Linker (computing): Difference between revisions

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[[File:Linker.svg|thumb|An illustration of the linking process. Object files and [[static library|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 file|object]] and [[library (computing)|library]] files into a single [[executable]] file such as a program or library. A linker is often part of a [[toolchain]] that includes a [[compiler]] and/or [[assembler (computing)|assembler]] that generates intermediate files that the linker processes. The linker may be integrated with other toolchain [[development tool|tools]] such that the user does not interact with the linker directly.

A simpler version that writes its [[Input/output|output]] directly to [[Computer memory|memory]] is called the ''loader'', though [[loader (computing)|loading]] is typically considered a separate process.<ref name="IBM_1972"/><ref name="Barron_1978_Consolidator"/>

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The executable output by the linker may need another relocation pass when it is finally loaded into memory (just before execution). This pass is usually omitted on [[computer hardware|hardware]] offering [[virtual memory]]: every program is put into its own address space, so there is no conflict even if all programs load at the same base address. This pass may also be omitted if the executable is a [[position independent]] executable.

On some [[Unix]] variants, such as [[SINTRAN&nbsp;III]], the process performed by a linker (assembling object files into a program) was called ''[[loader (computing)|loading]]'' (as in loading executable code onto a file).<ref name="BRF_1984"/> Additionally, in some operating systems, the same program handles both the jobs of linking and loading a program ([[dynamic linking]]).

== Dynamic linking ==

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== {{anchor|Consolidator}}Linkage editor ==

In IBM [[System/360]] through [[IBM Z]] [[Mainframe computer|mainframe]] environmentsoperating systems such as [[OS/360]], includingand [[z/successors|OS]]/360 forand theits [[z/Architecturesuccessors]] mainframes, this type of program is known as a ''linkage editor''. As the name implies a linkage ''editor'' has the additional capability of allowing the addition, replacement, and/or deletion of individual program sections. Operating systems such as OS/360 have format for executable load-modules containing supplementary data about the component sections of a program, so that an individual program section can be replaced, and other parts of the program updated so that relocatable addresses and other references can be corrected by the linkage editor, as part of the process.

One advantage of this is that it allows a program to be maintained without having to keep all of the intermediate object files, or without having to re-compile program sections that haven't changed. It also permits program updates to be distributed in the form of small files (originally [[card deck (computing)|card deck]]s), containing only the object module to be replaced. In such systems, object code is in the form and format of 80-byte punched-card images, so that updates can be introduced into a system using that medium. In later releases of OS/360 and in subsequent systems, load-modules contain additional data about versions of components modules, to create a traceable record of updates. It also allows one to add, change, or remove an [[overlay (programming)|overlay]] structure from an already linked load module.

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=== Unix & Unix-like ===

On Unix and Unix-like systems, the static linker is usually invoked via the command <code>ld</code> which is an abbreviation of ''LoaDer'' or ''Link eDitor''. The term "loader" was used to describe the process of loading external symbols from other programs during the process of linking.<ref name="UNIX_V6_manuals"/>

For example, on [[SINTRAN&nbsp;III]], linking (assembling object files into a program) was called <!--do not link to [[loader (computing)]] since that's different-->''loading'' {{endash}} as in loading executable code onto a file.<ref name="BRF_1984"/>

=== GNU ===

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<ref name="GNU_2018_Binutils">{{cite web |title=GNU Binutils: Linker Scripts |date=2018-07-18 |url=https://sourceware.org/binutils/docs/ld/Scripts.html |access-date=2019-01-18 |url-status=live |archive-url=https://web.archive.org/web/20200306230349/https://sourceware.org/binutils/docs/ld/Scripts.html |archive-date=2020-03-06}}</ref>

<ref name="UNIX_V6_manuals">{{cite book |title=UNIX PROGRAMMER'S MANUAL |date=May 1975 |edition=6 |chapter=1. ld }}</ref>

<ref name="Barron_1978_Consolidator">{{cite book |author-first=David William |author-last=Barron |author-link=David W. Barron |editor-first=J. John |editor-last=Floretin |title=Assemblers and Loaders |date=1978 |orig-year=1971, 1969 |edition=3 |publisher=[[Elsevier North-Holland Inc.]] |series=Computer Monographs |publication-place=New York, US |___location=[[University of Southampton]], Southampton, UK |isbn=0-444-19462-2 |lccn=78-19961 |chapter=5.7. Linkage editors and consolidators |pages=65–66}} (xii+100 pages)</ref>

<ref name="Salomon_1992">{{cite book |author-first=David |author-last=Salomon |editor-first=Ian D. |editor-last=Chivers |title=Assemblers and Loaders |date=February 1993 |chapter=8.2.3 Automatic jump-sizing |pages=237–238 |orig-year=1992 |edition=1 |series=Ellis Horwood Series In Computers And Their Applications |publisher=[[Ellis Horwood Limited]] / [[Simon & Schuster International Group]] |___location=California State University, Northridge, California, US |publication-place=Chicester, West Sussex, UK |isbn=0-13-052564-2 |chapter-url=http://www.davidsalomon.name/assem.advertis/asl.pdf |access-date=2008-10-01 |url-status=live |archive-url=https://web.archive.org/web/20200323010358/http://www.davidsalomon.name/assem.advertis/asl.pdf |archive-date=2020-03-23}} (xiv+294+4 pages)</ref>

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