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_CALLBACKimplementation
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.