When a child process creates its own windows, Presentation Manager messages can be used to signal events and/or pass information between the parent and child processes, provided the windows' handles are known to one another. One technique for passing window handles during process creation is described in Creating Another Process. Even where the child process does not create its own windows, it may use Presentation Manager messages to indicate events and pass information to its parent process.
The amount of information which can be passed in a Presentation Manager message is somewhat limited, due to the four-byte size of each message parameter. The use of Presentation Manager messages for passing information is therefore typically combined with other mechanisms such as shared memory or atoms. The message parameters are then used to carry pointers to shared memory objects, or string handles that are then used by the child process to access the required information.