# Overview of extended data structure substructures

The structures in this section are referenced in one or more of the service-specific structures and are defined in isdnacu.h.

Each of these structures contains an invoke field, indicating the presence or absence of useful data in the structure. If the field is set to ON, then the stack uses the information in the structure. Otherwise, the information in the structure is ignored. If the successful invocation of the service requires the inclusion of the parameter and the invoke field is OFF, the service request is rejected.

Some structures contain a reference to a string location and string length. The string location is specified using the offset field. This offset value is calculated from the address of the structure containing the offset field.

The len field in a structure contains the length of the data. There is no requirement to null-terminate the string. Note that structures sent from the stack do not include null terminations.

These structures are defined to be 8-byte aligned.