A filter index function should set values for at least the following in the parameter block before returning control to Directory Server:
SLAPI_PLUGIN_MR_INDEX_FN, the pointer to the indexer for generating the keys
SLAPI_PLUGIN_MR_OID
SLAPI_PLUGIN_MR_QUERY_OPERATOR
SLAPI_PLUGIN_MR_TYPE
SLAPI_PLUGIN_MR_VALUES
SLAPI_PLUGIN_OBJECT, if modified by your function
Refer to Chapter 18, Parameter Block Reference for details.