View source for Library (computing)

{{short description|Collection of resources used to develop a computer program}}
{{redirect-distinguish|Software library|library software}}
{{About|a software development concept|a repository of digital assets|Digital library}}
{{Use dmy dates|date=February 2020|cs1-dates=y}}
[[Image:Ogg vorbis libs and application dia.svg|thumb|277px|right|Illustration of an application which uses libvorbisfile to play an [[Ogg Vorbis]] file]]

In [[computing]], a '''library''' is a collection of [[System resource|resources]] that can be used during [[software development]] to implement a [[computer program]]. Commonly, a library consists of [[executable]] code such as [[compiled]] [[function (computer science)|functions]] and [[Class (programming)|classes]], or a library can be a collection of [[source code]]. A resource library may contain data such as [[image]]s and [[Text string|text]].

A library can be used by multiple, independent consumers (programs and other libraries). This differs from resources defined in a program which can usually only be used by that program. When a consumer uses a library resource, it gains the value of the library without having to implement it itself. Libraries encourage [[software reuse]] in a [[Modular programming|modular]] fashion. Libraries can use other libraries resulting in a hierarchy of libraries in a program.

When writing code that uses a library, a [[programmer]] only needs to know how to use that library's [[application programming interface]] (API) without understanding its internal mechanics. For example, a program could use a library that [[Abstraction (computer science)|abstracts]] a complicated [[system call]] so that the programmer can use the system feature without spending time to learn the intricacies of the system function.

== History ==

The idea of a computer library dates back to the first computers created by [[Charles Babbage]]. An 1888 paper on his [[Analytical Engine]] suggested that computer operations could be punched on separate cards from numerical input. If these operation punch cards were saved for reuse then "by degrees the engine would have a library of its own."<ref>{{cite journal |url=https://www.fourmilab.ch/babbage/hpb.html |first=H. P. |last=Babbage |journal=Proceedings of the British Association |date=September 12, 1888 |location=Bath|title=The Analytical Engine }}</ref>

[[File:FirstCodeLibrary-ESDAC-ThePreparationOfProgramsForAnElectronicDigitalComputer-1951.jpg|thumb|A woman working next to a filing cabinet containing the subroutine library on reels of punched tape for the EDSAC computer]]

In 1947 [[Herman Goldstine|Goldstine]] and [[John von Neumann|von Neumann]] speculated that it would be useful to create a "library" of [[subroutine]]s for their work on the [[IAS machine]], an early computer that was not yet operational at that time.<ref>{{Cite book |last=Goldstine |first=Herman H. |url=http://dx.doi.org/10.1515/9781400820139 |title=The Computer from Pascal to von Neumann |date=2008-12-31 |publisher=Princeton University Press |isbn=978-1-4008-2013-9 |location=Princeton |doi=10.1515/9781400820139}}</ref> They envisioned a physical library of [[magnetic wire recording]]s, with each wire storing reusable computer code.<ref>{{cite report |title=Planning and coding of problems for an electronic computing instrument |last1=Goldstine |first1=Herman |author-link1=Herman Goldstine |last2=von Neumann |first2=John |author-link2=John von Neumann |date=1947 |publisher=Institute for Advanced Study |pages=3, 21–22 |oclc=26239859 |quote=it will probably be very important to develop an extensive "library" of subroutines}}</ref>

Inspired by von Neumann, [[Maurice Wilkes|Wilkes]] and his team constructed [[EDSAC]]. A [[filing cabinet]] of [[punched tape]] held the subroutine library for this computer.<ref>{{Cite conference|last=Wilkes|first=M. V.| title=1951 International Workshop on Managing Requirements Knowledge |date=1951|chapter=The EDSAC Computer| page=79 |chapter-url=http://dx.doi.org/10.1109/afips.1951.13|conference=1951 International Workshop on Managing Requirements Knowledge|publisher=IEEE|doi=10.1109/afips.1951.13}}</ref> Programs for EDSAC consisted of a main program and a sequence of subroutines copied from the subroutine library.<ref>{{cite journal |last1=Campbell-Kelly |first1=Martin |date=September 2011 |title=In Praise of 'Wilkes, Wheeler, and Gill' |url=https://cacm.acm.org/magazines/2011/9/122802-in-praise-of-wilkes-wheeler-and-gill/fulltext |journal=Communications of the ACM |volume=54 |issue=9 |pages=25–27 |doi=10.1145/1995376.1995386|s2cid=20261972 |url-access=subscription }}</ref> In 1951 the team published the first textbook on programming, ''[[The Preparation of Programs for an Electronic Digital Computer]]'', which detailed the creation and the purpose of the library.<ref>{{cite book |last1=Wilkes |first1=Maurice |last2=Wheeler |first2=David |last3=Gill |first3=Stanley |author-link1=Maurice Wilkes |author-link2=David Wheeler (computer scientist) |author-link3=Stanley Gill |date=1951 |title=The Preparation of Programs for an Electronic Digital Computer |oclc=641145988 |url=https://archive.org/details/programsforelect00wilk/page/80/mode/2up?q=library |location= |publisher=Addison-Wesley |pages=45, 80–91, 100 |isbn=}}</ref>

