In computer science, a linker or link editor is a program that takes one or more objects generated by a compiler and combines them into a single executable program. In IBM mainframe environments such as OS/360 this program is known as a linkage editor.
On Unix variants the term loader is often used as a synonym for linker. Other terminology was in use, too. For example, on SINTRAN III the process performed by a linker (assembling object files into a program) was called loading (as in loading executable code onto a file). Because this usage blurs the distinction between the compile-time process and the run-time process, this article will use linking for the former and loading for the latter. However, in some operating systems the same program handles both the jobs of linking and loading a program.
Computer programs typically comprise several parts or modules; all these parts/modules need not be contained within a single object file, and in such case refer to each other by means of symbols. Typically, an object file can contain three kinds of symbols:
* defined symbols, which allow it to be called by other modules,
* undefined symbols, which call the other modules where these symbols are defined, and
* local symbols, used internally within the object file to facilitate relocation.
When a program comprises multiple object files, the linker combines these files into a unified executable program, resolving the symbols as it goes along.
Linkers can take objects from a collection called a library. Some linkers do not include the whole library in the output; they only include its symbols that are referenced from other object files or libraries. Libraries exist for diverse purposes, and one or more system libraries are usually linked in by default.
The linker also takes care of arranging the objects in a program’s address space. This may involve relocating code that assumes a specific base address to another base. Since a compiler seldom knows where an object will reside, it often assumes a fixed base location (for example, zero). Relocating machine code may involve re-targeting of absolute jumps, loads and stores.
The executable output by the linker may need another relocation pass when it is finally loaded into memory (just before execution). This pass is usually omitted on hardware offering virtual memory — every program is put into its own address space, so there is no conflict even if all programs load at the same base address. This pass may also be omitted if the executable is a position independent executable.
Many operating system environments allow dynamic linking, that is the postponing of the resolving of some undefined symbols until a program is run. That means that the executable code still contains undefined symbols, plus a list of objects or libraries that will provide definitions for these. Loading the program will load these objects/libraries as well, and perform a final linking.
This approach offers two advantages:
* Often-used libraries (for example the standard system libraries) need to be stored in only one location, not duplicated in every single binary.
* If an error in a library function is corrected by replacing the library, all programs using it dynamically will benefit from the correction after restarting them. Programs that included this function by static linking would have to be re-linked first.
There are also disadvantages:
* Known on the Windows platform as “DLL Hell”, an incompatible updated DLL will break executables that depended on the behavior of the previous DLL.
* A program, together with the libraries it uses, might be certified (eg as to correctness, documentation requirements, or performance) as a package, but not if components can be replaced. (This also argues against automatic OS updates in critical systems; in both cases, the OS and libraries form part of a qualified environment.)
As the compiler has no information on the layout of objects in the final output, it cannot take advantage of shorter or more efficient instructions that place a requirement on the address of another object. For example, a jump instruction can reference an absolute address or an offset from the current location, and the offset could be expressed with different lengths depending on the distance to the target. By generating the most conservative instruction (usually the largest relative or absolute variant, depending on platform) and adding relaxation hints, it is possible to substitute shorter or more efficient instructions during the final link. This step can be performed only after all input objects have been read and assigned temporary addresses; the relaxation pass subsequently re-assigns addresses, which may in turn allow more relaxations to occur. In general, the substituted sequences are shorter, which allows this process to always converge on the best solution given a fixed order of objects; if this is not the case, relaxations can conflict, and the linker needs to weigh the advantages of either option.
In a computer operating system , a loader is a component that locates a given program (which can be an application or, in some cases, part of the operating system itself) in offline storage (such as a hard disk ), loads it into main storage (in a personal computer, it’s called random access memory ), and gives that program control of the computer (allows it to execute its instructions).
A loader is a utility of an operating system. It copies programs from a storage device to a computer’s main memory, where the program can then be executed. Like linkers, loaders can also replace virtual addresses with real addresses. Most loaders function without user involvement. They are invisible to the user, but are a recognizable utility to the operating system.
