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In
computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...
, a library is a collection of non-volatile resources used by
computer program A computer program is a sequence or set of instructions in a programming language for a computer to execute. Computer programs are one component of software, which also includes documentation and other intangible components. A computer program ...
s, often for
software development Software development is the process of conceiving, specifying, designing, programming, documenting, testing, and bug fixing involved in creating and maintaining applications, frameworks, or other software components. Software development invol ...
. These may include configuration data, documentation, help data, message templates, pre-written code and
subroutine In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed. Functions may ...
s, classes,
values In ethics and social sciences, value denotes the degree of importance of something or action, with the aim of determining which actions are best to do or what way is best to live (normative ethics in ethics), or to describe the significance of di ...
or type specifications. In IBM's OS/360 and its successors they are referred to as partitioned data sets. A library is also a collection of implementations of behavior, written in terms of a language, that has a well-defined
interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...
by which the behavior is invoked. For instance, people who want to write a higher-level program can use a library to make
system call In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...
s instead of implementing those system calls over and over again. In addition, the behavior is provided for reuse by multiple independent programs. A program invokes the library-provided behavior via a mechanism of the language. For example, in a simple imperative language such as C, the behavior in a library is invoked by using C's normal function-call. What distinguishes the call as being to a library function, versus being to another function in the same program, is the way that the code is organized in the system. Library code is organized in such a way that it can be used by multiple programs that have no connection to each other, while code that is part of a program is organized to be used only within that one program. This distinction can gain a hierarchical notion when a program grows large, such as a multi-million-line program. In that case, there may be internal libraries that are reused by independent sub-portions of the large program. The distinguishing feature is that a library is organized for the purposes of being reused by independent programs or sub-programs, and the user only needs to know the interface and not the internal details of the library. The value of a library lies in the reuse of standardized program elements. When a program invokes a library, it gains the behavior implemented inside that library without having to implement that behavior itself. Libraries encourage the sharing of code in a
modular Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a s ...
fashion and ease the distribution of the code. The behavior implemented by a library can be connected to the invoking program at different
program lifecycle phase {{about, a technical topic, project management, software development process Program lifecycle phases are the stages a computer program undergoes, from initial creation to deployment and execution. The phases are edit time, compile time, link tim ...
s. If the code of the library is accessed during the build of the invoking program, then the library is called a
static library In computer science, a static library or statically-linked library is a set of routines, external functions and variables which are resolved in a caller at compile-time and copied into a target application by a compiler, linker, or binder, produ ...
. An alternative is to build the executable of the invoking program and distribute that, independently of the library implementation. The library behavior is connected after the executable has been invoked to be executed, either as part of the process of starting the execution, or in the middle of execution. In this case the library is called a dynamic library (loaded at runtime). A dynamic library can be loaded and linked when preparing a program for execution, by the
linker Linker or linkers may refer to: Computing * Linker (computing), a computer program that takes one or more object files generated by a compiler or generated by an assembler and links them with libraries, generating an executable program or shar ...
. Alternatively, in the middle of execution, an application may explicitly request that a module be loaded. Most compiled languages have a standard library, although programmers can also create their own custom libraries. Most modern software systems provide libraries that implement the majority of the system services. Such libraries have organized the services which a modern application requires. As such, most code used by modern applications is provided in these system libraries.


