About the Printing Functions(IM:XP)

Inside Macintosh: QuickDraw GX Printing Extensions and Drivers /: Chapter 5 - Printing Functions for Message Overrides

About the Printing Functions
You can only call the printing functions from within your implementations of printing message overrides. You call these functions to perform certain tasks while overriding a printing message. These functions are implemented by QuickDraw GX, just like the QuickDraw GX functions that you use to display shapes.

WARNING
You can only call the functions that are described in this chapter from within an override of a QuickDraw GX printing message. If you call one of these functions in any other context, your program will crash.
In its implementation of some of these functions, QuickDraw GX sends a corresponding printing message to the handlers in the message chain. For example, you can call the GXJobIdle function, the implementation of which sends the GXJobIdle message to the top of the message chain. Each message handler can then respond to the message
as required.
The printing cleanup functions work in a slighly different manner. The QuickDraw GX implementation of each of the printing cleanup functions sends a corresponding printing message; however, the message is sent to the next handler in the chain, not to the top
of the chain. For example, when you call the GXCleanupOpenConnection function, QuickDraw GX sends the GXCleanupOpenConnection message to the handler after your printing extension or printer driver in the message chain. QuickDraw GX implements the printing cleanup functions in this way to ensure that cleanup messages are sent to the same handlers in the same order as were the original printing messages.