When a device that is supported by the dynamic definition function powers on, VTAM® passes a dynamic definition request to the SDDLU exit. VTAM passes one dynamic definition request for each device that powers on. One dynamic definition request can represent one or more LUs being dynamically defined for the device.

A user-written exit can choose to fail the definition of specific LUs in the dynamic definition request. The exit can also choose to fail the definitions of all the LUs in the dynamic definition request. While the IBM® default exit does not fail dynamic definition requests for individual LUs in a multiple LU request, it does fail the entire request if the LUSEED operand is not compatible with the default exit. The LUSEED operand is not compatible with the default exit if any of the following occur:

• The LUSEED operand is blank.
• Two or three adjacent # characters are not found in the LUSEED operand.
• The LUSEED operand's first character is a # character or a number.

The exit, not system definition, performs this error checking.

When VTAM calls the exit with a dynamic definition request vector, VTAM might have already failed some of the LU definitions in the request. For example, if one of the LUs was predefined, VTAM would have already failed the dynamic definition for the LU. This is because a dynamic definition cannot override an existing definition. The exit cannot change any fields of information in a dynamic definition request for an LU definition that VTAM has already failed.

The topics that follow contain information you need to write an SDDLU exit routine.