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In
computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...
, a dynamic linker is the part of an
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
that loads and links the
shared libraries In computer science, a library is a collection of non-volatile resources used by computer programs, often for software development. These may include configuration data, documentation, help data, message templates, pre-written code and ...
needed by an
executable 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 ...
when it is executed (at " run time"), by copying the content of libraries from
persistent storage In computer science, persistence refers to the characteristic of state of a system that outlives (persists more than) the process that created it. This is achieved in practice by storing the state as data in computer data storage. Programs hav ...
to
RAM Ram, ram, or RAM may refer to: Animals * A male sheep * Ram cichlid, a freshwater tropical fish People * Ram (given name) * Ram (surname) * Ram (director) (Ramsubramaniam), an Indian Tamil film director * RAM (musician) (born 1974), Dutch * ...
, filling jump tables and relocating pointers. The specific operating system and executable format determine how the dynamic linker functions and how it is implemented. Linking is often referred to as a process that is performed when the executable is
compiled 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 tha ...
, while a dynamic linker is a special part of an operating system that loads external shared libraries into a running process and then binds those shared libraries dynamically to the running process. This approach is also called dynamic linking or late linking.


Implementations


Microsoft Windows

Dynamic-link library 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) ...
, or DLL, is
Microsoft Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washi ...
's implementation of the
shared library In computer science, a library is a collection of non-volatile resources used by computer programs, often for software development. These may include configuration data, documentation, help data, message templates, pre-written code and subro ...
concept in the
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 ...
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 ...
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
s. These libraries usually have the file extension DLL, OCX (for libraries containing ActiveX controls), or DRV (for legacy system drivers). The file formats for DLLs are the same as for Windows
EXE Exe or EXE may refer to: * .exe, a file extension * exe., abbreviation for executive Places * River Exe, in England * Exe Estuary, in England * Exe Island, in Exeter, England Transportation and vehicles * Exe (locomotive), a British locomotiv ...
files that is,
Portable Executable The Portable Executable (PE) format is a file format for executables, object code, DLLs and others used in 32-bit and 64-bit versions of Windows operating systems. The PE format is a data structure that encapsulates the information necessary fo ...
(PE) for
32-bit In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32- bit units. Compared to smaller bit widths, 32-bit computers can perform large calculati ...
and
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 ...
Windows, and
New Executable The New Executable (abbreviated NE or NewEXE) is a 16-bit .exe file format, a successor to the DOS MZ executable format. It was used in Windows 1.0–3.x, Windows 9x, multitasking MS-DOS 4.0, OS/2 1.x, and the OS/2 subset of Windows NT up to ver ...
(NE) for
16-bit 16-bit microcomputers are microcomputers that use 16-bit microprocessors. A 16-bit register can store 216 different values. The range of integer values that can be stored in 16 bits depends on the integer representation used. With the two ...
Windows. As with EXEs, DLLs can contain
code In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communicati ...
,
data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpret ...
, and
resources Resource refers to all the materials available in our environment which are technologically accessible, economically feasible and culturally sustainable and help us to satisfy our needs and wants. Resources can broadly be classified upon their av ...
, in any combination. Data files with the same
file format A file format is a standard way that information is encoded for storage in a computer file. It specifies how bits are used to encode information in a digital storage medium. File formats may be either proprietary or free. Some file format ...
as a DLL, but with different file extensions and possibly containing only resource sections, can be called resource DLLs. Examples of such DLLs include multi-language user interface libraries with extension MUI,
icon An icon () is a religious work of art, most commonly a painting, in the cultures of the Eastern Orthodox, Oriental Orthodox, and Catholic churches. They are not simply artworks; "an icon is a sacred image used in religious devotion". The mos ...
libraries, sometimes having the extension ICL, and
font In metal typesetting, a font is a particular size, weight and style of a typeface. Each font is a matched set of type, with a piece (a " sort") for each glyph. A typeface consists of a range of such fonts that shared an overall design. In mo ...
files, having the extensions FON and FOT.


