Dynamic linker
Encyclopedia
In computing
, a dynamic linker is the part of an operating system
(OS) that loads and links the shared libraries for an executable when it is executed. 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 at compile time
of the executable while a dynamic linker is in actuality a special loader that loads external shared libraries into a running process and then binds those shared libraries dynamically to the running process. The specifics of how a dynamic linker functions is operating system dependent.
platform see the more detailed article titled Dynamic-link library
.
systems that use ELF
for executable images and dynamic libraries, 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 newly constructed process address space. At compile time an executable has the path of the dynamic linker that should be used embedded into the
The dynamic linker can be influenced into modifying its behavior during either the program's execution or the program's linking. 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 the LD_LIBRARY_PATH and LD_PRELOAD environment variables. These variables adjust the runtime linking process by searching for shared libraries at alternative locations and by forcefully loading and linking libraries that would otherwise not be loaded and linked, respectively.
/Linux
based operating systems implement a dynamic linker model where a portion of the executable includes a very simple linker stub which causes the operating system to load an external library into memory. This linker stub is added at compile time for the target executable. The linker stub's purpose is to load the real dynamic linker machine code into memory and start the dynamic linker process by executing that newly loaded dynamic linker machine code. While the design of the operating system is to have the executable load the dynamic linker before the target executable's main function is started it however is implemented differently. The operating system knows the location of the dynamic linker and in turn loads that in memory during the process creation. Once the executable is loaded into memory the dynamic linker is already there and linker stub simply executes that code. The reason for this change is due to the fact that ELF binary format was designed for multiple Unix-like
operating systems and not just the GNU/Linux operating system.
The source code for the GNU/Linux linker is part of the glibc project and can be downloaded at the GNU website. The entire source code is available under the GNU LGPL.
operating system, and the Mac OS X
and iOS operating systems built atop it, implement a dynamic linker model where most of the machine code that make up the dynamic linker is actually an external executable that the operating system kernel
loads and executes first in a newly constructed process address space. At compile time an executable has the path of the dynamic linker that should be used embedded into one of the Mach-O
load commands. The operating system kernel reads this while creating the new process and in turn loads then executes this other executable binary. 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. Unlike other operating systems the executables on the Apple Mac OS X platform often interact with the dynamic linker during the execution of the process, it is even known that an executable will interact with the dynamic linker causing it to load more libraries and resolve more symbols hours after the initial launch of the executable. The reason a Mac OS X program interacts with the dynamic linker so often is due to Apple's Cocoa API and the language in which it is implemented, Objective-C. See the Cocoa
main article for more information. On the Darwin-based operating systems, the dynamic loaded shared libraries can be identified by either the filename suffix .dylib or by its placement inside the bundle for a framework.
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 with the dyld manual page . A typical modification of this behavior is the use of the DYLD_FRAMEWORK_PATH and DYLD_PRINT_LIBRARIES environment variables. The previously-mentioned variables adjust the executables' search path for the shared libraries, while another displays the names of the libraries as they are loaded and linked.
The source code for Apple's Mac OS X dynamic linker is open source and released as part of Darwin
and can be found in the dyld project at Apple's open source web site
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, a dynamic linker is the part of an operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
(OS) that loads and links the shared libraries for an executable when it is executed. 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 at compile time
Compile time
In computer science, compile time refers to either the operations performed by a compiler , programming language requirements that must be met by source code for it to be successfully compiled , or properties of the program that can be reasoned about at compile time.The operations performed at...
of the executable while a dynamic linker is in actuality a special loader that loads external shared libraries into a running process and then binds those shared libraries dynamically to the running process. The specifics of how a dynamic linker functions is operating system dependent.
Microsoft Windows
For the Microsoft WindowsMicrosoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
platform see the more detailed article titled Dynamic-link library
Dynamic-link library
Dynamic-link library , or DLL, is Microsoft's implementation of the shared library concept in the Microsoft Windows and OS/2 operating systems...
.
ELF-based Unix-like systems
In most Unix-likeUnix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
systems that use ELF
Executable and Linkable Format
In computing, the Executable and Linkable Format is a common standard file format for executables, object code, shared libraries, and core dumps. First published in the System V Application Binary Interface specification, and later in the Tool Interface Standard, it was quickly accepted among...
for executable images and dynamic libraries, 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 newly constructed process address space. At compile time an executable has the path of the dynamic linker that should be used embedded into the
.interp section. The operating system kernel reads this while creating the new process and in turn loads then executes this other executable binary. That binary then loads the executable image and all the dynamically-linked libraries on which it depends, and starts the executable. In Unix-like operating systems using ELF, dynamically-loaded shared libraries can be identified by the filename suffix .so.The dynamic linker can be influenced into modifying its behavior during either the program's execution or the program's linking. 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 the LD_LIBRARY_PATH and LD_PRELOAD environment variables. These variables adjust the runtime linking process by searching for shared libraries at alternative locations and by forcefully loading and linking libraries that would otherwise not be loaded and linked, respectively.