[[COBOL]] included "primitive capabilities for a library system" in 1959,<ref name="Wexelblat_1981_247">{{Cite book |last=Wexelblat |first=Richard |title=History of Programming Languages |publisher=Academic Press (A subsidiary of [[Harcourt Brace]]) |year=1981 |series=ACM Monograph Series |publication-place=New York, NY |isbn=0-12-745040-8 |page=[https://archive.org/details/historyofprogram0000hist/page/274 274] |url=https://archive.org/details/historyofprogram0000hist/page/274 }}</ref> but [[Jean E. Sammet|Jean Sammet]] described them as "inadequate library facilities" in retrospect.<ref name="Wexelblat_1981_258">Wexelblat, ''op. cit.'', p. 258</ref>

[[JOVIAL]] has a Communication Pool (COMPOOL), roughly a library of header files.

Another major contributor to the modern library concept came in the form of the [[subprogram]] innovation of [[FORTRAN]]. FORTRAN subprograms can be compiled independently of each other, but the compiler lacked a [[Linker (computing)|linker]]. So prior to the introduction of modules in Fortran-90, [[type checking]] between FORTRAN<ref group=NB>It was possible earlier between, e.g., Ada subprograms.</ref> subprograms was impossible.<ref name="Wilson_Clark_1988_126">{{Cite book |last1=Wilson |first1=Leslie B. |last2=Clark |first2=Robert G.|title=Comparative Programming Languages|publisher=Addison-Wesley |year=1988 |publication-place=Wokingham, England |isbn=0-201-18483-4 |page=126 }}</ref>

By the mid 1960s, copy and macro libraries for assemblers were common. Starting with the popularity of the [[IBM System/360]], libraries containing other types of text elements, e.g., system parameters, also became common.

In [[OS/360 and successors|IBM's OS/360 and its successors]] this is called a [[Data set (IBM mainframe)#Partitioned datasets|partitioned data set]].

The first [[object-oriented programming]] language, [[Simula]], developed in 1965, supported adding [[Class (computer science)|classes]] to libraries via its compiler.<ref name="Wilson_Clark_1988_52">Wilson and Clark, ''op. cit.'', p. 52</ref><ref name="Wexelblat_1981_716">Wexelblat, ''op. cit.'', p. 716</ref>

==Linking==
The ''linking'' (or ''binding'') process resolves references known as ''symbols'' (or ''links'') by searching for them in various locations including configured libraries. If a [[linker (computing)|linker]] (or binder) does not find a symbol, then it fails, but multiple matches may or may not cause failure.

''Static linking'' is linking at [[link time|build time]], such that the library executable code is included in the program. ''Dynamic linking'' is linking at [[Runtime (program lifecycle phase)|run time]]; it involves building the program with information that supports run-time linking to a dynamic link library (DLL). For dynamic linking, a compatible DLL file must be available to the program at run time, but for static linking, the program is standalone.

{{anchor|Smart linking}}''Smart linking'' is performed by a build tool that excludes unused code in the linking process. For example, a program that only uses integers for arithmetic, or does no arithmetic operations at all, can exclude floating-point library routines. This can lead to smaller program file size and reduced memory usage.

==Relocation==
Some references in a program or library module are stored in a relative or symbolic form which cannot be resolved until all code and libraries are assigned final static addresses. [[Relocation (computer science)|Relocation]] is the process of adjusting these references, and is done either by the linker or the [[loader (computing)|loader]]. In general, relocation cannot be done to individual libraries themselves because the addresses in memory may vary depending on the program using them and other libraries they are combined with. [[Position-independent code]] avoids references to absolute addresses and therefore does not require relocation.