Unix-like systems using ELF, and Darwin-based systems

In 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, most of the machine code that makes up the dynamic linker is actually an external executable that the operating system kernel loads and executes first in a process address space newly constructed as a result of calling exec or posix_spawn functions. At link time, the path of the dynamic linker that should be used is embedded into the executable image. When an executable file is loaded, the operating system kernel reads the path of the dynamic linker from it and then attempts to load and execute this other executable binary; if that attempt fails because, for example, there is no file with that path, the attempt to execute the original executable fails. The dynamic linker then loads the initial executable image and all the dynamically-linked libraries on which it depends and starts the executable. As a result, the pathname of the dynamic linker is part of the operating system's
application binary interface In computer software, an application binary interface (ABI) is an interface between two binary program modules. Often, one of these modules is a library or operating system facility, and the other is a program that is being run by a user. An ...
.


Systems using ELF

In Unix-like systems that use ELF for executable images and dynamic libraries, such as Solaris, 64-bit versions of
HP-UX HP-UX (from "Hewlett Packard Unix") is Hewlett Packard Enterprise's proprietary implementation of the Unix operating system, based on Unix System V (initially System III) and first released in 1984. Current versions support HPE Integrity Se ...
,
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whi ...
,
FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...
,
NetBSD NetBSD is a free and open-source Unix operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was forked. It continues to be actively developed and is ava ...
,
OpenBSD OpenBSD is a security-focused, free and open-source, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by forking NetBSD 1.0. According to the website, the OpenBSD project e ...
, and
DragonFly BSD DragonFly BSD is a free and open-source Unix-like operating system forked from FreeBSD 4.8. Matthew Dillon, an Amiga developer in the late 1980s and early 1990s and FreeBSD developer between 1994 and 2003, began working on DragonFly BSD ...
, the path of the dynamic linker that should be used is embedded at link time into the .interp section of the executable's PT_INTERP segment. In those systems, dynamically loaded shared libraries can be identified by the filename suffix .so (shared object). The dynamic linker can be influenced into modifying its behavior during either the program's execution or the program's linking, and the examples of this can be seen in the run-time linker manual pages for various Unix-like systems. A typical modification of this behavior is the use of LD_LIBRARY_PATH and LD_PRELOAD
environment variables 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 en ...
, which adjust the runtime linking process by searching for shared libraries at alternate locations and by forcibly loading and linking libraries that would otherwise not be, respectively. An example is zlibc, also known as uncompress.so, which facilitates transparent decompression when used through the LD_PRELOAD
hack Hack may refer to: Arts, entertainment, and media Games * ''Hack'' (Unix video game), a 1984 roguelike video game * ''.hack'' (video game series), a series of video games by the multimedia franchise ''.hack'' Music * ''Hack'' (album), a 199 ...
; consequently, it is possible to read pre-compressed (gzipped) file data on BSD and Linux systems as if the files were not compressed, essentially allowing a user to add transparent compression to the underlying filesystem, although with some caveats. The mechanism is flexible, allowing trivial adaptation of the same code to perform additional or alternate processing of data during the file read, prior to the provision of said data to the user process that has requested it.


macOS and iOS

In the Apple Darwin operating system, and in the
macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and la ...
and iOS operating systems built on top of it, the path of the dynamic linker that should be used is embedded at link time into one of the
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 Mac ...
load commands in the executable image. In those systems, dynamically loaded shared libraries can be identified either by the filename suffix .dylib or by their placement inside the
bundle Bundle or Bundling may refer to: * Bundling (packaging), the process of using straps to bundle up items Biology * Bundle of His, a collection of heart muscle cells specialized for electrical conduction * Bundle of Kent, an extra conduction path ...
for a framework. The dynamic linker not only links the target executable to the shared libraries but also places machine code functions at specific address points in memory that the target executable knows about at link time. When an executable wishes to interact with the dynamic linker, it simply executes the machine-specific call or jump instruction to one of those well-known address points. The executables on the macOS and iOS platforms often interact with the dynamic linker during the execution of the process; it is even known that an executable might interact with the dynamic linker, causing it to load more libraries and resolve more symbols, hours after it initially launches. The reason that a macOS or iOS program interacts with the dynamic linker so often is due both to Apple's
Cocoa Cocoa may refer to: Chocolate * Chocolate * ''Theobroma cacao'', the cocoa tree * Cocoa bean, seed of ''Theobroma cacao'' * Chocolate liquor, or cocoa liquor, pure, liquid chocolate extracted from the cocoa bean, including both cocoa butter an ...
and
Cocoa Touch Cocoa Touch is the application development environment for building software programs to run on iOS for the iPhone and iPod Touch, iPadOS for the iPad, watchOS for the Apple Watch, and tvOS for the Apple TV, from Apple Inc. Cocoa Touch ...
APIs and
Objective-C Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was selected by NeXT for its NeXT ...
, the language in which they are implemented (see their main articles for more information). The dynamic linker can be coerced into modifying some of its behavior; however, unlike other Unix-like operating systems, these modifications are hints that can be (and sometimes are) ignored by the dynamic linker. Examples of this can be seen in dyld's manual page. A typical modification of this behavior is the use of the DYLD_FRAMEWORK_PATH and DYLD_PRINT_LIBRARIES environment variables. The former of the previously-mentioned variables adjusts the executables' search path for the shared libraries, while the latter displays the names of the libraries as they are loaded and linked. Apple's macOS dynamic linker is an open-source project released as part of Darwin and can be found in the Apple's open-source dyld project.


