Callbacks are user-supplied routines that are automatically called by Client-Library when certain triggering events, known as callback events, occur.
Some callback events are the result of a server response arriving for an application. For example, a notification callback event occurs when a registered procedure notification arrives from an Open Server.
Other callback events occur at the internal Client-Library level. For example, a client message callback event occurs when Client-Library generates an error message.
When Client-Library recognizes a callback event, it calls the appropriate callback routine.
Client-Library must be actively engaged in reading from the network to recognize some callback events. Most callback events of this type are raised automatically when Client-Library is reading results from the network.
However, for applications that use Client-Library’s asynchronous modes, or that use Open Server registered procedure notifications, two types of callback events may require special handling:
The completion callback event, which occurs in asynchronous mode applications when an asynchronous Client-Library routine completes. Depending on the operating system, applications either receive completions automatically or by polling. See “Completions”.
The notification callback event, which occurs when an Open Server notification arrives for an application. Applications must take special steps to ensure that they receive notification events. See “Receiving notifications asynchronously”.
Because some types of callback routines are executed
from within a system interrupt handler or from a Client-Library
worker thread, you must code applications so that data accessed
by both the application’s mainline code and the callbacks
is safely shared.