== Categories ==

=== Executable ===

An executable library consists of code that has been converted from source code into [[machine code]] or an intermediate form such as [[bytecode]]. A linker allows for using library objects by associating each reference with an address at which the object is located. For example, in [[C (programming language)|C]], a library function is invoked via C's normal function call [[Syntax (programming languages)|syntax]] and [[Semantics (computer science)|semantics]].<ref>{{Cite thesis|title=Metamorphic Detection Using Function Call Graph Analysis|publisher=San Jose State University Library|first=Prasad|last=Deshpande| year=2013 |doi=10.31979/etd.t9xm-ahsc|doi-access=free}}</ref>

A variant is a library containing compiled code (object code in IBM's nomenclature) in a form that cannot be loaded by the OS but that can be read by the linker.

=== Static ===
A [[static library]] is an executable library that is linked into a program at build-time by a linker (or whatever the build tool is called that does linking).<ref name="Static Libraries">{{cite web|title=Static Libraries|url=http://tldp.org/HOWTO/Program-Library-HOWTO/static-libraries.html|publisher=TLDP|access-date=3 October 2013|url-status=live|archive-url=https://web.archive.org/web/20130703011904/http://tldp.org/HOWTO/Program-Library-HOWTO/static-libraries.html|archive-date=3 July 2013}}</ref><ref>{{cite book|last=Kaminsky|first=Dan|chapter=Chapter 3 - Portable Executable and Executable and Linking Formats|date=2008|chapter-url=http://dx.doi.org/10.1016/b978-1-59749-237-9.00003-x|title=Reverse Engineering Code with IDA Pro|pages=37–66|publisher=Elsevier|doi=10.1016/b978-1-59749-237-9.00003-x|isbn=978-1-59749-237-9|access-date=2021-05-27}}</ref> This process, and the resulting stand-alone file, is known as a [[static build]] of the program. A static build may not need any further [[relocation (computer science)|relocation]] if [[virtual memory]] is used and no [[address space layout randomization]] is desired.<ref>{{cite conference|url=http://usenix.org/legacy/publications/library/proceedings/usenix05/tech/general/full_papers/collberg/collberg_html/main.html|title=SLINKY: Static Linking Reloaded|conference=USENIX '05|first1=Christian |last1=Collberg |first2=John H. |last2=Hartman |first3=Sridivya |last3=Babu |first4=Sharath K. |last4=Udupa|publisher=Department of Computer Science, [[University of Arizona]]|access-date=2016-03-17|year=2003|url-status=live|archive-url=https://web.archive.org/web/20160323214637/https://www.usenix.org/legacy/publications/library/proceedings/usenix05/tech/general/full_papers/collberg/collberg_html/main.html|archive-date=23 March 2016}}</ref>

A static library is sometimes called an ''archive'' on Unix-like systems.

=== Dynamic ===

A [[dynamic library]] is linked when the program is run {{endash}} either at [[load-time]] or [[Runtime (program lifecycle phase)|runtime]]. The dynamic library was intended after the static library to support additional [[software deployment]] flexibility.

=== Sources ===
A source library consists of source code, not compiled code.

=== Shared ===
A [[shared library]] is a library that contains [[executable code]] designed to be used by multiple [[computer program]]s or other libraries at [[Runtime (program lifecycle phase)|runtime]], with only one copy of that code in memory, shared by all programs using the code.<ref>{{cite book |title=Linkers and Loaders |last=Levine |first=John R. |chapter=9. Shared Libraries |isbn=1-55860-496-0 |date=2000}}</ref><ref>{{cite book |title=UNIX System V/386 Release 3.2 Programmers Guide, Vol. 1 |url=http://www.bitsavers.org/pdf/att/unix/System_V_386_Release_3.2/UNIX_System_V_386_Release_3.2_Programmers_Guide_Vol1_1989.pdf |page=8{{hyp}}2 |isbn=0-13-944877-2 |date=1989|publisher=Prentice Hall }}</ref><ref>{{cite web |url=https://www.cs.cornell.edu/courses/cs414/2001FA/sharedlib.pdf |title=Shared Libraries in SunOS |pages=1, 3}}</ref>

=== Object ===

Although generally an obsolete technology today, an object library exposes resources for [[object-oriented programming]] (OOP) and a distributed object is a remote object library. Examples include: [[Component Object Model|COM]]/DCOM, [[System Object Model|SOM]]/DSOM, [[Distributed Objects Everywhere|DOE]], [[Portable Distributed Objects|PDO]] and various [[CORBA]]-based systems.

