What is linking in computer programming?

Answer 1

In order to execute a high-level program, the source code must be converted to a machine code executable. To achieve this, each translation unit is precompiled by the preprocessor. The resultant code is then compiled by the compiler to generate an object file. Once all the object files are generated they are interwoven and linked together to produce the final executable. This final stage is known as linking.

Compiling

A computer source program, written as human readable text, needs to be converted to something the computer can use as instructions as part of a program. A compiler (or assembler) performs this function by examining the text and creating a machine language version of the source program. If the program is simple enough and you have the right compiler/assembler, the results may be a program that is immediately usable as an executable program. Otherwise the results of a compilation is a file called an object module, object program, or program object and the contents of the file is called object code. This has nothing to do with object oriented programming.

Objects (not OOP)

Modern day programming involves larger complex programs made up of multiple source programs that are compiled separately into object modules (or functions or subroutine). These programs often reference each other to do things required for the computer program to work properly. They also may reference system supplied and other modules that may already be available, or need to be made available on the computer.

Referencing these external modules is done with special codes inserted by the compiler into the results of the program compilation. These unresolved external references is one of the reasons the result of compiling a program is not immediately executable.

Linking

Unresolved references have to be identified and made available in the executable program. Most systems refer to this as linking. It is also called binding or object resolution. The program that does this can be called (depending on your computer or system) a linker, linkage editor, binder, loader, or task builder. There are even systems that can do the work "on the fly" as the program running.

Linkers get a list of files of object modules and locations on the computer to look for more modules, put them together and change the unresolved references to point to the actual code in the object modules. Any remaining unknowns are searched for in system areas and are resolved similarly. Once done the result should be a program that is executable in the system.