A library is also a collection of implementations of behavior, written in terms of a language, that has a well-defined
Library code is organized in such a way that it can be used by multiple programs that have no connection to each other, while code that is part of a program is organized to be used only within that one program. This distinction can gain a hierarchical notion when a program grows large, such as a multi-million-line program. In that case, there may be internal libraries that are reused by independent sub-portions of the large program. The distinguishing feature is that a library is organized for the purposes of being reused by independent programs or sub-programs, and the user only needs to know the interface and not the internal details of the library.
The value of a library lies in the reuse of standardized program elements. When a program invokes a library, it gains the behavior implemented inside that library without having to implement that behavior itself. Libraries encourage the sharing of code in a modular fashion and ease the distribution of the code.
The behavior implemented by a library can be connected to the invoking program at different
Most compiled languages have a standard library, although programmers can also create their own custom libraries. Most modern software systems provide libraries that implement the majority of the system services. Such libraries have organized the services which a modern application requires. As such, most code used by modern applications is provided in these system libraries.
The idea of a computer library dates back to the first computers created by
Inspired by von Neumann, Wilkes and his team constructed EDSAC. A filing cabinet of punched tape held the subroutine library for this computer. Programs for EDSAC consisted of a main program and a sequence of subroutines copied from the subroutine library. In 1951 the team published the first textbook on programming, The Preparation of Programs for an Electronic Digital Computer, which detailed the creation and the purpose of the library.
JOVIAL had a Communication Pool (COMPOOL), roughly a library of header files.
Another major contributor to the modern library concept came in the form of the
By the mid 1960s, copy and macro libraries for assemblers were common. Starting with the popularity of the IBM System/360, libraries containing other types of text elements, e.g., system parameters, also became common.
Libraries are important in the program linking or binding process, which resolves references known as links or symbols to library modules. The linking process is usually automatically done by a
The references being resolved may be addresses for jumps and other routine calls. They may be in the main program, or in one module depending upon another. They are resolved into fixed or relocatable addresses (from a common base) by allocating runtime memory for the
Some programming languages use a feature called smart linking whereby 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 floating-point library routines. This smart-linking feature can lead to smaller application file sizes and reduced memory usage.
Some references in a program or library module are stored in a relative or symbolic form which cannot be resolved until all code and libraries are assigned final static addresses. Relocation is the process of adjusting these references, and is done either by the linker or the loader. In general, relocation cannot be done to individual libraries themselves because the addresses in memory may vary depending on the program using them and other libraries they are combined with. Position-independent code avoids references to absolute addresses and therefore does not require relocation.
When linking is performed during the creation of an executable or another object file, it is known as static linking or early binding. In this case, the linking is usually done by a linker, but may also be done by the compiler. A static library, also known as an archive, is one intended to be statically linked. Originally, only static libraries existed. Static linking must be performed when any modules are recompiled.
All of the modules required by a program are sometimes statically linked and copied into the executable file. This process, and the resulting stand-alone file, is known as a
A shared library or shared object is a file that is intended to be shared by
Shared libraries can be statically linked during compile-time, meaning that references to the library modules are resolved and the modules are allocated memory when the executable file is created. But often linking of shared libraries is postponed until they are loaded.[dubious ]
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 do not supply. In addition to the names and entry points of the code located within, they also require a list of the objects they depend on. This is a side-effect of one of OOP's core concepts, inheritance, which means that parts of the complete definition of any method may be in different 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 (RPC) 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 of COM, 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
Class libraries are the rough OOP equivalent of older types of code libraries. They contain
Today most class libraries are stored in a
Another library technique uses completely separate executables (often in some lightweight form) and calls them using a remote procedure call (RPC) over a network to another computer. This 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 that has already been loaded on the same machine. This approach is commonly used in a
Code generation libraries
Code generation libraries are high-level
Most modern Unix-like systems
The system stores
libfoo.so files in directories such as
/usr/local/lib. The filenames always start with
lib, and end with a suffix of
The system inherits static library conventions from
.afile, and can use
.so-style dynamically linked libraries (with the
.dylibsuffix instead). Most libraries in macOS, however, consist of "frameworks", placed inside special directories called "bundles
MyFrameworkwould be implemented in a bundle called
MyFramework.framework/MyFrameworkbeing either the dynamically linked library file or being a symlink to the dynamically linked library file in
.LIBfile in Windows
.DLLfile must be present at runtime.
- Code reuse – Use of existing software to build new software
- Linker (computing) – Computer program which combines multiple object files into a single file
- Loader (computing) – Part of an operating system
- Dynamic-link library – Microsoft's implementation of the shared library concept in Windows and OS/2
- Object file – File containing relocatable format machine code
- Plug-in – Software component that adds a specific feature to an existing software application
- Prelink, also known as Prebinding
- Static library – set of routines, external functions and variables in computer science
- Runtime library – set of routines that can be invoked by a compiled software binary during its execution
- Visual Component Library – Visual Library (VCL)
- Component Library for Cross Platform (CLX)
- C standard library – Standard library for the C programming language
- Java Class Library
- Framework Class Library – Standard library of Microsoft's .NET Framework
- Generic programming – Style of computer programming (used by the C++ Standard Library)
- soname – Field of data in a shared object file
- Method stub – term in programming
- It was possible earlier between, e.g., Ada subprograms.
- "Static Libraries". TLDP. Archived from the original on 2013-07-03. Retrieved 2013-10-03.
- Babbage, H. P. (1888-09-12). "The Analytical Engine". Proceedings of the British Association. Bath.
- Wexelblat, op. cit., p. 258
- Wilson and Clark, op. cit., p. 52
- Wexelblat, op. cit., p. 716
- Collberg, Christian; Hartman, John H.; Babu, Sridivya; Udupa, Sharath K. (2003). "SLINKY: Static Linking Reloaded". Department of Computer Science, University of Arizona. Archived from the original on 2016-03-23. Retrieved 2016-03-17.
Bresnahan, Christine; Blum, Richard (2015-04-27). LPIC-1 Linux Professional Institute Certification Study Guide: Exam 101-400 and Exam 102-400. John Wiley & Sons (published 2015). p. 82. . Retrieved 2015-09-03.
Linux shared libraries are similar to the dynamic link libraries (DLLs) of Windows. Windows DLLs are usually identified by
- Article Beginner's Guide to Linkers by David Drysdale
- Article Faster C++ program startups by improving runtime linking efficiency by Léon Bottou and John Ryland
- How to Create Program Libraries by Baris Simsek
- BFD - the Binary File Descriptor Library
- 1st Library-Centric Software Design Workshop LCSD'05 Archived 2019-08-28 at the Wayback Machine at OOPSLA'05
- 2nd Library-Centric Software Design Workshop LCSD'06 at OOPSLA'06
- How to create shared library by Ulrich Drepper (with much background info)
- Anatomy of Linux dynamic libraries at IBM.com