The object library technology was developed since as OOP became popular, it became apparent that OOP runtime binding required information than contemporary libraries did not provide. In addition to the names and entry points of the code located within, due to inheritance, OOP binding also requires a list of dependencies {{endash}} since the full definition of a method may be in different places. Further, this requires more than listing that one library requires the services of another. In OOP, the libraries themselves may not be known at [[compile time]], and vary from system to system.

The remote object technology was developed in parallel to support multi-tier programs with a [[user interface]] application running on a [[personal computer]] (PC) using services of a [[Mainframe computer|mainframe]] or [[minicomputer]] such as data storage and processing. For instance, a program on a PC would send messages to a minicomputer via [[remote procedure call]] (RPC) to retrieve relatively small samples from a relatively large dataset. In response, distributed object technology was developed.

=== Class ===

A class library contains [[Class (computer science)|classes]] that can be used to create [[object (computer science)|objects]]. In [[Java (programming language)|Java]], for example, classes are contained in [[jar (file format)|JAR files]] and objects are created at runtime from the classes. However, in [[Smalltalk]], a class library is the starting point for a [[system image]] that includes the entire state of the environment, classes and all instantiated objects. Most class libraries are stored in a [[package repository]] (such as Maven Central for Java). Client code explicitly specifies dependencies to external libraries in build configuration files (such as a Maven Pom in Java).

=== Remote ===
A remote library runs on another computer and its assets are accessed via [[remote procedure call]] (RPC) over a network. This [[Distributed computing|distributed architecture]] allows for minimizing installation of the library and support for it on each consuming system and ensuring consistent versioning. A significant downside is that each library call entails significantly more overhead than for a local library.

=== Runtime ===
A [[runtime library]] provides access to the [[runtime environment]] that is available to a program {{endash}} tailored to the host [[computer platform|platform]].

=== Language standard ===
Many modern [[programming languages]] specify a [[standard library]] that provides a base level of functionality for the language environment.

=== Code generation ===
A code generation library has a high-level [[Application Programming Interface|API]] generating or transforming [[byte code]] for [[Java (programming language)|Java]]. They are used by [[aspect-oriented programming]], some data access frameworks, and for testing to generate dynamic proxy objects. They also are used to intercept field access.<ref>{{cite web
 |access-date  = 2010-03-03
 |publisher   = [[Source Forge]]
 |title       = Code Generation Library
 |url         = https://sourceforge.net/projects/cglib/
 |quote       = Byte Code Generation Library is high level API to generate and transform JAVA byte code. It is used by AOP, testing, data access frameworks to generate dynamic proxy objects and intercept field access.
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20100112154306/http://sourceforge.net/projects/cglib/
 |archive-date = 12 January 2010
}}</ref>

==File naming==
===Unix-like===
On most modern [[Unix-like]] systems, library files are stored in directories such as <code>/lib</code>, <code>/usr/lib</code> and <code>/usr/local/lib</code>. A filename typically starts with <code>lib</code>, and ends with <code>.a</code> for a static library ([[Ar (file format)|archive]]) or <code>.so</code> for a shared object (dynamically linked library). For example, <code>libfoo.a</code> and <code>libfoo.so</code>.

Often, [[symbolic link]] files are used to manage versioning of a library by providing a link file named without a version that links to a file named with a version. For example, <code>libfoo.so.2</code> might be version 2 of library ''foo'' and a link file named <code>libfoo.so</code> provides a version independent name to that file that programs link to. The link file could be changed to a refer to a version 3 (<code>libfoo.so.3</code>) such that consuming programs will then use version 3 without having to change the program.

Files with the extension <code>.la</code> are [[libtool]] archives; they are not usable by the system.

===macOS===
{{see also|Dynamic linker#macOS and iOS}}
The [[macOS]] system inherits static library conventions from [[BSD]], with the library stored in a <code>.a</code> file. It uses either <code>.so</code> or <code>.dylib</code> for dynamic libraries. Most libraries in macOS, however, consist of "frameworks", placed inside special directories called "[[Bundle (macOS)|bundles]]" which wrap the library's required files and metadata. For example, a framework called <code>Abc</code> would be implemented in a bundle called <code>Abc.framework</code>, with <code>Abc.framework/Abc</code> being either the dynamically linked library file or a symlink to the dynamically linked library file in <code>Abc.framework/Versions/Current/Abc</code>.