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." In 1947 Goldstine and
von Neumann Von Neumann may refer to: * John von Neumann (1903–1957), a Hungarian American mathematician * Von Neumann family * Von Neumann (surname), a German surname * Von Neumann (crater), a lunar impact crater See also * Von Neumann algebra * Von Ne ...
speculated that it would be useful to create a "library" of
subroutine In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed. Functions may ...
s for their work on the
IAS machine The IAS machine was the first electronic computer built at the Institute for Advanced Study (IAS) in Princeton, New Jersey. It is sometimes called the von Neumann machine, since the paper describing its design was edited by John von Neumann, a ...
, an early computer that was not yet operational at that time. They envisioned a physical library of magnetic wire recordings, with each wire storing reusable computer code. Inspired by von Neumann, Wilkes and his team constructed EDSAC. A
filing cabinet A filing cabinet (or sometimes file cabinet in American English) is a piece of office furniture for storing paper documents in file folders. In the most simple context, it is an enclosure for drawers in which items are stored. The two most comm ...
of
punched tape Five- and eight-hole punched paper tape Paper tape reader on the Harwell computer with a small piece of five-hole tape connected in a circle – creating a physical program loop Punched tape or perforated paper tape is a form of data storage ...
held the subroutine library for this computer. Programs for EDSAC consisted of a main program and a sequence of subroutines copied from the subroutine library. In 1951 the team published the first textbook on programming, ''
The Preparation of Programs for an Electronic Digital Computer ''The Preparation of Programs for an Electronic Digital Computer'' (sometimes called ''WWG'', after its authors' initials) was the first book on computer programming. Published in 1951, it was written by Maurice Wilkes, David Wheeler, and Stanl ...
'', which detailed the creation and the purpose of the library. COBOL included "primitive capabilities for a library system" in 1959, but
Jean Sammet Jean may refer to: People * Jean (female given name) * Jean (male given name) * Jean (surname) Fictional characters * Jean Grey, a Marvel Comics character * Jean Valjean, fictional character in novel ''Les Misérables'' and its adaptations * Jea ...
described them as "inadequate library facilities" in retrospect.Wexelblat, ''op. cit.'', p. 258
JOVIAL JOVIAL is a high-level programming language based on ALGOL 58, specialized for developing embedded systems (specialized computer systems designed to perform one or a few dedicated functions, usually embedded as part of a larger, more complete dev ...
had 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 In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed. Functions may ...
innovation of FORTRAN. FORTRAN subprograms can be compiled independently of each other, but the compiler lacked a
linker Linker or linkers may refer to: Computing * Linker (computing), a computer program that takes one or more object files generated by a compiler or generated by an assembler and links them with libraries, generating an executable program or shar ...
. So prior to the introduction of modules in Fortran-90,
type checking In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progra ...
between FORTRANIt was possible earlier between, e.g., Ada subprograms. subprograms was impossible. 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. Simula was the first
object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pr ...
language, and its classes were nearly identical to the modern concept as used in
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
,
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
, and C#. The ''class'' concept of Simula was also a progenitor of the ''package'' in
Ada Ada may refer to: Places Africa * Ada Foah, a town in Ghana * Ada (Ghana parliament constituency) * Ada, Osun, a town in Nigeria Asia * Ada, Urmia, a village in West Azerbaijan Province, Iran * Ada, Karaman, a village in Karaman Province, ...
and the ''module'' of Modula-2.Wilson and Clark, ''op. cit.'', p. 52 Even when developed originally in 1965, Simula classes could be included in library files and added at compile time.Wexelblat, ''op. cit.'', p. 716


Linking

Libraries are important in the program ''linking'' or ''binding'' process, which resolves references known as ''links'' or ''symbols'' to library modules. The linking process is usually automatically done by a ''
linker Linker or linkers may refer to: Computing * Linker (computing), a computer program that takes one or more object files generated by a compiler or generated by an assembler and links them with libraries, generating an executable program or shar ...
'' or ''binder'' program that searches a set of libraries and other modules in a given order. Usually it is not considered an error if a link target can be found multiple times in a given set of libraries. Linking may be done when an executable file is created, or whenever the program is used at runtime. The references being resolved may be addresses for jumps and other routine calls. They may be in the main program, or in one module depending upon another. They are resolved into fixed or relocatable addresses (from a common base) by allocating runtime memory for the
memory segment Memory segmentation is an operating system memory management technique of division of a computer's primary memory into segments or sections. In a computer system using segmentation, a reference to a memory location includes a value that identifie ...
s of each module referenced. Some programming languages use a feature called ''smart linking'' whereby the linker is aware of or integrated with the compiler, such that the linker knows how external references are used, and code in a library that is never actually ''used'', even though internally referenced, can be discarded from the compiled application. For example, a program that only uses integers for arithmetic, or does no arithmetic operations at all, can exclude floating-point library routines. This smart-linking feature can lead to smaller application file sizes 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'' is the process of adjusting these references, and is done either by the linker or the
loader Loader can refer to: * Loader (equipment) * Loader (computing) ** LOADER.EXE, an auto-start program loader optionally used in the startup process of Microsoft Windows ME * Loader (surname) * Fast loader * Speedloader * Boot loader ** LOADER.COM ...
. 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 In computing, position-independent code (PIC) or position-independent executable (PIE) is a body of machine code that, being placed somewhere in the primary memory, executes properly regardless of its absolute address. PIC is commonly used fo ...
avoids references to absolute addresses and therefore does not require relocation.


