementation.