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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.
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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
453:; 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.
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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
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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
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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
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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
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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
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594:. 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
504:
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
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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
442:, 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
517:
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.
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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
561:. 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.
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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
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load commands in the executable image. In those systems, dynamically loaded shared libraries can be identified either by the filename suffix
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PARMLIB (specified at system startup time), or in the "link pack area" where specific reentrant modules are loaded at system startup time.
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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
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systems, most of the machine code that makes up the dynamic linker is actually an external executable that the operating system
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functions. At link time, the path of the dynamic linker that should be used is embedded into the executable image.
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segment. In those systems, dynamically loaded shared libraries can be identified by the filename suffix
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1040:. The Morgan Kaufmann Series in Software Engineering and Programming (1 ed.). San Francisco, USA:
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loads and executes first in a process address space newly constructed as a result of calling
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those shared libraries dynamically to the running process. This approach is also called
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Linking is often referred to as a process that is performed when the executable is
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Apple's macOS dynamic linker is an open-source project released as part of
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is done typically using a LINK macro instruction containing a
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904:: Darwin/Mac OS X dynamic linker/loader –
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267:). The file formats for DLLs are the same as for Windows
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operating system all files, including executables, are
818:: OpenBSD dynamic linker/loader –
772:: FreeBSD dynamic linker/loader –
725:: Solaris dynamic linker/loader –
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for executable images and dynamic libraries, such as
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Unix-like systems using ELF, and Darwin-based systems
795:: NetBSD dynamic linker/loader –
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Dynamic linking from
Assembler language programs in
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949:"Operating Systems Principles: Linking and Loading"
836:"ZLIBC - Transparent access to compressed files"
1102:Dynamic Linking in Linux and Windows, part one
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164:"), by copying the content of libraries from
8:
525:and can be found in the Apple's open-source
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323:libraries, sometimes having the extension
120:Learn how and when to remove this message
756:– Administration and Privileged Commands
291:Windows. As with EXEs, DLLs can contain
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669:
56:Please improve this article by adding
537:In Unix-like operating systems using
27:Part of a computer's operating system
7:
977:"Anatomy of Linux dynamic libraries"
706:Microsoft Developer Network Library
244:. These libraries usually have the
1111:Anatomy of Linux dynamic libraries
25:
481:or by their placement inside the
271:files – that is,
617:
34:
1066:from the original on 2012-12-05
1000:Sivilotti, Paul (August 2012).
956:North Carolina State University
702:"Creating a Resource-Only DLL"
383:In Unix-like systems that use
1:
1002:"Dynamic Linking and Loading"
658:Dynamic dead code elimination
571:IBM OS/360 and its successors
533:XCOFF-based Unix-like systems
465:operating system, and in the
331:files, having the extensions
58:secondary or tertiary sources
1151:Application binary interface
676:Not to be confused with the
419:section of the executable's
373:application binary interface
1093:Dynamic Linking and Loading
625:Computer programming portal
575:Supervisor Call instruction
1330:
1309:Computer security exploits
1256:Foreign function interface
255:(for libraries containing
215:
1251:Binary-code compatibility
1220:Position-independent code
229:'s implementation of the
160:when it is executed (at "
975:Jones, M. (2008-08-28).
927:"Open Source - Releases"
729:User Commands Reference
700:Microsoft Corporation.
947:Xuxian, Jiang (2009).
303:, in any combination.
45:relies excessively on
1009:Ohio State University
565:OS/360 and successors
440:environment variables
391:, 64-bit versions of
1235:Virtual method table
680:compression library.
598:) is the exception.
559:environment variable
515:DYLD_PRINT_LIBRARIES
223:Dynamic-link library
218:Dynamic-link library
1200:Memory segmentation
1037:Linkers and Loaders
511:DYLD_FRAMEWORK_PATH
273:Portable Executable
1304:Computer libraries
1173:Calling convention
166:persistent storage
140:is the part of an
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1290:
1283:Year 2038 problem
912:General Commands
822:General Commands
799:General Commands
776:General Commands
485:for a framework.