Static libraries

When linking is performed during the creation of an executable or another object file, it is known as ''static linking'' or ''early binding''. In this case, the linking is usually done by a
linker Linker or linkers may refer to: Computing * Linker (computing), a computer program that takes one or more object files generated by a compiler or generated by an assembler and links them with libraries, generating an executable program or shar ...
, but may also be done by the
compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...
. A ''static library'', also known as an ''archive'', is one intended to be statically linked. Originally, only static libraries existed. Static linking must be performed when any modules are recompiled. All of the modules required by a program are sometimes statically linked and copied into the executable file. This process, and the resulting stand-alone file, is known as a
static build A static build is a compiled version of a program which has been statically linked against libraries. Linking In computer science, linking means taking one or more objects generated by compilers and assembling them into a single executable progr ...
of the program. A static build may not need any further relocation if
virtual memory In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...
is used and no
address space layout randomization Address space layout randomization (ASLR) is a computer security technique involved in preventing exploitation of memory corruption vulnerabilities. In order to prevent an attacker from reliably jumping to, for example, a particular exploited f ...
is desired.


Shared libraries

A shared library or shared object is a file that is intended to be shared by
executable files In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instructions", as opposed to a data file ...
and further shared
object file An object file is a computer file containing object code, that is, machine code output of an assembler or compiler. The object code is usually relocatable, and not usually directly executable. There are various formats for object files, and the ...
s. Modules used by a program are loaded from individual shared objects into memory at
load time In computer systems a loader is the part of an operating system that is responsible for loading programs and libraries. It is one of the essential stages in the process of starting a program, as it places programs into memory and prepares them ...
or runtime, rather than being copied by a linker when it creates a single monolithic executable file for the program. Shared libraries can be statically linked during compile-time, meaning that references to the library modules are resolved and the modules are allocated memory when the executable file is created. But often linking of shared libraries is postponed until they are loaded. Most modern
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
sSome older systems, e.g., Burroughs MCP,
Multics Multics ("Multiplexed Information and Computing Service") is an influential early time-sharing operating system based on the concept of a single-level memory.Dennis M. Ritchie, "The Evolution of the Unix Time-sharing System", Communications of ...
, also have only a single format for executable files, regardless of whether they are shared.
can have shared library files of the same format as the executable files. This offers two main advantages: first, it requires making only one loader for both of them, rather than two (having the single loader is considered well worth its added complexity). Secondly, it allows the executables also to be used as shared libraries, if they have a
symbol table In computer science, a symbol table is a data structure used by a language translator such as a compiler or interpreter, where each identifier (or symbols), constants, procedures and functions in a program's source code is associated with info ...
. Typical combined executable and shared library formats are
ELF An elf () is a type of humanoid supernatural being in Germanic mythology and folklore. Elves appear especially in North Germanic mythology. They are subsequently mentioned in Snorri Sturluson's Icelandic Prose Edda. He distinguishes "ligh ...
and
Mach-O Mach-O, short for Mach object file format, is a file format for executables, object code, shared libraries, dynamically-loaded code, and core dumps. It was developed to replace the a.out format. Mach-O is used by some systems based on the ...
(both in Unix) and PE (Windows). In some older environments such as 16-bit Windows or MPE for the
HP 3000 The HP 3000 series is a family of 16-bit and 32-bit minicomputers from Hewlett-Packard. It was designed to be the first minicomputer with full support for time-sharing in the hardware and the operating system, features that had mostly been limite ...
, only stack-based data (local) was allowed in shared-library code, or other significant restrictions were placed on shared-library code.


