Shared objects can be defined to act as filters. This technique involves associating the interfaces that the filter provides with an alternative shared object. At runtime, the alternative shared object supplies one or more of the interfaces provided by the filter. This alternative shared object is referred to as a filtee. A filtee is built in the same manner as any shared object is built.
Filtering provides a mechanism of abstracting the compilation environment from the runtime environment. At link-edit time, a symbol reference that binds to a filter interface is resolved to the filters symbol definition. At runtime, a symbol reference that binds to a filter interface can be redirected to an alternative shared object.
Individual interfaces that are defined within a shared object can be defined as filters by using the mapfile keywords FILTER or AUXILIARY. Alternatively, a shared object can define all of the interfaces the shared object offers as filters by using the link-editor's –F or –f options. These techniques are typically used individually, but can also be combined within the same shared object.
Two forms of filtering exist.
This filtering requires only a symbol table entry for the interface being filtered. At runtime, the implementation of a filter symbol definition must be provided from a filtee.
Interfaces are defined to act as standard filters by using the link-editor's mapfile keyword FILTER, or by using the link-editor's –F option. This mapfile keyword or option, is qualified with the name of one or more filtees that must supply the symbol definition at runtime.
A filtee that cannot be processed at runtime is skipped. A standard filter symbol that cannot be located within the filtee, also causes the filtee to be skipped. In both of these cases, the symbol definition provided by the filter is not used to satisfy this symbol lookup.
This filtering provides a similar mechanism to standard filtering, except the filter provides a fall back implementation corresponding to the auxiliary filter interfaces. At runtime, the implementation of the symbol definition can be provided from a filtee.
Interfaces are defined to act as auxiliary filters by using the link-editor's mapfile keyword AUXILIARY, or by using the link-editor's –f option. This mapfile keyword or option, is qualified with the name of one or more filtees that can supply the symbol definition at runtime.
A filtee that cannot be processed at runtime is skipped. An auxiliary filter symbol that cannot be located within the filtee, also causes the filtee to be skipped. In both of these cases, the symbol definition provided by the filter is used to satisfy this symbol lookup.