Runtime Linking

Runtime linking involves the binding of objects, usually generated from one or more previous link-edits, to generate a runable process. During the generation of these objects by the link-editor, the binding requirements are verified and appropriate bookkeeping information is added to each object to allow the runtime linker to map, relocate, and complete the binding process.

During the execution of the process, the facilities of the runtime linker are also made available and can be used to extend the process' address space by adding additional shared objects on demand. The two most common components involved in runtime linking are dynamic executables and shared objects.

Dynamic Executables

Dynamic executables are applications that are executed under the control of a runtime linker. These applications usually have dependencies in the form of shared objects, which are located and bound by the runtime linker to create a runable process. Dynamic executables are the default output file generated by the link-editor.

Shared Objects

Shared objects provide the key building block to a dynamically linked system. Basically, a shared object is similar to a dynamic executable; however, shared objects have not yet been assigned a virtual address.

Dynamic executables usually have dependencies on one or more shared objects. That is, the shared object(s) must be bound to the dynamic executable to produce a runable process. Because shared objects can be used by many applications, aspects of their construction directly affect shareability, versioning, and performance.

You can distinguish the processing of shared objects by either the link-editor or the runtime linker by referring to the environments in which the shared objects are being used:

compilation environment

Shared objects are processed by the link-editor to generate dynamic executables or other shared objects. The shared objects become dependencies of the output file being generated.

runtime environment

Shared objects are processed by the runtime linker, together with a dynamic executable, to produce a runable process.