# PR_CALLBACKimplementation

Used to define pointers to functions that will be implemented by the
client but called from a (different) shared library.

## Syntax

```{code}
#include <prtypes.h>type PR_CALLBACKimplementation
```

## Description

Functions that are implemented in an application (or shared library)
that are intended to be called from another shared library (such as
NSPR) must be declared with the `PR_CALLBACK` attribute. Normally such
functions are passed by reference (pointer to function). The
`PR_CALLBACK` attribute is included as part of the function's
definition between its return value type and the function's name.