Mac OS 8 Developer Documentation
You can use the Program-to-Program Communications (PPC) Toolbox to send and receive low-level message blocks between applications. The PPC Toolbox can be used by multiple applications running on the same computer or across a network.
Inside Macintosh:Interapplication Communication
(Chapter 11 - Program-to-Program Communications Toolbox)
This document describes the parts of the Macintosh system software that allow your application to communicate and work with other applications.
describes how to use the Program-to-Program Communications Toolbox and includes an overview, a tutorial, and a complete API reference.
Mac OS X