GNU/Linux
The GNUGNU
GNU is a Unix-like computer operating system developed by the GNU project, ultimately aiming to be a "complete Unix-compatible software system"...
/Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
based operating systems implement a dynamic linker model where a portion of the executable includes a very simple linker stub which causes the operating system to load an external library into memory. This linker stub is added at compile time for the target executable. The linker stub's purpose is to load the real dynamic linker machine code into memory and start the dynamic linker process by executing that newly loaded dynamic linker machine code. While the design of the operating system is to have the executable load the dynamic linker before the target executable's main function is started it however is implemented differently. The operating system knows the location of the dynamic linker and in turn loads that in memory during the process creation. Once the executable is loaded into memory the dynamic linker is already there and linker stub simply executes that code. The reason for this change is due to the fact that ELF binary format was designed for multiple Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
operating systems and not just the GNU/Linux operating system.
The source code for the GNU/Linux linker is part of the glibc project and can be downloaded at the GNU website. The entire source code is available under the GNU LGPL.
Mac OS X and iOS
The Apple DarwinDarwin (operating system)
Darwin is an open source POSIX-compliant computer operating system released by Apple Inc. in 2000. It is composed of code developed by Apple, as well as code derived from NeXTSTEP, BSD, and other free software projects....
operating system, and the Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
and iOS operating systems built atop it, implement a dynamic linker model where most of the machine code that make up the dynamic linker is actually an external executable that the operating system kernel
Kernel (computing)
In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources...
loads and executes first in a newly constructed process address space. At compile time an executable has the path of the dynamic linker that should be used embedded into one of the Mach-O
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. A replacement for the a.out format, Mach-O offered more extensibility and faster access to information in the symbol table.Mach-O was once used by...
load commands. The operating system kernel reads this while creating the new process and in turn loads then executes this other executable binary. 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. Unlike other operating systems the executables on the Apple Mac OS X platform often interact with the dynamic linker during the execution of the process, it is even known that an executable will interact with the dynamic linker causing it to load more libraries and resolve more symbols hours after the initial launch of the executable. The reason a Mac OS X program interacts with the dynamic linker so often is due to Apple's Cocoa API and the language in which it is implemented, Objective-C. See the Cocoa
Cocoa (API)
Cocoa is Apple's native object-oriented application programming interface for the Mac OS X operating system and—along with the Cocoa Touch extension for gesture recognition and animation—for applications for the iOS operating system, used on Apple devices such as the iPhone, the iPod Touch, and...
main article for more information. On the Darwin-based operating systems, the dynamic loaded shared libraries can be identified by either the filename suffix .dylib or by its placement inside the bundle for a framework.
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 with the dyld manual page . A typical modification of this behavior is the use of the DYLD_FRAMEWORK_PATH and DYLD_PRINT_LIBRARIES environment variables. The previously-mentioned variables adjust the executables' search path for the shared libraries, while another displays the names of the libraries as they are loaded and linked.
The source code for Apple's Mac OS X dynamic linker is open source and released as part of Darwin
Darwin (operating system)
Darwin is an open source POSIX-compliant computer operating system released by Apple Inc. in 2000. It is composed of code developed by Apple, as well as code derived from NeXTSTEP, BSD, and other free software projects....
and can be found in the dyld project at Apple's open source web site
See also
- Dynamic loadingDynamic loadingDynamic loading is a mechanism by which a computer program can, at run time, load a library into memory, retrieve the addresses of functions and variables contained in the library, execute those functions or access those variables, and unload the library from memory...
- Direct bindingDirect bindingDirect 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 out which library contains the symbol.- Theory :When...
- Linker (computing)
- Loader (computing)Loader (computing)In computing, a loader is the part of an operating system that is responsible for loading programs. It is one of the essential stages in the process of starting a program, as it places programs into memory and prepares them for execution...
- Compile timeCompile timeIn computer science, compile time refers to either the operations performed by a compiler , programming language requirements that must be met by source code for it to be successfully compiled , or properties of the program that can be reasoned about at compile time.The operations performed at...
- Dynamic-link libraryDynamic-link libraryDynamic-link library , or DLL, is Microsoft's implementation of the shared library concept in the Microsoft Windows and OS/2 operating systems...
- DLL HellDLL hellIn computing, DLL Hell is a term for the complications that arise when working with dynamic link libraries used with Microsoft Windows operating systems, particularly legacy 16-bit editions which all run in a single memory space....