In computer science, a library is a collection of subprograms used to develop software. Libraries are distinguished from executables in that they are not independent programss; rather, they are "helper" code that provides services to some other independent program.

Well-known libraries include:

Library linking describes the inclusion of one or more of these software libraries into a new program. There are multiple types of linking: static linking and dynamic linking. These are described below.

Table of contents
1 Static Linking
2 Dynamic Linking
3 Shared Library

Static Linking

Static linking is linking in which a library is embedded into the program executable at compile time by a linker. A linker is a separate utility which takes one or more libraries and object files (which are previously generated by a compiler or an assembler) and produces an actual executable file.

One of the biggest disadvantages of static linking is that the resulting executable file becomes larger and therefore consumes a larger amount of system resources and takes longer to load into memory.

Examples of libraries which are traditionally designed to be statically linked include the ANSI C standard library and the ALIB assembler library.

Dynamic Linking

Dynamic linking is linking in which a library is loaded by the operating system's loader separately from the executable file at loadtime or runtime.

Most operating systems resolve external dependencies like libraries (called imports) as part of the loading process. For these systems, the executables contain a table called an import directory which is a variable-length array of imports. Each element in the array contains a name of a library. The loader searches the hard disk for the needed library, loads it into memory at an unpredictable location and updates the executable with the library's location in memory. The executable then uses this information to call functions and access data stored in the library. This type of dynamic linking is called loadtime linking and is used by most operating systems including Windows and Linux. Loadtime linking is one of the most complex routines the loader performs while loading an application.

Other operating systems resolve dependencies at runtime. For these systems, the executable calls an operating system API, passing it the name of a library file, a function number within the library and the function's parameters. The operating system resolves the import immediately and calls the appropriate function on behalf of the application. This type of dynamic linking is called runtime linking. Because of the overhead added to each call, runtime linking is incredibly slow and negatively affects an executable's performance. As a result, runtime linking is rarely used by modern operating systems.

In dynamic linking the library, commonly referred to as a dynamic link library (DLL) or shared library, is a pre-compiled and linked executable file which is stored separately on the computer's hard disk. It is loaded only when needed by an application. In most cases, multiple applications can use the same copy of the library at the same time and there is no need for the operating system to load multiple instances of the library into memory concurrently. In these cases, the libraries are stateless. That is, any data which must be stored by the library is stored by the application(s) it is serving. For this reason, these dynamic libraries are considered in-process.

One of the largest disadvantages of dynamic linking is that the executables depend on the separately stored libraries in order to function properly. If the library is deleted, moved, renamed or replaced with an incompatible version, the executable could malfunction. This is commonly known as DLL-hell.

All Windows *.DLL files are dynamically linked libraries.

Shared Library

Libraries can be linked dynamically. In Microsoft Windows, those are called dynamically linked library, or DLL. Conventional libraries are often called static library to distinguish from shared libraries.

A DLL is a software library (often stored in a file) consisting of a collection of resources or routines that are available to other programs. A program that wants to use these routines is linked (see linker) with the DLL at the time it is actually started or later. Oppose this with a static library, the contents of which are copied into the program when the program is compiled and linked.

A program performing the former task is called a loader, while the latter task is accomplished by a linker. However, to link a program against a DLL, thus making the program request that a particular DLL be loaded when it is started, the linker also needs to look into the DLL to verify that all symbols (routines and variables) used by the program are actually provided by the DLL, thus leaving the impression that dynamic linking is performed at compile time, while it actually happens at run time (in most cases, at program start time).

The process of making resources available to other programs is called exporting. Most common forms of exports include procedures (functions, routines, subroutines), variables, and some sorts of static data, e.g. icons. Exported procedures are also called entry points, because invoking them is akin to "entering" the library. In order to allow access to them, the resources receive names, which are written down inside a table, also containing their offsets inside the file. These names (and sometimes, by analogy, the resources they represent) are called symbols. Similarly, the table is called symbol table.

In most modern operating systems, shared libraries can be of the same format as the "regular" executables. This allows two main advantages: first, it requires making only one loader for them, rather than two. Secondly, it allows the executables also to be used as DLLs, if they have a symbol table (see below). Typical executable/DLL formats are ELF (UNIX) and PE (Microsoft Windows). In Windows, the concept was taken one step further, even system resources such as fonts being bundled in the DLL file format.

Executables are less likely to have a symbol table (they are not mandatory and are usually stripped down to save space), as opposed to DLLs which need one to serve their purpose. Aside from that, from most other aspects, the difference between DLLs and executables in modern operating systems is mostly conventional, as the other data structures are shared between the two types of files. Both have a record pointing at a main entry point. While an executable's main entry point is used by the operating system to launch it, the operating system uses a DLL's main entry point only when it is loaded by some application, to initialize that DLL. In other words, the user of the operating system cannot directly cause the invokation of the main (or indeed any other) entry point of a DLL.

The term DLL is mostly used on Windows products. On the UNIX platform, the term shared library is more commonly used. In some cases, an operating system can become overloaded with different versions of DLLs, which impedes its performance and stability. Such a scenario is known as DLL-hell.