A program that is loaded may itself contain components that are not initially loaded into main storage, but can be loaded if and when their logic is needed. In a multitasking operating system, a program that is sometimes called a dispatcher juggles the computer processor’s time among different tasks and calls the loader when a program associated with a task is not already in main storage. (By program here, we mean a binary file that is the result of a programming language compilation, linkage editing, or some other program preparation process.)
In Unix, the loader is the handler for the system call execve(). The Unix loader’s tasks include:
1. validation (permissions, memory requirements etc.);
2. copying the program image from the disk into main memory;
3. copying the command-line arguments on the stack;
4. initializing registers (e.g., the stack pointer);
5. jumping to the program entry point (_start).
Some computers need relocating loaders, which adjust addresses (pointers) in the executable to compensate for variations in the address at which loading starts. The computers which need relocating loaders are those in which pointers are absolute addresses rather than offsets from the program’s base address. One well-known example is IBM’s System/360 mainframes and their descendants, including the System z9 series.
Dynamic linking loaders are another type of loader that load and link shared libraries (like .dll files) to already loaded running programs
In computer science, a library is a collection of resources used to develop software. These may include subroutines, classes, values or type specifications.
Libraries contain code and data that provide services to independent programs. This allows the sharing and changing of code and data in a modular fashion. Some executables are both standalone programs and libraries, but most libraries are not executable. Executables and libraries make references known as links to each other through the process known as linking, which is typically done by a linker.
As of 2009, most modern software systems provide libraries that implement the majority of system services. Such libraries have commoditized the services which a modern application requires. As such, most code used by modern applications is provided in these system libraries.
The earliest programming concepts analogous to libraries were intended to separate data definitions from the program implementation. JOVIAL brought the “COMPOOL” (Communication Pool) concept to popular attention in 1959, although it adopted the idea from the large-system SAGE software. Following the computer science principles of separation of concerns and information hiding, “Comm Pool’s purpose was to permit the sharing of System Data among many programs by providing a centralized data description.
COBOL also included “primitive capabilities for a library system” in 1959, but Jean Sammet described them as “inadequate library facilities” in retrospect.
Another major contributor to the modern library concept came in the form of the subprogram innovation of FORTRAN. FORTRAN subprograms can be compiled independently of each other, but the compiler lacks a linker. So prior to the introduction of modules in Fortran-90, type checking between subprograms was impossible.
Finally, historians of the concept should remember the influential Simula 67. Simula was the first object-oriented programming language, and its classes are nearly identical to the modern concept as used in Java, C++, and C#. The class concept of Simula was also a progenitor of the package in Ada and the module of Modula-2. Even when developed originally in 1965, Simula classes could be included in library files and added at compile time.
Originally, only static libraries existed. A static library, also known as an archive, consists of a set of routines which are copied into a target application by the compiler, linker, or binder, producing object files and a stand-alone executable file. This process, and the stand-alone executable file, are known as a static build of the target application. Actual addresses for jumps and other routine calls are stored in a relative or symbolic form which cannot be resolved until all code and libraries are assigned final static addresses.
The linker resolves all of the unresolved addresses into fixed or relocatable addresses (from a common base) by loading all code and libraries into actual runtime memory locations. This linking process can take as much, or more, time than the compilation process, and must be performed when any of the modules is recompiled. Most compiled languages have a standard library, but programmers can also create their own custom libraries.
A linker may work on specific types of object files, and thus require specific (compatible) types of libraries. Collecting object files into a static library may ease their distribution and encourage their use. A client, either a program or a library subroutine, accesses a library object by referencing just its name. The linking process resolves references by searching the libraries in the order given. Usually, it is not considered an error if a name can be found multiple times in a given set of libraries.
Some programming languages may use a feature called “smart linking” where the linker is aware of or integrated with the compiler, such that the linker “knows” how external references are used, and code in a library that is never actually used, even though internally referenced, can be discarded from the compiled application. For example, a program that only uses integers for arithmetic, or does no arithmetic operations at all, can exclude the floating-point library routines. This smart-linking feature can lead to smaller application file sizes and reduced memory usage.
