Linker (computing) - Wikipedia, the free encyclopedia http://en.wikipedia.

org/wiki/Linker_(computing)

From Wikipedia, the free encyclopedia

In computer science, a linker or link editor is a program that takes one

or more objects generated by a compiler and combines them into a
single executable program.

In IBM mainframe environments such as OS/360 this program is known

as a linkage editor.

On Unix variants the term loader is often used as a synonym for linker.
Other terminology was in use, too. For example, on SINTRAN III the
process performed by a linker (assembling object files into a program)
was called loading (as in loading executable code onto a file)[1].
Because this usage blurs the distinction between the compile-time
process and the run-time process, this article will use linking for the
former and loading for the latter. However, in some operating systems An illustration of the linking
the same program handles both the jobs of linking and loading a process. Object files and static
program; see dynamic linking. libraries are assembled into a
new library or executable.

1 Overview
2 Dynamic linking
3 Relaxation
4 References
5 See also
6 External links

Computer programs typically comprise several parts or modules; all these parts/modules need not be
contained within a single object file, and in such case refer to each other by means of symbols. Typically, an
object file can contain three kinds of symbols:

defined symbols, which allow it to be called by other modules,

undefined symbols, which call the other modules where these symbols are defined, and
local symbols, used internally within the object file to facilitate relocation.

When a program comprises multiple object files, the linker combines these files into a unified executable
program, resolving the symbols as it goes along.

Linkers can take objects from a collection called a library. Some linkers do not include the whole library in
the output; they only include its symbols that are referenced from other object files or libraries. Libraries
exist for diverse purposes, and one or more system libraries are usually linked in by default.

The linker also takes care of arranging the objects in a program's address space. This may involve relocating
code that assumes a specific base address to another base. Since a compiler seldom knows where an object
will reside, it often assumes a fixed base location (for example, zero). Relocating machine code may involve
re-targeting of absolute jumps, loads and stores.

1 of 3 1/6/2011 12:29 AM
Linker (computing) - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Linker_(computing)

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 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.

See also: Dynamic linker

Many operating system environments allow dynamic linking, that is the postponing of the resolving of some
undefined symbols until a program is run. That means that the executable code still contains undefined
symbols, plus a list of objects or libraries that will provide definitions for these. Loading the program will
load these objects/libraries as well, and perform a final linking.

This approach offers two advantages:

Often-used libraries (for example the standard system libraries) need to be stored in only one location,
not duplicated in every single binary.
If an error in a library function is corrected by replacing the library, all programs using it dynamically
will benefit from the correction after restarting them. Programs that included this function by static
linking would have to be re-linked first.

There are also disadvantages:

Known on the Windows platform as "DLL Hell", an incompatible updated DLL will break
executables that depended on the behavior of the previous DLL.
A program, together with the libraries it uses, might be certified (eg as to correctness, documentation
requirements, or performance) as a package, but not if components can be replaced. (This also argues
against automatic OS updates in critical systems; in both cases, the OS and libraries form part of a
qualified environment.)

As the compiler has no information on the layout of objects in the final output, it cannot take advantage of
shorter or more efficient instructions that place a requirement on the address of another object. For example,
a jump instruction can reference an absolute address or an offset from the current location, and the offset
could be expressed with different lengths depending on the distance to the target. By generating the most
conservative instruction (usually the largest relative or absolute variant, depending on platform) and adding
relaxation hints, it is possible to substitute shorter or more efficient instructions during the final link. This
step can be performed only after all input objects have been read and assigned temporary addresses; the
relaxation pass subsequently re-assigns addresses, which may in turn allow more relaxations to occur. In
general, the substituted sequences are shorter, which allows this process to always converge on the best
solution given a fixed order of objects; if this is not the case, relaxations can conflict, and the linker needs to
weight the advantages of either option.

1. ^ BRF-LINKER User Manual. ND-60.196.01. 08/84.

David William Barron, Assemblers and Loaders. 1972, Elsevier.

C. W. Fraser and D. R. Hanson, A Machine Independent Linker. Software-Practice and Experience
12, 4 (April 1982).
IBM Corporation, Operating System 360, Linkage Editor, Program Logic Manual, 1967 [1]

2 of 3 1/6/2011 12:29 AM
Linker (computing) - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Linker_(computing)

(http://www.bitsavers.org/pdf/ibm/360/Y28-6610_LinkEdit(E)_PLM.pdf)
D.W. Jones, Assembly Language as Object Code. Software-Practice and Experience 13, 8 (August
1983)
John R. Levine: Linkers and Loaders, Morgan-Kauffman, ISBN 1-55860-496-0
Leon Presser, John R. White: Linkers and Loaders. ACM Computing Surveys, Volume 4, Number 3,
September 1972, pp.149-167 [2] (http://www-inst.eecs.berkeley.edu/~cs162/sp06/hand-outs/p149-
presser-linker-loader.pdf)
Norman Ramsey, Relocating Machine Instructions by Currying. (1996) [3]
(http://citeseer.csail.mit.edu/58313.html)
David Salomon, Assemblers and Loaders. 1993 [4] (http://www.davidsalomon.name/assem.advertis
/asl.pdf)

Dynamic library
GNU linker
Library
Name decoration
Object file
Relocation
Relocation table
Prelinking
Static library

Ian Lance Taylor's Linkers blog entries (http://www.google.fr

/search?q=site%3Awww.airs.com%2Fblog%2Farchives+%22linkers+part%22)
Linkers and Loaders by Sandeep Grover (http://www.linuxjournal.com/article/6463)
Another Listing of Where to Get a Complete Collection of Free Tools for Assembly Language
Development (http://www.dpgraph.com/assembly.html)
GoLink: a free linker for Windows programming (http://www.godevtool.com/)
Retrieved from "http://en.wikipedia.org/wiki/Linker_(computing)"
Categories: Compilers | Computer libraries | Programming language implementation

This page was last modified on 27 August 2010 at 15:05.

Text is available under the Creative Commons Attribution-ShareAlike License; additional terms may
apply. See Terms of Use for details.
Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization.

3 of 3 1/6/2011 12:29 AM