Memory sharing

Library code may be shared in memory by multiple
process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business process, activities that produce a specific se ...
es, and on disk. If virtual memory is used, processes would execute the same physical page of RAM that is mapped into the different address spaces of the processes. This has advantages. For instance, on the
OpenStep OpenStep is a defunct object-oriented application programming interface (API) specification for a legacy object-oriented operating system, with the basic goal of offering a NeXTSTEP-like environment on non-NeXTSTEP operating systems. OpenStep wa ...
system, applications were often only a few hundred kilobytes in size and loaded quickly; most of their code was located in libraries that had already been loaded for other purposes by the operating system. Programs can accomplish RAM sharing by using
position-independent code In computing, position-independent code (PIC) or position-independent executable (PIE) is a body of machine code that, being placed somewhere in the primary memory, executes properly regardless of its absolute address. PIC is commonly used fo ...
, as in
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and ot ...
, which leads to a complex but flexible architecture, or by using common virtual addresses, as in Windows and
OS/2 OS/2 (Operating System/2) is a series of computer operating systems, initially created by Microsoft and IBM under the leadership of IBM software designer Ed Iacobucci. As a result of a feud between the two companies over how to position OS/2 r ...
. These systems ensure, by various means, like pre-mapping the address space and reserving slots for each shared library, that code has a high probability of being shared. A third alternative is
single-level store Single-level storage (SLS) or single-level memory is a computer storage term which has had two meanings. The two meanings are related in that in both, pages of memory may be in primary storage (RAM) or in secondary storage (disk), and that the ph ...
, as used by the
IBM System/38 The System/38 is a discontinued minicomputer and midrange computer manufactured and sold by IBM. The system was announced in 1978. The System/38 has 48-bit addressing, which was unique for the time, and a novel integrated database system. It w ...
and its successors. This allows position-dependent code, but places no significant restrictions on where code can be placed or how it can be shared. In some cases, different versions of shared libraries can cause problems, especially when libraries of different versions have the same file name, and different applications installed on a system each require a specific version. Such a scenario is known as
DLL hell In computing, DLL Hell is a term for the complications that arise when one works with dynamic-link libraries (DLLs) used with Microsoft Windows operating systems, particularly legacy 16-bit editions, which all run in a single memory space. DLL Hel ...
, named after the Windows and OS/2
DLL file Dynamic-link library (DLL) is Microsoft's implementation of the shared library concept in the Microsoft Windows and OS/2 operating systems. These libraries usually have the file extension DLL, OCX (for libraries containing ActiveX controls), o ...
. Most modern operating systems after 2001 have clean-up methods to eliminate such situations or use application-specific "private" libraries.


Dynamic linking

Dynamic linking or
late binding In computing, late binding or dynamic linkage—though not an identical process to dynamically linking imported code libraries—is a computer programming mechanism in which the method being called upon an object, or the function being called ...
is linking performed while a program is being loaded (
load time In computer systems a loader is the part of an operating system that is responsible for loading programs and libraries. It is one of the essential stages in the process of starting a program, as it places programs into memory and prepares them ...
) or executed ( runtime), rather than when the executable file is created. A dynamically linked library ( dynamic-link library, or DLL, under
Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
and
OS/2 OS/2 (Operating System/2) is a series of computer operating systems, initially created by Microsoft and IBM under the leadership of IBM software designer Ed Iacobucci. As a result of a feud between the two companies over how to position OS/2 r ...
; shareable image under OpenVMS; dynamic shared object, or DSO, under
Unix-like A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
systems) is a library intended for dynamic linking. Only a minimal amount of work is done by the
linker Linker or linkers may refer to: Computing * Linker (computing), a computer program that takes one or more object files generated by a compiler or generated by an assembler and links them with libraries, generating an executable program or shar ...
when the executable file is created; it only records what library routines the program needs and the index names or numbers of the routines in the library. The majority of the work of linking is done at the time the application is loaded (load time) or during execution (runtime). Usually, the necessary linking program, called a "dynamic linker" or "linking loader", is actually part of the underlying
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
. (However, it is possible, and not exceedingly difficult, to write a program that uses dynamic linking and includes its own dynamic linker, even for an operating system that itself provides no support for dynamic linking.) Programmers originally developed dynamic linking in the
Multics Multics ("Multiplexed Information and Computing Service") is an influential early time-sharing operating system based on the concept of a single-level memory.Dennis M. Ritchie, "The Evolution of the Unix Time-sharing System", Communications of ...
operating system, starting in 1964, and the MTS (
Michigan Terminal System The Michigan Terminal System (MTS) is one of the first time-sharing computer operating systems.. Developed in 1967 at the University of Michigan for use on IBM S/360-67, S/370 and compatible mainframe computers, it was developed and used by a con ...
), built in the late 1960s.


