Filtering Combinations

Individual interfaces that define standard filters, together with individual interfaces that define auxiliary filters, can be defined within the same shared object. This combination of filter definitions is achieved by using the mapfile keywords FILTER and AUXILIARY to assign the required filtees.

A shared object that defines all of its interfaces to be filters by using the -F, or -f option, or the mapfile FILTER directive, is either a standard weak, or auxiliary filter.

A shared object can define individual interfaces to act as filters, together with defining all the interfaces of the object to act as a filters. In this case, the individual filtering defined for an interface is processed first. When a filtee for an individual interface filter can not be established, the filtee defined for all he interfaces of the filter provides a fallback if appropriate.

For example, consider the filter filter.so.1. This filter defines that all interfaces act as auxiliary filters against the filtee filtee.so.1 using the link-editor's -f option. filter.so.1 also defines the individual interface foo to be a standard filter against the filtee foo.so.1 using the mapfile FILTER symbol attribute. filter.so.1 also defines the individual interface bar to be an auxiliary filter against the filtee bar.so.1 using the mapfile AUXILIARY symbol attribute.

An external reference to foo results in processing the filtee foo.so.1. If foo can not be found from foo.so.1, then no further processing of the filter is carried out. In this case, no fallback processing is performed because foo is defined to be a standard filter.

An external reference to bar results in processing the filtee bar.so.1. If bar can not be found from bar.so.1, then processing falls back to the filtee filtee.so.1. In this case, fallback processing is performed because bar is defined to be an auxiliary filter. If bar can not be found from filtee.so.1, then the definition of bar within the filter filter.so.1 is finally used to resolve the external reference.