Microsoft Windows Dynamic-link library, or DLL, is
Microsoft's implementation of the
shared library concept in the
Microsoft Windows and
OS/2 operating systems. These libraries usually have the
file extension DLL, OCX (for libraries containing
ActiveX controls), or DRV (for legacy
system drivers). The file formats for DLLs are the same as for Windows
EXE files that is,
Portable Executable (PE) for
32-bit and
64-bit Windows, and
New Executable (NE) for
16-bit Windows. As with EXEs, DLLs can contain
code,
data, and
resources, in any combination. Data
files with the same
file format 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 MUI,
icon libraries, sometimes having the extension ICL, and
font files, having the extensions FON and FOT.
Unix-like systems using ELF, and Darwin-based systems In most
Unix-like systems, 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 process address space newly constructed as a result of calling
exec or
posix_spawn functions. At link time, the path of the dynamic linker that should be used is embedded into the executable image. 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 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
application binary interface.
Systems using ELF In Unix-like systems that use
ELF for executable images and dynamic libraries, such as
Solaris, 64-bit versions of
HP-UX,
Linux,
FreeBSD,
NetBSD,
OpenBSD, and
DragonFly BSD, the path of the dynamic linker that should be used is embedded at link time into the .interp section of the executable's PT_INTERP segment. In those systems, dynamically loaded shared libraries can be identified by the filename suffix .so (shared object). 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 LD_LIBRARY_PATH and LD_PRELOAD
environment variables, 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 uncompress.so, which facilitates transparent decompression when used through the LD_PRELOAD
hack; 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.
macOS and iOS In the Apple
Darwin operating system, and in the
macOS and
iOS 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
Mach-O load commands in the executable image. In those systems, dynamically loaded shared libraries can be identified either by the filename suffix .dylib or by their placement inside the
bundle for a framework. 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. 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
Cocoa and
Cocoa Touch APIs and
Objective-C, the language in which they are implemented (see their main articles for more information). 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 dyld's manual page. A typical modification of this behavior is the use of the DYLD_FRAMEWORK_PATH and DYLD_PRINT_LIBRARIES 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. Apple's macOS dynamic linker is an open-source project released as part of
Darwin and can be found in the Apple's open-source dyld project.
XCOFF-based Unix-like systems In Unix-like operating systems using
XCOFF, such as
AIX, dynamically-loaded shared libraries use the filename suffix .a. 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
LIBPATH environment variable. 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.
OS/360 and successors Dynamic linking from Assembler language programs in
IBM OS/360 and its successors is done typically using a LINK macro instruction containing a
Supervisor Call instruction 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 PARMLIB (specified at system startup time), or in the "link pack area" where specific reentrant modules are loaded at system startup time.
Multics In the
Multics operating system all files, including executables, are
segments. 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
binder) is the exception. ==Efficiency==