Dynamic linking involves loading the subroutines of a library (which may be referred to as a DLL, especially under Windows, or as a DSO (dynamic shared object) under Unix-like systems) into an application program at load time or runtime, rather than linking them in at compile time. Only a minimum amount of work is done at compile time by the linker; it only records what library routines the program needs and the index names or numbers of the routines in the library. The majority of the work of linking is done at the time the application is loaded (loadtime) or during execution (runtime). The necessary linking code, called a loader, is actually part of the underlying operating system. At the appropriate time the loader finds the relevant DLLs or DSOs and adds the relevant data to the process’s memory space.
The increasing use of dynamic linkage has implications for software licensing. For example, the GPL linking exception allows programs which do not license themselves under GPL to link to libraries licensed under GPL without thereby becoming subject to GPL requirements.
Programmers originally developed dynamic linking in the Multics operating system, starting in 1964, and the MTS (Michigan Terminal System), built in the late 1960s.
The loader must solve one problem: the actual location in memory of the library data cannot be known until the executable and all dynamically linked libraries have been loaded into memory. This is because the memory locations used depend on which specific dynamic libraries have been loaded. It is not possible to depend on the absolute location of the data in the executable, nor even in the library, since conflicts between different libraries would result: if two of them specified the same or overlapping addresses, it would be impossible to use both in the same program.
However, in practice, the shared libraries on most systems do not change often. Therefore, systems can compute a likely load address for each shared library on the system before it is needed, and store that information in the libraries and executables. If every shared library that is loaded has undergone this process, then each will load at its predetermined address, which speeds up the process of dynamic linking. This optimization is known as prebinding in Mac OS X and prelinking in Linux. Disadvantages of this technique include the time required to precompute these addresses every time the shared libraries change, the inability to use address space layout randomization, and the requirement of sufficient virtual address space for use (a problem that will be alleviated by the adoption of 64-bit architectures, at least for the time being).
The library itself contains a jump table of all the methods within it, known as entry points. Calls into the library “jump through” this table, looking up the location of the code in memory, then calling it. This introduces overhead in calling into the library, but the delay is so small as to be negligible.
Locating libraries at runtime
Dynamic linkers/loaders vary widely in functionality. Some depend on the executable storing explicit paths to the libraries. Any change to the library naming or layout of the file system will cause these systems to fail. More commonly, only the name of the library (and not the path) is stored in the executable, with the operating system supplying a method to find the library on-disk based on some algorithm.
One of the biggest disadvantages of dynamic linking involves the executables depending on the separately stored libraries in order to function properly. If the library is deleted, moved, or renamed, or if an incompatible version of the DLL is copied to a place that is earlier in the search, the executable would fail to load. On Windows this is commonly known as DLL hell.
Most Unix-like systems have a “search path” specifying file system directories in which to look for dynamic libraries. Some systems specify the default path in a configuration file; others hard-code it into the dynamic loader. Some executable file formats can specify additional directories in which to search for libraries for a particular program. This can usually be overridden with an environment variable, although it is disabled for setuid and setgid programs, so that a user can’t force such a program to run arbitrary code with root permissions. Developers of libraries are encouraged to place their dynamic libraries in places in the default search path. On the downside, this can make installation of new libraries problematic, and these “known” locations quickly become home to an increasing number of library files, making management more complex.
Microsoft Windows will check the registry to determine the proper place to find an ActiveX DLL, but for other DLLs it will check the directory where it loaded the program from; the current working directory; any directories set by calling the SetDllDirectory() function; the System32, System, and Windows directories; and finally the directories specified by the PATH environment variable. Applications written for the .NET Framework framework (since 2002), also check the Global Assembly Cache as the primary store of shared dll files to remove the issue of DLL hell.
OpenStep used a more flexible system, collecting a list of libraries from a number of known locations (similar to the PATH concept) when the system first starts. Moving libraries around causes no problems at all, although users incur a time cost when first starting the system.
AmigaOS stores generic system libraries in a directory defined by the LIBS: path assignment. Application-specific libraries can go in the same directory as the application’s executable. AmigaOS will search these locations when an executable attempts to launch a shared library. An application may also supply an explicit path when attempting to launch a library.
