Dynamically Linked Library

A dynamically linked library (DLL) is not part of an absolute file, but is linked in at load time or runtime. The primary motivation of a DLL is to allow multiple programs to share a single copy of a large library, such as X Windows. DLLs also allow a library update (e.g. security patch) to be effective for all programs without re-linking.

Ways to link in the DLL include:

• Let the loader do the linking at load time. The absolute file contains information on what external symbols are needed and where they go.

• Let the program make calls to initialize itself on first call.

• Leave the calls undefined, so that they generate error traps at runtime. The OS figures out what the calls should be and fixes the code at the point of call.