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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
442:; 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
359:
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
595:
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
477:
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
1090:
583:. 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
493:
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
360:
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
431:, 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
506:
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.
303:
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
550:. 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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39:
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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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89:
169:. The specific operating system and executable format determine how the dynamic linker functions and how it is implemented.
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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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1029:. 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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893:: Darwin/Mac OS X dynamic linker/loader –
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256:). The file formats for DLLs are the same as for Windows
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operating system all files, including executables, are
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807:: OpenBSD dynamic linker/loader –
761:: FreeBSD dynamic linker/loader –
714:: 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
784:: NetBSD dynamic linker/loader –
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Dynamic linking from
Assembler language programs in
938:"Operating Systems Principles: Linking and Loading"
688:
1284:
825:"ZLIBC - Transparent access to compressed files"
521:
1091:Dynamic Linking in Linux and Windows, part one
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153:"), by copying the content of libraries from
514:and can be found in the Apple's open-source
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1117:
312:libraries, sometimes having the extension
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109:Learn how and when to remove this message
745:– Administration and Privileged Commands
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280:Windows. As with EXEs, DLLs can contain
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1019:
935:
913:
45:Please improve this article by adding
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963:
526:In Unix-like operating systems using
16:Part of a computer's operating system
966:"Anatomy of Linux dynamic libraries"
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200:
18:
695:Microsoft Developer Network Library
233:. These libraries usually have the
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1100:Anatomy of Linux dynamic libraries
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14:
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470:or by their placement inside the
260:files – that is,
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1055:from the original on 2012-12-05
989:Sivilotti, Paul (August 2012).
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945:North Carolina State University
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771:
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691:"Creating a Resource-Only DLL"
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372:In Unix-like systems that use
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991:"Dynamic Linking and Loading"
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647:Dynamic dead code elimination
590:
560:IBM OS/360 and its successors
522:XCOFF-based Unix-like systems
454:operating system, and in the
320:files, having the extensions
47:secondary or tertiary sources
1140:Application binary interface
665:Not to be confused with the
408:section of the executable's
362:application binary interface
7:
1082:Dynamic Linking and Loading
614:Computer programming portal
599:
564:Supervisor Call instruction
10:
1319:
1298:Computer security exploits
1245:Foreign function interface
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244:(for libraries containing
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1240:Binary-code compatibility
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1209:Position-independent code
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218:'s implementation of the
149:when it is executed (at "
964:Jones, M. (2008-08-28).
916:"Open Source - Releases"
718:User Commands Reference
652:
689:Microsoft Corporation.
936:Xuxian, Jiang (2009).
292:, in any combination.
34:relies excessively on
998:Ohio State University
554:OS/360 and successors
429:environment variables
380:, 64-bit versions of
1224:Virtual method table
669:compression library.
587:) is the exception.
548:environment variable
504:DYLD_PRINT_LIBRARIES
212:Dynamic-link library
207:Dynamic-link library
1189:Memory segmentation
1026:Linkers and Loaders
500:DYLD_FRAMEWORK_PATH
262:Portable Executable
1293:Computer libraries
1162:Calling convention
155:persistent storage
129:is the part of an
1280:
1279:
1272:Year 2038 problem
901:General Commands
811:General Commands
788:General Commands
765:General Commands
474:for a framework.
416:(shared object).
368:Systems using ELF
231:operating systems
224:Microsoft Windows
201:Microsoft Windows
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196:Implementations
186:dynamic linking
165:and relocating
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40:primary sources
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1233:Related topics
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1204:Opaque pointer
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299:with the same
274:New Executable
254:system drivers
248:controls), or
235:file extension
220:shared library
205:Main article:
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450:In the Apple
446:macOS and iOS
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433:uncompress.so
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402:DragonFly BSD
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60: –
59:
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54:Find sources:
48:
42:
41:
37:
32:This article
30:
26:
21:
20:
1259:
1184:Machine code
1103:
1095:Symantec.com
1094:
1085:
1057:. Retrieved
1025:
1001:. Retrieved
984:
973:. Retrieved
959:
948:. Retrieved
931:
920:. Retrieved
909:
882:
871:. Retrieved
867:"zlibc.conf"
861:
850:. Retrieved
846:
837:
829:the original
819:
796:
780:ld.elf_so(1)
773:
750:
726:
716:Solaris 11.4
703:
694:
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637:Late binding
594:
584:
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540:
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252:(for legacy
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190:late linking
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33:
1219:System call
1199:Object code
1150:conventions
918:. apple.com
914:Apple Inc.
847:delorie.com
488:Objective-C
484:Cocoa Touch
353:posix_spawn
301:file format
163:jump tables
1287:Categories
1214:Relocation
1167:Call stack
1059:2020-01-12
1003:2020-09-24
975:2020-09-24
950:2020-09-24
922:2014-07-04
873:2014-07-04
852:2014-07-04
710:ld.so.1(1)
677:References
591:Efficiency
530:, such as
437:LD_PRELOAD
426:LD_PRELOAD
161:, filling
147:executable
99:March 2011
69:newspapers
36:references
1303:Compilers
1157:Alignment
1023:(2000) .
518:project.
486:APIs and
410:PT_INTERP
338:Unix-like
290:resources
276:(NE) for
264:(PE) for
216:Microsoft
180:and then
123:computing
1086:IECC.com
1069:Errata:
1053:Archived
1049:42413382
803:ld.so(1)
627:DLL Hell
600:See also
581:segments
336:In most
174:compiled
167:pointers
151:run time
1260:dynamic
1172:Library
1104:IBM.com
889:dyld(1)
809:OpenBSD
763:FreeBSD
757:rtld(1)
642:prelink
577:Multics
575:In the
571:Multics
544:LIBPATH
406:.interp
398:OpenBSD
390:FreeBSD
378:Solaris
246:ActiveX
178:process
83:scholar
1267:Loader
1255:Linker
1177:static
1148:Parts,
1064:Code:
1047:
1037:
903:Manual
895:Darwin
813:Manual
790:Manual
786:NetBSD
767:Manual
743:Manual
720:Manual
585:binder
512:Darwin
472:bundle
468:.dylib
464:Mach-O
452:Darwin
400:, and
394:NetBSD
342:kernel
316:, and
288:, and
278:16-bit
270:64-bit
266:32-bit
85:
78:
71:
64:
56:
1142:(ABI)
994:(PDF)
941:(PDF)
899:macOS
739:Linux
653:Notes
528:XCOFF
480:Cocoa
456:macOS
386:Linux
382:HP-UX
297:files
295:Data
182:binds
139:links
135:loads
133:that
90:JSTOR
76:books
1045:OCLC
1035:ISBN
897:and
667:zlib
516:dyld
502:and
496:dyld
482:and
458:and
440:hack
424:and
347:exec
324:and
318:font
310:icon
286:data
282:code
268:and
228:OS/2
226:and
141:the
137:and
125:, a
62:news
970:IBM
532:AIX
460:iOS
414:.so
374:ELF
350:or
326:FOT
322:FON
314:ICL
306:MUI
258:EXE
250:DRV
242:OCX
238:DLL
188:or
159:RAM
157:to
121:In
38:to
1289::
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1051:.
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996:.
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1125:t
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978:.
953:.
925:.
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855:.
697:.
112:)
106:(
101:)
97:(
87:·
80:·
73:·
66:·
43:.
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