In addition to identifying static and dynamic loading, computer scientists also often classify libraries according to how they are shared among programs. Dynamic libraries almost always offer some form of sharing, allowing the same library to be used by multiple programs at the same time. Static libraries, by definition, cannot be shared. The term “linker” comes from the process of copying procedures or subroutines which may come from “relocatable” libraries and adjusting or “linking” the machine address to the final locations of each module.
The term shared library conveys some ambiguity because it covers at least two different concepts. First, it is the sharing of code located on disk by unrelated programs. The second concept is the sharing of code in memory, when programs execute the same physical page of RAM, mapped into different address spaces. It would seem that the latter would be preferable, and indeed it has a number of advantages. For instance on the OpenStep system, applications were often only a few hundred kilobytes in size and loaded almost instantly; the vast majority of their code was located in libraries that had already been loaded for other purposes by the operating system. There is a cost, however; shared code must be specifically written to run in a multitasking environment. In some older environments such as 16-bit Windows or MPE for the HP 3000, only stack based data (local) was allowed, or other significant restrictions were placed on writing a DLL.
Programs can accomplish RAM sharing by using position independent code as in Unix, which leads to a complex but flexible architecture, or by using position dependent code as in Windows and OS/2. These systems make sure, by various tricks like pre-mapping the address space and reserving slots for each DLL, that code has a great probability of being shared. Windows DLLs are not shared libraries in the Unix sense. (A third alternative is single-level store, as used by the IBM System/38 and its successors. This allows position-dependent code but places no significant restrictions on where code can be placed or how it can be shared.) The rest of this section concentrates on aspects common to both variants.
As of 2009, most modern operating systems can have shared libraries of the same format as the “regular” executables. This offers two main advantages: first, it requires making only one loader for both of them, rather than two (having the single loader is considered well worth its added complexity). Secondly, it allows the executables also to be used as DLLs, if they have a symbol table. Typical executable/DLL formats are ELF and Mach-O (both in Unix) and PE (Windows). In Windows, the concept was taken one step further, with even system resources such as fonts being bundled in the DLL file format. The same is true under OpenStep, where the universal “bundle” format is used for almost all system resources.
The term DLL occurs most frequently in Microsoft Windows and OS/2 environments. Unix and Unix-like platforms more commonly use the term shared library or shared object; consequently, the .so filename extension occurs most commonly to identify shared library files in such environments — often followed by another dot and a version number (for example libc.so.6). The different semantics provide technical justification for this usage. For further explanations, see the position independent code article.
In some cases different versions of DLLs can cause problems, especially when DLLs of different versions have the same file name, and different applications installed on a system each require a specific version. Such a scenario is known as DLL hell. Most modern operating systems, after 2001, have clean-up methods to eliminate such situations.
Dynamic loading, a subset of dynamic linking, involves a dynamically linked library loading and unloading at run-time on request. Such a request may be made implicitly at compile-time or explicitly at run-time. Implicit requests are made at compile-time when a linker adds library references that include file paths or simply file names. Explicit requests are made when applications make direct calls to an operating system’s API at runtime.
Most operating systems that support dynamically linked libraries also support dynamically loading such libraries via a run-time linker API. For instance, Microsoft Windows uses the API functions LoadLibrary, LoadLibraryEx, FreeLibrary and GetProcAddress with Microsoft Dynamic Link Libraries; POSIX based systems, including most UNIX and UNIX-like systems, use dlopen, dlclose and dlsym. Some development systems automate this process.
Another solution to the library issue comes from using completely separate executables (often in some lightweight form) and calling them using a remote procedure call (RPC) over a network to another computer. This approach maximizes operating system re-use: the code needed to support the library is the same code being used to provide application support and security for every other program. Additionally, such systems do not require the library to exist on the same machine, but can forward the requests over the network.
However, such an approach means that every library call requires a considerable amount of overhead. RPC calls are much more expensive than calling a shared library which has already been loaded on the same machine. This approach is commonly used in a distributed architecture which makes heavy use of such remote calls, notably client-server systems and application servers such as Enterprise JavaBeans.
