When you connect to the list structure and indicate your interest in using the list structure monitoring function, the system allocates a list notification vector for your use and returns a token to you representing this vector. The list notification vector shows the state (empty or nonempty) of each list or event queue you are monitoring. Each connector to the list structure that indicates interest in list monitoring (by coding the VECTORLEN parameter on the IXLCONN macro) or event queue monitoring (by coding the VECTORLEN and EMCSTGPCT parameters on the IXLCONN macro) is allocated a list notification vector.

A list notification vector consists of an array of entries, each of which can be associated with a particular list header or with the user's event queue. The number of entries must be a multiple of 32. The assignment of particular vector entries to monitor particular lists or to monitor the user's event queue is under the user's control with the IXLLIST MONITOR_LIST and MONITOR_EVENTQ request types. Note that the user can change this monitoring assignment dynamically over time (so that at any given point in time, none, some, or all of the allocated vector entries might be actively in use for monitoring purposes). However, the user should take care to manage the assignment of monitoring to particular vector entries such that any given vector entry is never monitoring more than one thing at a time. In such a case, the results are unpredictable.

When a transition occurs for a monitored list or event queue, the system automatically updates the associated entry in the list notification vector to reflect the empty or nonempty state of the list or event queue. The IXLVECTR macro provides the interface to the list notification vector. To determine whether a list or event queue you are monitoring is empty or non-empty, invoke the IXLVECTR macro with either the TESTLISTSTATE or LTVECENTRIES parameter. You can use the IXLVECTR macro with the MODIFYVECTORSIZE parameter to change the size of your list notification vector, so you can, for instance, monitor more lists.

See Using the IXLVECTR Macro for more information.