# Shared library

A shared library is a library that contains executable code designed to be used by multiple computer programs or other libraries at runtime, with only one copy of that code in memory, shared by all programs using the code.

## Library types

A program that is configured to use a library can use either static-linking or dynamic-linking. Historically, libraries could only be static. For static-linking (.lib), the library is effectively embedded into the programs executable file, while for dynamic-linking the library can be loaded at runtime from a shared location, such as system files.
A program requires a compiler and linker step to access a library. In static-linking, those references are fully resolved at compile-time, whereas in dynamic-linking those references are not fully resolved until runtime by the operating system.

## Formats

Many modern operating systems now use a unified format for their shared libraries and executable files. For example:
* Microsoft Windows uses the Portable Executable (PE) format for .dll files;
* operating systems such as Solaris and other System V Release 4-based systems, Linux, and the free-software BSD operating systems use the Executable and Linkable Format (ELF) for .so files;
* Apple's Darwin-based operating systems, such as macOS and iOS, use the Mach-O format for .dylib files.
This offers two main advantages: first, it requires only one loader (building and maintaining a single loader is considered well worth any added complexity). Secondly, it allows an executable file to be used as a shared library (if it has a symbol table).
In some older environments such as 16-bit Windows or MPE for the HP 3000, significant restrictions were placed on shared library code, such as only allowing stack-based (local) data.

## Memory sharing

Library code may be shared in memory by multiple processes, and on disk. If virtual memory is used, processes would execute the same physical page of RAM that is mapped into the different address spaces of the processes. This has advantages. For instance, on the OpenStep system, applications were often only a few hundred kilobytes in size and loaded quickly; most of their code was located in libraries that had already been loaded for other purposes by the operating system.
Programs can accomplish RAM sharing by using position-independent code, as in Unix, which leads to a complex but flexible architecture, or by using common virtual addresses, as in Windows and OS/2. These systems ensure, by various means, like pre-mapping the address space and reserving slots for each shared library, that code has a high probability of being shared. A third alternative is single-level store with a single address space, as used by the IBM System/38 and its successors. This allows position-dependent code, with programs and libraries assigned a permanent address in that address space.
In some cases, different versions of shared libraries can cause problems, especially when libraries 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, named after the Windows and OS/2 DLL file. Most modern operating systems after 2001 have clean-up methods to eliminate such situations or use application-specific "private" libraries.

## Dynamic linking

Dynamic linking or late binding is linking performed while a program is being loaded (load time) or executed (runtime), rather than when the executable file is created. A dynamically linked library (dynamic-link library, or DLL, under Windows and OS/2; shareable image under OpenVMS; dynamic shared object, or DSO, under Unix-like systems) is a library intended for dynamic linking. Only a minimal amount of work is done by the linker when the executable file is created; 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 (load time) or during execution (runtime). Usually, the necessary linking program, called a dynamic linker or linking loader, is actually part of the underlying operating system. (However, it is possible, and not exceedingly difficult, to write a program that uses dynamic linking and includes its own dynamic linker, even for an operating system that itself provides no support for dynamic linking.)
Programmers originally developed dynamic linking in the Multics operating system, starting in 1964, and the MTS (Michigan Terminal System), built in the late 1960s.

## Optimizations

Since shared libraries on most systems do not change often, 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 or prelinking on macOS and Linux, respectively. IBM z/VM uses a similar technique, called "Discontinuous Saved Segments" (DCSS). 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).

## Locating libraries at runtime

Loaders for shared libraries 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.
If a shared library that an executable depends on is deleted, moved, or renamed, or if an incompatible version of the library is copied to a place that is earlier in the search, the executable would fail to load. This is called dependency hell, existing on many platforms. The (infamous) Windows variant is commonly known as DLL hell. This problem cannot occur if each version of each library is uniquely identified and each program references libraries only by their full unique identifiers. The "DLL hell" problems with earlier Windows versions arose from using only the names of libraries, which were not guaranteed to be unique, to resolve dynamic links in programs. (To avoid "DLL hell", later versions of Windows rely largely on options for programs to install private DLLs—essentially a partial retreat from the use of shared libraries—along with mechanisms to prevent replacement of shared system DLLs with earlier versions of them.)

### Microsoft Windows

Microsoft Windows checks the registry to determine the proper place to load DLLs that implement COM objects, but for other DLLs it will check the directories in a defined order. First, Windows checks the directory where it loaded the program (private DLL); any directories set by calling the SetDllDirectory function; the System32, System, and Windows directories; then the current working directory; and finally the directories specified by the PATH environment variable. Applications written for the .NET 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

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.

### Unix-like systems

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.

## Dynamic loading

Dynamic loading, a subset of dynamic linking, involves a dynamically linked library loading and unloading at runtime on request. Such a request may be made implicitly or explicitly. Implicit requests are made when a compiler or static 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.
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.