Object and class libraries
Although originally pioneered in the 1960s, dynamic linking did not reach operating systems used by consumers until the late 1980s. It was generally available in some form in most operating systems by the early 1990s. During this same period, object-oriented programming (OOP) was becoming a significant part of the programming landscape. OOP with runtime binding requires additional information that traditional libraries don’t supply. In addition to the names and entry points of the code located within, they also require a list of the objects on which they depend. This is a side-effect of one of OOP’s main advantages, inheritance, which means that the complete definition of any method may be defined in a number of places. This is more than simply listing that one library requires the services of another: in a true OOP system, the libraries themselves may not be known at compile time, and vary from system to system.
At the same time many developers worked on the idea of multi-tier programs, in which a “display” running on a desktop computer would use the services of a mainframe or minicomputer for data storage or processing. For instance, a program on a GUI-based computer would send messages to a minicomputer to return small samples of a huge dataset for display. Remote procedure calls already handled these tasks, but there was no standard RPC system.
Soon the majority of the minicomputer and mainframe vendors instigated projects to combine the two, producing an OOP library format that could be used anywhere. Such systems were known as object libraries, or distributed objects if they supported remote access (not all did). Microsoft’s COM is an example of such a system for local use, DCOM a modified version that supports remote access.
For some time object libraries held the status of the “next big thing” in the programming world. There were a number of efforts to create systems that would run across platforms, and companies competed to try to get developers locked into their own system. Examples include IBM’s System Object Model (SOM/DSOM), Sun Microsystems’ Distributed Objects Everywhere (DOE), NeXT’s Portable Distributed Objects (PDO), Digital’s ObjectBroker, Microsoft’s Component Object Model (COM/DCOM), and any number of CORBA-based systems.
After the inevitable cooling of marketing hype, object libraries continue to be used in both object-oriented programming and distributed information systems. Class libraries are the rough OOP equivalent of older types of code libraries. They contain classes, which describe characteristics and define actions (methods) that involve objects. Class libraries are used to create instances, or objects with their characteristics set to specific values. In some OOP languages, like Java, the distinction is clear, with the classes often contained in library files (like Java’s JAR file format) and the instantiated objects residing only in memory (although potentially able to be made persistent in separate files). In others, like Smalltalk, the class libraries are merely the starting point for a system image that includes the entire state of the environment, classes and all instantiated objects.
The system identifies libraries with a “.library” suffix (for example: mathieeedoubtrans.library). These are separate files (shared libraries) that are invoked by the program that is running, and are dynamically-loaded but are not dynamically linked. Once the library has been invoked usually it could not deallocate memory and resources it asked for. Users can force a “flushlibs” option by using AmigaDOS command Avail (that checks free memory in the system and has the option Avail flush that frees memory from libraries left open by programs). Since AmigaOS 4.0 July 2007 First Update, support for shared objects and dynamic linking has been introduced. Now “.so” objects can exist also on Amiga, together with “.library files”.
* Most modern Unix-like systems
The system stores libfoo.a and libfoo.so files in directories such as /lib, /usr/lib or /usr/local/lib. The filenames always start with lib, and end with .a (archive, static library) or .so (shared object, dynamically linked library). Some systems might have multiple names for the dynamically linked library, with most of the names being names for symbolic links to the remaining name; those names might include the major version of the library, or the full version number; or example, on some systems libfoo.so.2 would be the filename for the second major interface revision of the dynamically linked library libfoo. The .la files sometimes found in the library directories are libtool archives, not usable by the system as such.
* Mac OS X
The system inherits static library conventions from BSD, with the library stored in a .a file, and can use .so-style dynamically-linked libraries (with the .dylib suffix instead). Most libraries in Mac OS X, however, consist of “frameworks”, placed inside special directories called “bundles” which wrap the library’s required files and metadata.
* Microsoft Windows
*.DLL files are dynamically linkable libraries. Other file name extensions may be used for specific purpose DLLs, e.g. *.OCX for OLE libraries. The interface revisions are either encoded in the file names, or abstracted away using COM-object interfaces. Depending on how they are compiled, *.LIB files can be either static libraries or representations of dynamically linkable libraries needed only during compilation, known as “Import Libraries”. Unlike in the UNIX world, where different file extensions are used, when linking against .LIB file in Windows one must first know if it is a regular static library or an import library. In the latter case, a .DLL file must be present at runtime.