Optimizations

Since shared libraries on most systems do not change often, systems can compute a likely load address for each shared library on the system before it is needed and store that information in the libraries and executables. If every shared library that is loaded has undergone this process, then each will load at its predetermined address, which speeds up the process of dynamic linking. This optimization is known as prebinding or prelinking on macOS and Linux, respectively. IBM
z/VM z/VM is the current version in IBM's VM family of virtual machine operating systems. z/VM was first released in October 2000 and remains in active use and development . It is directly based on technology and concepts dating back to the 1960s, wi ...
uses a similar technique, called "Discontinuous Saved Segments" (DCSS). Disadvantages of this technique include the time required to precompute these addresses every time the shared libraries change, the inability to use
address space layout randomization Address space layout randomization (ASLR) is a computer security technique involved in preventing exploitation of memory corruption vulnerabilities. In order to prevent an attacker from reliably jumping to, for example, a particular exploited f ...
, and the requirement of sufficient virtual address space for use (a problem that will be alleviated by the adoption of
64-bit In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit CPUs and ALUs are those that are based on processor registers, address buses, or data buses of that size. A compu ...
architectures, at least for the time being).


Locating libraries at runtime

Loaders for shared libraries vary widely in functionality. Some depend on the executable storing explicit paths to the libraries. Any change to the library naming or layout of the file system will cause these systems to fail. More commonly, only the name of the library (and not the path) is stored in the executable, with the operating system supplying a method to find the library on disk, based on some algorithm. If a shared library that an executable depends on is deleted, moved, or renamed, or if an incompatible version of the library is copied to a place that is earlier in the search, the executable would fail to load. This is called ''
dependency hell Dependency hell is a colloquial term for the frustration of some software users who have installed software packages which have dependencies on specific versions of other software packages. The dependency issue arises when several packages have ...
'', existing on many platforms. The (infamous) Windows variant is commonly known as
DLL hell In computing, DLL Hell is a term for the complications that arise when one works with dynamic-link libraries (DLLs) used with Microsoft Windows operating systems, particularly legacy 16-bit editions, which all run in a single memory space. DLL Hel ...
. This problem cannot occur if each version of each library is uniquely identified and each program references libraries only by their full unique identifiers. The "DLL hell" problems with earlier Windows versions arose from using only the names of libraries, which were not guaranteed to be unique, to resolve dynamic links in programs. (To avoid "DLL hell", later versions of Windows rely largely on options for programs to install private DLLs—essentially a partial retreat from the use of shared libraries—along with mechanisms to prevent replacement of shared system DLLs with earlier versions of them.)


Microsoft Windows

Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
checks the
registry Registry may refer to: Computing * Container registry, an operating-system-level virtualization registry * Domain name registry, a database of top-level internet domain names * Local Internet registry * Metadata registry, information system for re ...
to determine the proper place to load DLLs that implement COM objects, but for other DLLs it will check the directories in a defined order. First, Windows checks the directory where it loaded the program (''private DLL''); any directories set by calling the SetDllDirectory() function; the System32, System, and Windows directories; then the current working directory; and finally the directories specified by the PATH
environment variable An environment variable is a dynamic-named value that can affect the way running processes will behave on a computer. They are part of the environment in which a process runs. For example, a running process can query the value of the TEMP envi ...
. Applications written for the
.NET Framework The .NET Framework (pronounced as "''dot net"'') is a proprietary software framework developed by Microsoft that runs primarily on Microsoft Windows. It was the predominant implementation of the Common Language Infrastructure (CLI) until bein ...
(since 2002), also check the
Global Assembly Cache The Global Assembly Cache (GAC) is a machine-wide CLI assembly cache for the Common Language Infrastructure (CLI) in Microsoft's .NET Framework. The approach of having a specially controlled central repository addresses the flaws in the shared lib ...
as the primary store of shared dll files to remove the issue of
DLL hell In computing, DLL Hell is a term for the complications that arise when one works with dynamic-link libraries (DLLs) used with Microsoft Windows operating systems, particularly legacy 16-bit editions, which all run in a single memory space. DLL Hel ...
.


