413:. 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.)
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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
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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
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Most modern operating systems use the same format for both shared libraries and executable files. This offers two main advantages: first, it requires only one loader (building and maintaining a single loader is considered well worth any added complexity). Secondly, it allows an executable file to be
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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
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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
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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.
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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.
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Shared libraries can be statically linked at compile-time, meaning that references to the library are resolved and the library is allocated memory when the executable file is created. But often linking of shared libraries is postponed until they are loaded.
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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
201:. For perspective, a program can alternatively be monolithic -- built to include the executable code of the library in its executable file, but the library code embedded in the program's executable file is not usable by other programs.
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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
353:. (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.)
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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
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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.
258:, 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
278:. 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
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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.
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function; the System32, System, and
Windows directories; then the current working directory; and finally the directories specified by the PATH
298:. Most modern operating systems after 2001 have clean-up methods to eliminate such situations or use application-specific "private" libraries.
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formats can specify additional directories in which to search for libraries for a particular program. This can usually be overridden with an
240:, only stack-based data (local) was allowed in shared library code, or other significant restrictions were placed on shared library code.
432:, but for other DLLs it will check the directories in a defined order. First, Windows checks the directory where it loaded the program (
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Most operating systems that support dynamically linked libraries also support dynamically loading such libraries via a
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will load at its predetermined address, which speeds up the process of dynamic linking. This optimization is known as
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the executable, with the operating system supplying a method to find the library on disk, based on some algorithm.
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501:"known" locations quickly become home to an increasing number of library files, making management more complex.
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274:, which leads to a complex but flexible architecture, or by using common virtual addresses, as in Windows and
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Dynamic loading, a subset of dynamic linking, involves a dynamically linked library loading and unloading at
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necessary linking program, called a "dynamic linker" or "linking loader", is actually part of the underlying
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Private DLLs are DLLs that are installed with a specific application and used only by that application.
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systems) is a library intended for dynamic linking. Only a minimal amount of work is done by the
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loads the shared library from a file (other than the program's executable file) into memory at
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213:). Examples of file formats use for both shared libraries and executable files include
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in which to look for dynamic libraries. Some systems specify the default path in a
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409:, existing on many platforms. The (infamous) Windows variant is commonly known as
324:), rather than when the executable file is created. A dynamically linked library (
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When running a program that is configured to use a shared library, the
151:"Shared object" redirects here. For the synchronization mechanism, see
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as the primary store of shared dll files to remove the issue of
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to determine the proper place to load DLLs that implement
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Code library designed for mutual use by multiple programs
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Programmers originally developed dynamic linking in the
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is linking performed while a program is being loaded (
488:, others hard-code it into the dynamic loader. Some
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systems have a "search path" specifying file-system
79:. Unsourced material may be challenged and removed.
571:. Some development systems automate this process.
360:operating system, starting in 1964, and the MTS (
254:Library code may be shared in memory by multiple
758:
393:
703:
627:
625:
611:by Ulrich Drepper (with much background info)
390:architectures, at least for the time being).
266:Programs can accomplish RAM sharing by using
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707:Saved Segments Planning and Administration
139:Learn how and when to remove this message
631:
340:; dynamic shared object, or DSO, under
34:may need to be cleaned up or summarized
759:
436:); any directories set by calling the
377:on macOS and Linux, respectively. IBM
209:used as a shared library (if it has a
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416:
77:adding citations to reliable sources
48:
18:
736:Microsoft Developer Network Library
732:"Dynamic-Link Library Search Order"
662:"VSI OpenVMS Linker Utility Manual"
294:, named after the Windows and OS/2
228:In some older environments such as
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384:address space layout randomization
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36:because it has been split from/to
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553:Microsoft Dynamic Link Libraries
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174:designed to be used by multiple
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23:
742:from the original on 9 May 2012
636:. microsoft.com. Archived from
444:. Applications written for the
64:needs additional citations for
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697:
678:
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496:, although it is disabled for
1:
687:Information Technology Digest
632:Anderson, Rick (2000-01-11).
615:
609:How To Write Shared Libraries
448:(since 2002), also check the
394:Locating libraries at runtime
364:), built in the late 1960s.
7:
772:Operating system technology
595:, also have a single format
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10:
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587:Some older systems, e.g.,
508:
305:
247:
150:
738:. Microsoft. 2012-03-06.
268:position-independent code
153:Monitor (synchronization)
704:IBM Corporation (2011).
574:
375:prebinding or prelinking
362:Michigan Terminal System
336:; shareable image under
32:This article or section
535:uses the API functions
634:"The End of DLL Hell"
450:Global Assembly Cache
685:"A History of MTS".
494:environment variable
442:environment variable
326:dynamic-link library
73:improve this article
312:Dynamic linking or
39:Library (computing)
767:Computer libraries
667:. VSI. August 2019
486:configuration file
280:single-level store
533:Microsoft Windows
472:Unix-like systems
438:SetDllDirectory()
422:Microsoft Windows
417:Microsoft Windows
282:, as used by the
176:computer programs
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531:. For instance,
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351:operating system
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88:"Shared library"
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603:Further reading
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511:Dynamic loading
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505:Dynamic loading
490:executable file
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406:dependency hell
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320:) or executed (
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302:Dynamic linking
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172:executable code
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549:GetProcAddress
509:Main article:
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446:.NET Framework
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308:Dynamic linker
306:Main article:
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248:Main article:
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244:Memory sharing
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230:16-bit Windows
170:that contains
160:shared library
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640:on 2001-06-05
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589:Burroughs MCP
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368:Optimizations
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90: –
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84:Find sources:
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62:This article
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35:
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744:. Retrieved
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714:. Retrieved
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669:. Retrieved
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642:. Retrieved
638:the original
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314:late binding
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211:symbol table
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71:Please help
66:verification
63:
37:
33:
545:FreeLibrary
537:LoadLibrary
482:directories
434:private DLL
430:COM objects
424:checks the
761:Categories
746:2012-05-20
671:2021-01-31
644:2012-01-15
616:References
99:newspapers
478:Unix-like
342:Unix-like
318:load time
256:processes
195:load time
180:libraries
178:or other
129:July 2023
740:Archived
525:run-time
465:OpenStep
460:OpenStep
454:DLL hell
426:registry
411:DLL hell
296:DLL file
292:DLL hell
270:, as in
260:OpenStep
236:for the
716:Jan 29,
593:Multics
565:dlclose
527:linker
517:runtime
358:Multics
338:OpenVMS
330:Windows
322:runtime
238:HP 3000
199:runtime
184:runtime
113:scholar
561:dlopen
498:setuid
388:64-bit
346:linker
221:, and
219:Mach-O
115:
108:
101:
94:
86:
711:(PDF)
665:(PDF)
575:Notes
569:dlsym
557:POSIX
551:with
476:Most
166:is a
120:JSTOR
106:books
718:2022
693:(5).
567:and
547:and
379:z/VM
334:OS/2
332:and
276:OS/2
272:Unix
92:news
529:API
234:MPE
232:or
215:ELF
197:or
182:at
162:or
75:by
763::
734:.
689:.
647:.
624:^
591:,
563:,
555:;
543:,
539:,
456:.
225:.
223:PE
217:,
186:.
158:A
749:.
720:.
691:5
674:.
155:.
142:)
136:(
131:)
127:(
117:·
110:·
103:·
96:·
69:.
42:.
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