VI_EVENT_EXCEPTION

Resource Classes

All I/O session types

Description

This event notifies the application that an error condition has occurred during an operation invocation. In VISA, exceptions are defined as events. The exception-handling model follows the event-handling model for callbacks, and is like any other event in VISA, except that the queueing and suspended handler mechanisms are not allowed.

A VISA operation generating an exception blocks until the exception handler execution is completed. However, an exception handler sometimes may prefer to terminate the program prematurely without returning the control to the operation generating the exception. VISA does not preclude an application from using a platform-specific or language-specific exception handling mechanism from within the VISA exception handler. For example, the C++ try/catch block can be used in an application in conjunction with the C++ throw mechanism from within the VISA exception handler.

When using the C++ try/catch/throw or other exception-handling mechanisms, the control will not return to the VISA system. This has some important repercussions:

• If multiple handlers were installed on the exception event, the handlers that were not invoked prior to the current handler will not be invoked for the current exception.
• The exception context will not be deleted by the VISA system when a C++ exception is used. In this case, the application should delete the exception context as soon as the application has no more use for the context, before terminating the session. An application should use the viClose() operation to delete the exception context.

One situation in which an exception event will not be generated is in the case of asynchronous operations. If the error is detected after the operation is posted—once the asynchronous portion has begun—the status is returned normally via the I/O completion event. However, if an error occurs before the asynchronous portion begins—the error is returned from the asynchronous operation itself—then the exception event will still be raised. This deviation is due to the fact that asynchronous operations already raise an event when they complete, and this I/O completion event may occur in the context of a separate thread previously unknown to the application. In summary, a single application event handler can easily handle error conditions arising from both exception events and failed asynchronous operations.

Event Attributes

Related Topics

VI_ATTR_EVENT_TYPE

VI_ATTR_OPER_NAME

VI_ATTR_STATUS

viEnableEvent