OpenStep

OpenStep OpenStep is a defunct object-oriented application programming interface (API) specification for a legacy object-oriented operating system, with the basic goal of offering a NeXTSTEP-like environment on non-NeXTSTEP operating systems. OpenStep wa ...
used a more flexible system, collecting a list of libraries from a number of known locations (similar to the PATH concept) when the system first starts. Moving libraries around causes no problems at all, although users incur a time cost when first starting the system.


Unix-like systems

Most
Unix-like A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
systems have a "search path" specifying file-system
directories Directory may refer to: * Directory (computing), or folder, a file system structure in which to store computer files * Directory (OpenVMS command) * Directory service, a software application for organizing information about a computer network's ...
in which to look for dynamic libraries. Some systems specify the default path in a
configuration file In computing, configuration files (commonly known simply as config files) are files used to configure the parameters and initial settings for some computer programs. They are used for user applications, server processes and operating system ...
, others hard-code it into the dynamic loader. Some
executable file In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instructions", as opposed to a data fil ...
formats can specify additional directories in which to search for libraries for a particular program. This can usually be overridden with an
environment variable An environment variable is a dynamic-named value that can affect the way running processes will behave on a computer. They are part of the environment in which a process runs. For example, a running process can query the value of the TEMP envi ...
, although it is disabled for
setuid The Unix access rights flags setuid and setgid (short for ''set user identity'' and ''set group identity'') allow users to run an executable with the file system permissions of the executable's owner or group respectively and to change behaviour ...
and setgid programs, so that a user can't force such a program to run arbitrary code with root permissions. Developers of libraries are encouraged to place their dynamic libraries in places in the default search path. On the downside, this can make installation of new libraries problematic, and these "known" locations quickly become home to an increasing number of library files, making management more complex.


Dynamic loading

Dynamic loading, a subset of dynamic linking, involves a dynamically linked library loading and unloading at runtime on request. Such a request may be made implicitly or explicitly. Implicit requests are made when a compiler or static linker adds library references that include file paths or simply file names. Explicit requests are made when applications make direct calls to an operating system's API. Most operating systems that support dynamically linked libraries also support dynamically loading such libraries via a run-time linker
API An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...
. For instance,
Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
uses the API functions LoadLibrary, LoadLibraryEx, FreeLibrary and GetProcAddress with Microsoft Dynamic Link Libraries;
POSIX The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inter ...
-based systems, including most UNIX and UNIX-like systems, use dlopen, dlclose and dlsym. Some development systems automate this process.


