The following information is provided as is, and the authors take no responsibility for the correctness.
Print processors are user-mode DLLs that are responsible for converting a print job's spooled data into a format that can be sent to a print monitor. They are also responsible for handling application requests to pause, resume and cancel print jobs.
The print job's spooled data is contained in a spool file. The print processor reads the file, performs conversion operations on the data stream, and writes the converted data to the spooler. The spooler then sends the data stream to the appropriate print monitor.
A set of user-mode GDI functions is exported by gdi32.dll, for use by print processors that handle NT-based-operating system EMF as an input format. The following table lists the functions that are provided.
Access to the data of a SPL - Microsoft® Windows Spool File Format spool file, can be implemented quiet easy from within a print processor.
This is described on the Windows Spool Files page.