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An illustration of the linking process. Object files and static libraries are assembled into a new library or executable. A linker or link editor is a computer program that combines intermediate software build files such as object and library files into a single executable file such a program or library.
The program had to explicitly call the CHAIN subroutine to load a new link, and the new link replaced all of the old link's storage except for the Fortran COMMON area. IBM introduced more general overlay handling [7] in IBSYS/IBJOB, including a tree structure and automatic loading of links as part of CALL processing.
The system's Linkage Editor application is named IEWL. [3] IEWL's main function is to associate load modules (executable programs) and object modules (the output from, say, assemblers and compilers), including "automatic calls" to libraries (high-level language "built-in functions"), into a format which may be most efficiently loaded by IEWFETCH.
Relocation is the process of assigning load addresses for position-dependent code and data of a program and adjusting the code and data to reflect the assigned addresses. [1] [2] Prior to the advent of multiprocess systems, and still in many embedded systems, the addresses for objects are absolute starting at a known location, often zero.
Linking is often referred to as a process that is performed when the executable is compiled, while a dynamic linker is a special part of an operating system that loads external shared libraries into a running process and then binds those shared libraries dynamically to the running process. This approach is also called dynamic linking or late ...
Dynamic loading is the process of loading a dynamic library at run-time and also unloading a library. A load can be initiated implicitly or explicitly by a program. An implicit request occurs if the program is configured to automatically load the dynamic library and this is setup at link-time.
An example of load-and-go systems is the loader Anthony J. Barr wrote for the University Computing Corporation in 1968 that was replaced in the market by the IBM OS/360 loader in 1972. These OS/360 loaders performed many of the functions of the Linkage Editor but placed the linked program in memory rather than creating an executable on disk. [9]
[9] [10] The code segment contains only code and the linkage section serves as a template for a new linkage segment. Pointer register 4 (PR4) points to the linkage segment of the procedure. A call to a procedure saves PR4 in the stack before loading it with a pointer to the callee's linkage segment.