Object libraries

Although originally pioneered in the 1960s, dynamic linking did not reach
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
s used by consumers until the late 1980s. It was generally available in some form in most operating systems by the early 1990s. During this same period,
object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pr ...
(OOP) was becoming a significant part of the programming landscape. OOP with runtime binding requires additional information that traditional libraries don't supply. In addition to the names and entry points of the code located within, they also require a list of the objects they depend on. This is a side-effect of one of OOP's core concepts, inheritance, which means that parts of the complete definition of any method may be in different places. This is more than simply listing that one library requires the services of another: in a true OOP system, the libraries themselves may not be known at
compile time In computer science, compile time (or compile-time) describes the time window during which a computer program is compiled. The term is used as an adjective to describe concepts related to the context of program compilation, as opposed to concept ...
, and vary from system to system. At the same time many developers worked on the idea of multi-tier programs, in which a "display" running on a desktop computer would use the services of a mainframe or minicomputer for data storage or processing. For instance, a program on a GUI-based computer would send messages to a minicomputer to return small samples of a huge dataset for display.
Remote procedure call In distributed computing, a remote procedure call (RPC) is when a computer program causes a procedure ( subroutine) to execute in a different address space (commonly on another computer on a shared network), which is coded as if it were a normal ( ...
s (RPC) already handled these tasks, but there was no standard RPC system. Soon the majority of the minicomputer and mainframe vendors instigated projects to combine the two, producing an OOP library format that could be used anywhere. Such systems were known as object libraries, or distributed objects, if they supported remote access (not all did). Microsoft's COM is an example of such a system for local use. DCOM, a modified version of COM, supports remote access. For some time object libraries held the status of the "next big thing" in the programming world. There were a number of efforts to create systems that would run across platforms, and companies competed to try to get developers locked into their own system. Examples include IBM's
System Object Model In computing, the System Object Model (SOM) is an Object-oriented programming, object-oriented shared library system developed by IBM. DSOM, a distributed version based on CORBA, allowed objects on different computers to communicate. SOM defin ...
(SOM/DSOM),
Sun Microsystems Sun Microsystems, Inc. (Sun for short) was an American technology company that sold computers, computer components, software, and information technology services and created the Java programming language, the Solaris operating system, ZFS, the ...
'
Distributed Objects Everywhere Distributed Objects Everywhere (DOE) was a long-running Sun Microsystems project to build a distributed computing environment based on the CORBA system in the 'back end' and OpenStep as the user interface. First started in 1990 and announced soon t ...
(DOE),
NeXT Next may refer to: Arts and entertainment Film * ''Next'' (1990 film), an animated short about William Shakespeare * ''Next'' (2007 film), a sci-fi film starring Nicolas Cage * '' Next: A Primer on Urban Painting'', a 2005 documentary film Lit ...
's
Portable Distributed Objects Portable Distributed Objects (PDO) is an application programming interface (API) for creating object-oriented code that can be executed remotely on a network of computers. It was created by NeXT Computer, Inc. using their OpenStep system, whose us ...
(PDO), Digital's ObjectBroker, Microsoft's Component Object Model (COM/DCOM), and any number of
CORBA The Common Object Request Broker Architecture (CORBA) is a standard defined by the Object Management Group (OMG) designed to facilitate the communication of systems that are deployed on diverse platforms. CORBA enables collaboration between s ...
-based systems.


Class libraries

Class libraries are the rough OOP equivalent of older types of code libraries. They contain classes, which describe characteristics and define actions (
methods Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to: *Scien ...
) that involve objects. Class libraries are used to create instances, or objects with their characteristics set to specific values. In some OOP languages, like
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
, the distinction is clear, with the classes often contained in library files (like Java's JAR file format) and the instantiated objects residing only in memory (although potentially able to be made persistent in separate files). In others, like Smalltalk, the class libraries are merely the starting point for a system image that includes the entire state of the environment, classes and all instantiated objects. Today most class libraries are stored in a package repository (such as Maven Central for Java). Client code explicitly declare the dependencies to external libraries in build configuration files (such as a Maven Pom in Java).


Remote libraries

Another library technique uses completely separate executables (often in some lightweight form) and calls them using a
remote procedure call In distributed computing, a remote procedure call (RPC) is when a computer program causes a procedure ( subroutine) to execute in a different address space (commonly on another computer on a shared network), which is coded as if it were a normal ( ...
(RPC) over a network to another computer. This maximizes operating system re-use: the code needed to support the library is the same code being used to provide application support and security for every other program. Additionally, such systems do not require the library to exist on the same machine, but can forward the requests over the network. However, such an approach means that every library call requires a considerable amount of overhead. RPC calls are much more expensive than calling a shared library that has already been loaded on the same machine. This approach is commonly used in a
distributed architecture A distributed system is a system whose components are located on different networked computers, which communicate and coordinate their actions by passing messages to one another from any system. Distributed computing is a field of computer sci ...
that makes heavy use of such remote calls, notably client-server systems and
application server An application server is a server that hosts applications or software that delivers a business application through a communication protocol. An application server framework is a service layer model. It includes software components available to a ...
s such as
Enterprise JavaBean Jakarta Enterprise Beans (EJB; formerly Enterprise JavaBeans) is one of several Java APIs for modular construction of enterprise software. EJB is a server-side software component that encapsulates business logic of an application. An EJB web ...
s.


