Function libraries also implemented a very important security feature, linkage classes. All normal libraries have a linkage class of zero. Libraries used by the MCP or other privileged system modules may not be usable from normal programs. They are accessed by function and forced in linkage class one. A client in linkage class zero cannot link to linkage class one entry-points. A library with linkage class one that needs to offer entry-points to normal programs can do so if it is designated as ‘trusted’. It can offer selected entry-points in linkage class zero.