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56 Agilent VISA User’s Guide
3
Programming with VISA
The userHandle parameter allows you to assign a value to be used with
the handler on the specified session. Thus, you can install the same
handler for the same event type on several sessions with different
userHandle values. The same handler is called for the specified event
type.
However, the value passed to userHandle is different. Therefore the
handlers are uniquely identified by the combination of the handler and
the userHandle . This may be useful when you need a different handling
method depending on the userHandle .
Example: Installing an Event Handler This example shows how
to install an event handler to call my_handler when a Service Request
occurs. Note that VI_EVENT_SERVICE_REQ must also be an enabled
event with the
viEnableEvent function for the service request event to
be delivered.
viInstallHandler( vi , VI_EVENT_SERVICE_REQ,
my_handler , addr );
Use the viUninstallHandler function to uninstall a specific handler, or
you can use wildcards (VI_ANY_HNDLR in the handler parameter) to
uninstall groups of handlers. See viUninstallHandler in the VISA
Online Help for more details on this function.
Writing the Handler
The handler installed needs to be written by the programmer. The event
handler typically reads an associated attribute and performs some sort of
action. See the event handler in the example program later in this
section.
Enabling Events
Before an event can be delivered, it must be enabled using the
viEnableEvent function. This function causes the application to be
notified when the enabled event has occurred, where the parameters are:
viEnableEvent( vi , eventType , mechanism ,
context );