Dynamic linker

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.

Implementations

Microsoft Windows

For the Microsoft Windows platform see the more detailed article titled Dynamic-link library.

GNU/Linux

The GNU/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. On the GNU/Linux operating system dynamic loaded shared libraries can be identified by the filename's suffix ".so".

The dynamic linker can be influenced into modifying it's behavior during either the programs execution or the programs linking. Examples of this can be seen with the ld (unix) manual page ^[1]. A typical modification of this behavior is the use of the LD_PRELOAD and LD_LIBRARY_PATH environment variables. The previously mentioned variables adjust the executables 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.

The source code for the GNU/Linux linker is part of the glibc project and can be downloaded at the GNU^[2] website. The entire source code is available under a variant of the GNU's GPL, (General Public License), license.

Mac OS X

The Apple Mac OS X based operating systems 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 (computing) 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 programming language. See the Cocoa (API) main article for more information. On the Apple Mac OS X and its open source alternative (Darwin (operating system)) the dynamic loaded shared libraries can be identified by either the filename suffix ".dylib" or by its placement inside a folder with the suffice ".framework".

The dynamic linker can be influenced into modifying some of its behavior, however unlike other Unix like operating systems these modifications are hints that can and sometimes are ignored by the dynamic linker. Examples of this can be seen with the dyld manual page ^[3]. 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.

While the source code to the Apple Mac OS X's dynamic linker is not available an open source and similar version is. The open source version of the Apple Mac OS X platform is often called simply Darwin. While at one time the source code for both were the same with small sections of the code omitted by Apple the source code today has diverged and are actually very different with patches and changes kept in sync between the two different source code trees, (SourceCaches). The open source version of dynamic linker (dyld) can be found at Apple's open source web site ^[4]

Implementations

Microsoft Windows

GNU/Linux

Mac OS X

See also