===Windows===
Often, a [[Microsoft Windows|Windows]] [[dynamic-link library]] (DLL) has the file extension <code>.dll</code>,<ref>
{{cite book
 |last1            = Bresnahan
 |first1           = Christine
 |last2            = Blum
 |first2           = Richard
 |title            = LPIC-1 Linux Professional Institute Certification Study Guide: Exam 101-400 and Exam 102-400
 |url              = https://books.google.com/books?id=jf3zBgAAQBAJ
 |publisher        = John Wiley & Sons
 |publication-date = 2015
 |page             = 82
 |isbn             = 9781119021186
 |access-date       = 2015-09-03
 |quote            = Linux shared libraries are similar to the dynamic link libraries (DLLs) of Windows. Windows DLLs are usually identified by <code>.dll</code> filename extensions.
 |date             = 2015-04-27
 |url-status          = live
 |archive-url       = https://web.archive.org/web/20150924234210/https://books.google.com/books?id=jf3zBgAAQBAJ
 |archive-date      = 24 September 2015
}}
</ref> although sometimes different extensions are used to indicate general content, e.g. <code>.ocx</code> for a [[Object Linking and Embedding|OLE]] library.

A <code>.lib</code> file can be either a static library or contain the information needed to build an application that consumes the associated DLL. In the latter case, the associated DLL file must be present at runtime.

==See also==

* {{annotated link|Code reuse}}
* {{annotated link|Object file}}
* {{annotated link|Plug-in (computing)|Plug-in}}
* {{annotated link|Prelink|aka=prebinding}}
* {{annotated link|Runtime library}}
* {{annotated link|Visual Component Library}} (VCL)
* {{annotated link|Component Library for Cross Platform}} (CLX)
* {{annotated link|C standard library}}
* {{annotated link|Java Class Library}}
* {{annotated link|Framework Class Library}}
* {{annotated link|Generic programming}} (used by the [[C++ Standard Library]])
* {{annotated link|soname}}
* {{annotated link|Method stub}}
* [[List of open source code libraries]]

==Notes==
{{reflist|group=NB}}

==References==
{{reflist}}

==Further reading==
* {{cite book |author-last=Levine |author-first=John R. |author-link=John R. Levine |title=Linkers and Loaders |date=2000 |orig-year=October 1999 |edition=1 |publisher=[[Morgan Kaufmann]] |series=The Morgan Kaufmann Series in Software Engineering and Programming |location=San Francisco, USA |isbn=1-55860-496-0 |oclc=42413382 |chapter=Chapter 9: Shared Libraries & Chapter 10: Dynamic Linking and Loading |url=https://www.iecc.com/linker/ |access-date=2020-01-12 |url-status=live |archive-url=https://archive.today/20121205032107/http://www.iecc.com/linker/ |archive-date=2012-12-05}} Code: [https://archive.today/20200114225034/https://linker.iecc.com/code.html][ftp://ftp.iecc.com/pub/linker/]{{dead link|date=May 2025|bot=medic}}{{cbignore|bot=medic}} Errata: [https://linker.iecc.com/<!-- https://archive.today/20200114224817/https://linker.iecc.com/ 2020-01-14 -->]
* Article ''[http://www.lurklurk.org/linkers/linkers.html Beginner's Guide to Linkers]'' by David Drysdale
* Article ''[https://objprelink.sourceforge.net/objprelink.html Faster C++ program startups by improving runtime linking efficiency]'' by Léon Bottou and John Ryland
* {{usurped|1=[https://web.archive.org/web/20060628062553/http://www.enderunix.org/simsek/articles/libraries.pdf How to Create Program Libraries]}} by Baris Simsek
* [https://sourceware.org/binutils/docs-2.30/bfd/index.html BFD] - the Binary File Descriptor Library
* [http://lcsd05.cs.tamu.edu 1st Library-Centric Software Design Workshop LCSD'05] {{Webarchive|url=https://web.archive.org/web/20190828045251/http://lcsd05.cs.tamu.edu/ |date=2019-08-28  }} at OOPSLA'05
* [https://web.archive.org/web/20060618065018/http://lcsd.cs.tamu.edu/2006/ 2nd Library-Centric Software Design Workshop LCSD'06] at OOPSLA'06
* [http://people.redhat.com/drepper/dsohowto.pdf How to create shared library] by Ulrich Drepper (with much background info)
* [http://www.ibm.com/developerworks/linux/library/l-dynamic-libraries/ Anatomy of Linux dynamic libraries] at IBM.com

{{Computer science}}
{{Application binary interface}}
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