XCOFF-based Unix-like systems

In Unix-like operating systems using XCOFF, such as
AIX Aix or AIX may refer to: Computing * AIX, a line of IBM computer operating systems *An Alternate Index, for a Virtual Storage Access Method Key Sequenced Data Set * Athens Internet Exchange, a European Internet exchange point Places Belgiu ...
, dynamically-loaded shared libraries use the filename suffix .a. The dynamic linker can be influenced into modifying its behavior during either the program's execution or the program's linking. A typical modification of this behavior is the use of the
LIBPATH An environment variable is a Name resolution (programming languages), dynamic-named Value (computer science), value that can affect the way running process (computing), processes will behave on a computer. They are part of the environment in wh ...
environment variable. This variable adjusts the runtime linking process by searching for shared libraries at alternate locations and by forcibly loading and linking libraries that would otherwise not be, respectively.


OS/360 and successors

Dynamic linking from Assembler language programs in IBM OS/360 and its successors is done typically using a LINK macro instruction containing a Supervisor Call instruction that activates the operating system routines that makes the library module to be linked available to the program. Library modules may reside in a "STEPLIB" or "JOBLIB" specified in control cards and only available to a specific execution of the program, in a library included in the LINKLIST in the PARMLIB (specified at system startup time), or in the "link pack area" where specific reentrant modules are loaded at system startup time.


Multics

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 all files, including executables, are segments. A call to a routine not part of the current segment will cause the system to find the referenced segment, in memory or on disk, and add it to the address space of the running process. Dynamic linking is the normal method of operation, and static linking (using the ''binder'') is the exception.


Efficiency

Dynamic linking is generally slower (requires more CPU cycles) than linking during compilation time, as is the case for most processes executed at runtime. However, dynamic linking is often more space-efficient (on disk and in memory at runtime). When a library is linked statically, every process being run is linked with its own copy of the library functions being called upon. Therefore, if a library is called upon many times by different programs, the same functions in that library are duplicated in several places in the system's memory. Using shared, dynamic libraries means that, instead of linking each file to its own copy of a library at compilation time and potentially wasting memory space, only one copy of the library is ever stored in memory at a time, freeing up memory space to be used elsewhere. Additionally, in dynamic linking, a library is only loaded if it is actually being used.


See also

*
Direct binding Direct binding is a feature of the linker and dynamic linker on Solaris and OpenSolaris. It provides a method to allow libraries to directly bind symbols to other libraries, rather than weakly bind to them and leave the dynamic linker to figure ...
* DLL Hell *
Dynamic loading Dynamic loading is a mechanism by which a computer program can, at run time, load a library (or other binary) into memory, retrieve the addresses of functions and variables contained in the library, execute those functions or access those var ...
* Late binding * prelink *
Dynamic dead code elimination In compiler theory, dead-code elimination (also known as DCE, dead-code removal, dead-code stripping, or dead-code strip) is a compiler optimization to remove code which does not affect the program results. Removing such code has several benefits: ...


Notes


References


Further reading

* Code

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


External links



''IECC.com''
Dynamic Linking in Linux and Windows, part one
''Symantec.com''
Anatomy of Linux dynamic libraries
''IBM.com'' {{Application binary interface Computer libraries Computer security exploits Compilers