Dynamically Linked Library

library (DLL) A library which is linked to applicationprograms when they are loaded or run rather than as the final

phase of compilation. This means that the same block of

library code can be shared between several tasks rather than

each task containing copies of the routines it uses. The

executable is compiled with a library of "stubs" which allow

link errors to be detected at compile-time. Then, at runtime, either the system loader or the task's entry code

must arrange for library calls to be patched with the

addresses of the real shared library routines, possibly via a

jump table.

The alternative is to make library calls part of the

operating system kernel and enter them via some kind of

trap instruction. This is generally less efficient than an

ordinary subroutine call.

It is important to ensure that the version of a dynamically

linked library is compatible with what the executable expects.

Examples of operating systems using dynamic linking are

SunOS (.so - shared object files), Microsoft Windows

(.dll) and RISC OS on the Acorn Archimedes (relocatable

modules).

(1995-12-12)