427:(shared object).
379:Systems using ELF
242:operating systems
235:Microsoft Windows
212:Microsoft Windows
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265:system drivers
259:controls), or
246:file extension
231:shared library
216:Main article:
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461:In the Apple
457:macOS and iOS
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71: –
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65:Find sources:
59:
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43:This article
41:
37:
32:
31:
19:
1270:
1195:Machine code
1114:
1106:Symantec.com
1105:
1096:
1068:. Retrieved
1036:
1012:. Retrieved
995:
984:. Retrieved
970:
959:. Retrieved
942:
931:. Retrieved
920:
893:
882:. Retrieved
878:"zlibc.conf"
872:
861:. Retrieved
857:
848:
840:the original
830:
807:
791:ld.elf_so(1)
784:
761:
737:
727:Solaris 11.4
714:
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648:Late binding
605:
595:
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568:
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263:(for legacy
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201:late linking
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64:
44:
1230:System call
1210:Object code
1161:conventions
929:. apple.com
925:Apple Inc.
858:delorie.com
499:Objective-C
495:Cocoa Touch
364:posix_spawn
312:file format
174:jump tables
1298:Categories
1225:Relocation
1178:Call stack
1070:2020-01-12
1014:2020-09-24
986:2020-09-24
961:2020-09-24
933:2014-07-04
884:2014-07-04
863:2014-07-04
721:ld.so.1(1)
688:References
602:Efficiency
541:, such as
448:LD_PRELOAD
437:LD_PRELOAD
172:, filling
158:executable
110:March 2011
80:newspapers
47:references
1314:Compilers
1168:Alignment
1034:(2000) .
529:project.
497:APIs and
421:PT_INTERP
349:Unix-like
301:resources
287:(NE) for
275:(PE) for
227:Microsoft
191:and then
134:computing
1097:IECC.com
1080:Errata:
1064:Archived
1060:42413382
814:ld.so(1)
638:DLL Hell
611:See also
592:segments
347:In most
185:compiled
178:pointers
162:run time
1271:dynamic
1183:Library
1115:IBM.com
900:dyld(1)
820:OpenBSD
774:FreeBSD
768:rtld(1)
653:prelink
588:Multics
586:In the
582:Multics
555:LIBPATH
417:.interp
409:OpenBSD
401:FreeBSD
389:Solaris
257:ActiveX
189:process
94:scholar
1278:Loader
1266:Linker
1188:static
1159:Parts,
1075:Code:
1058:
1048:
914:Manual
906:Darwin
824:Manual
801:Manual
797:NetBSD
778:Manual
754:Manual
731:Manual
596:binder
523:Darwin
483:bundle
479:.dylib
475:Mach-O
463:Darwin
411:, and
405:NetBSD
353:kernel
327:, and
299:, and
289:16-bit
281:64-bit
277:32-bit
96:
89:
82:
75:
67:
1153:(ABI)
1005:(PDF)
952:(PDF)
910:macOS
750:Linux
664:Notes
539:XCOFF
491:Cocoa
467:macOS
397:Linux
393:HP-UX
308:files
306:Data
193:binds
150:links
146:loads
144:that
101:JSTOR
87:books
1056:OCLC
1046:ISBN
908:and
678:zlib
527:dyld
513:and
507:dyld
493:and
469:and
451:hack
435:and
358:exec
335:and
329:font
321:icon
297:data
293:code
279:and
239:OS/2
237:and
152:the
148:and
136:, a
73:news
981:IBM
543:AIX
471:iOS
425:.so
385:ELF
361:or
337:FOT
333:FON
325:ICL
317:MUI
269:EXE
261:DRV
253:OCX
249:DLL
199:or
170:RAM
168:to
132:In
49:to
1300::
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117:(
112:)
108:(
98:·
91:·
84:·
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20:)
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