Part of a computer's operating system
In
computing
, a
dynamic linker
is the part of an
operating system
that
loads
and
links
the
shared libraries
needed by an
executable
when it is executed (at "
run time
"), by copying the content of libraries from
persistent storage
to
RAM
, filling
jump tables
and relocating
pointers
. 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 when the executable is
compiled
, while a dynamic linker is a special part of an operating system that loads external shared libraries into a running
process
and then
binds
those shared libraries dynamically to the running process. This approach is also called
dynamic linking
or
late linking
.
Implementations
[
edit
]
Microsoft Windows
[
edit
]
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
.
[1]
Unix-like systems using ELF, and Darwin-based systems
[
edit
]
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
[
edit
]
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.
[2]
[3]
[4]
[5]
[6]
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,
[7]
also known as
uncompress.so
,
[a]
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.
[8]
[9]
macOS and iOS
[
edit
]
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.
[10]
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.
[11]
XCOFF-based Unix-like systems
[
edit
]
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
[
edit
]
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
[
edit
]
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
[
edit
]
Dynamic linking is generally slower (requires more CPU cycles) than linking during compilation time,
[12]
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 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.
[13]
Additionally, in dynamic linking, a library is only loaded if it is actually being used.
[14]
See also
[
edit
]
Notes
[
edit
]
- ^
Not to be confused with the
zlib
compression library.
References
[
edit
]
Further reading
[
edit
]
External links
[
edit
]
|
---|
Parts and
conventions
| |
---|
Related topics
| |
---|