Used to define pointers to functions that will be implemented by the client but called from a (different) shared library.
#include <prtypes.h> type PR_CALLBACK implementation
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.