Code generation libraries

Code generation libraries are high-level
API An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...
s that can generate or transform
byte code Bytecode (also called portable code or p-code) is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (norma ...
for
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
. They are used by
aspect-oriented programming In computing, aspect-oriented programming (AOP) is a programming paradigm that aims to increase modularity by allowing the separation of cross-cutting concerns. It does so by adding behavior to existing code (an advice) ''without'' modifying th ...
, some data access frameworks, and for testing to generate dynamic proxy objects. They also are used to intercept field access.


File naming


Most modern Unix-like systems

The system stores libfoo.a and libfoo.so files in directories such as /lib, /usr/lib or /usr/local/lib. The filenames always start with lib, and end with a suffix of .a (
archive An archive is an accumulation of historical records or materials – in any medium – or the physical facility in which they are located. Archives contain primary source documents that have accumulated over the course of an individual or ...
, static library) or of .so (shared object, dynamically linked library). Some systems might have multiple names for a dynamically linked library. These names typically share the same prefix and have different suffixes indicating the version number. Most of the names are names for
symbolic links In computing, a symbolic link (also symlink or soft link) is a file whose purpose is to point to a file or directory (called the "target") by specifying a path thereto. Symbolic links are supported by POSIX and by most Unix-like operating syst ...
to the latest version. For example, on some systems libfoo.so.2 would be the filename for the second major interface revision of the dynamically linked library libfoo. The .la files sometimes found in the library directories are
libtool In computer programming, GNU Libtool is a software development tool, part of the GNU build system, consisting of a shell script created to address the software portability problem when compiling shared libraries from source code. It hides t ...
archives, not usable by the system as such.


macOS

The system inherits static library conventions from
BSD The Berkeley Software Distribution or Berkeley Standard Distribution (BSD) is a discontinued operating system based on Research Unix, developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berk ...
, with the library stored in a .a file, and can use .so-style dynamically linked libraries (with the .dylib suffix instead). Most libraries in macOS, however, consist of "frameworks", placed inside special directories called " bundles" which wrap the library's required files and metadata. For example, a framework called MyFramework would be implemented in a bundle called MyFramework.framework, with MyFramework.framework/MyFramework being either the dynamically linked library file or being a symlink to the dynamically linked library file in MyFramework.framework/Versions/Current/MyFramework.


Microsoft Windows

Dynamic-link libraries Dynamic-link library (DLL) is Microsoft's implementation of the shared library concept in the Microsoft Windows and OS/2 operating systems. These libraries usually have the file extension DLL, OCX (for libraries containing ActiveX controls), o ...
usually have the suffix *.DLL, although other file name extensions may identify specific-purpose dynamically linked libraries, e.g. *.OCX for OLE libraries. The interface revisions are either encoded in the file names, or abstracted away using COM-object interfaces. Depending on how they are compiled, *.LIB files can be either static libraries or representations of dynamically linkable libraries needed only during compilation, known as " import libraries". Unlike in the
UNIX Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and ot ...
world, which uses different file extensions, when linking against .LIB file in
Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
one must first know if it is a regular static library or an import library. In the latter case, a .DLL file must be present at runtime.


See also

* * * * * * * * * * (VCL) * (CLX) * * * * (used by the
C++ Standard Library The C standard library or libc is the standard library for the C programming language, as specified in the ISO C standard. ISO/IEC (2018). '' ISO/IEC 9899:2018(E): Programming Languages - C §7'' Starting from the original ANSI C standard, it was ...
) * *


Notes


References


Further reading

* Code

ftp://ftp.iecc.com/pub/linker/] Errata
https://archive.today/20200114224817/https://linker.iecc.com/ 2020-01-14 -->
* Article

' by David Drysdale * Article

' by Léon Bottou and John Ryland
How to Create Program Libraries
by Baris Simsek

- the Binary File Descriptor Library
1st Library-Centric Software Design Workshop LCSD'05
at OOPSLA'05
2nd Library-Centric Software Design Workshop LCSD'06
at OOPSLA'06
How to create shared library
by Ulrich Drepper (with much background info)
Anatomy of Linux dynamic libraries
at IBM.com {{DEFAULTSORT:Library (Computing) Computer libraries, Operating system technology