Static library
Encyclopedia
In computer science
, a static library or statically-linked library is a set of routines, external functions and variables which are resolved in a caller at compile-time and copied into a target application by a compiler
, linker, or binder, producing an object file
and a stand-alone executable
. This executable and the process of compiling it are both known as a static build
of the program. Historically, libraries could only be static. Static libraries are either merged with other static libraries and object file
s during building/linking to form a single executable
, or they may be loaded at run-time into the address space
of the loaded executable at a static memory offset
determined at compile-time/link-time.
With static linking, it is enough to include those parts of the library that are directly and indirectly referenced by the target executable (or target library). With dynamic libraries, the entire library is loaded, as it is not known in advance which functions will be invoked by applications. Whether this advantage is significant in practice depends on the structure of the library.
it is common to include the library files an application needs with the application. On Unix-like
systems this is less common as package management system
s can be used to ensure the correct library files are available. This allows the library files to be shared between many applications leading to space savings. It also allows the library to be updated to fix bugs and security flaws without updating the applications that use the library. These advantages are not the sole consideration, and many executables (especially those targeting Microsoft Windows) use both static and dynamic libraries.
. Static library files may also be linked/loaded at run-time by a linker or linking loader (e.g., the X11
module loader). However, whether such a process can be called static linking is controversial.
or in C++
. These two languages provide storage-class specifiers for indicating external or internal linkage, in addition to providing other features.
To create such a library, the exported functions/procedures and other objects variables must be specified for external linkage (i.e. not by using the C static keyword).
Static libraries usually have a ".lib" extension on Windows and a ".a" on UNIX
Computer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
, a static library or statically-linked library is a set of routines, external functions and variables which are resolved in a caller at compile-time and copied into a target application by a compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
, linker, or binder, producing an object file
Object file
An object file is a file containing relocatable format machine code that is usually not directly executable. Object files are produced by an assembler, compiler, or other language translator, and used as input to the linker....
and a stand-alone executable
Executable
In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...
. This executable and the process of compiling it are both known as a static build
Static build
A static build is a compiled version of a program which has been statically linked against libraries.In computer science, linking means taking one or more objects generated by compilers and assemble them into a single executable program...
of the program. Historically, libraries could only be static. Static libraries are either merged with other static libraries and object file
Object file
An object file is a file containing relocatable format machine code that is usually not directly executable. Object files are produced by an assembler, compiler, or other language translator, and used as input to the linker....
s during building/linking to form a single executable
Executable
In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...
, or they may be loaded at run-time into the address space
Address space
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...
of the loaded executable at a static memory offset
Static memory allocation
Static memory allocation refers to the process of allocating memory at compile-time before the associated program is executed, unlike dynamic memory allocation or automatic memory allocation where memory is allocated as required at run-time....
determined at compile-time/link-time.
Advantages of static linking
There are several advantages to statically linking libraries with an executable instead of dynamically linking them. The most significant is that the application can be certain that all its libraries are present and that they are the correct version. This avoids dependency problems. In some cases, static linking can result in a performance improvement. Static linking can also allow the application to be contained in a single executable file, simplifying distribution and installation.With static linking, it is enough to include those parts of the library that are directly and indirectly referenced by the target executable (or target library). With dynamic libraries, the entire library is loaded, as it is not known in advance which functions will be invoked by applications. Whether this advantage is significant in practice depends on the structure of the library.
File size
In static linking, the size of the executable becomes greater than in dynamic linking, as the library code is stored within the executable rather than in separate files. If you include the size of the library files in the application then the total size will be similar. On Microsoft WindowsMicrosoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
it is common to include the library files an application needs with the application. On Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
systems this is less common as package management system
Package management system
In software, a package management system, also called package manager, is a collection of software tools to automate the process of installing, upgrading, configuring, and removing software packages for a computer's operating system in a consistent manner...
s can be used to ensure the correct library files are available. This allows the library files to be shared between many applications leading to space savings. It also allows the library to be updated to fix bugs and security flaws without updating the applications that use the library. These advantages are not the sole consideration, and many executables (especially those targeting Microsoft Windows) use both static and dynamic libraries.
Link/load
It is important to note that any static library function can call a function or procedure in another static library. The linker/linking loader/loader handles this the same way as for any "normal" object fileObject file
An object file is a file containing relocatable format machine code that is usually not directly executable. Object files are produced by an assembler, compiler, or other language translator, and used as input to the linker....
. Static library files may also be linked/loaded at run-time by a linker or linking loader (e.g., the X11
X Window System
The X window system is a computer software system and network protocol that provides a basis for graphical user interfaces and rich input device capability for networked computers...
module loader). However, whether such a process can be called static linking is controversial.
Creating static libraries in C/C++
Static libraries can be easily created in CC (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
or in C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
. These two languages provide storage-class specifiers for indicating external or internal linkage, in addition to providing other features.
To create such a library, the exported functions/procedures and other objects variables must be specified for external linkage (i.e. not by using the C static keyword).
Static libraries usually have a ".lib" extension on Windows and a ".a" on UNIX
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
See also
- Dynamic link library (.DLL)
- External variableExternal variableIn the C programming language, an external variable is a variable defined outside any function block. On the other hand, a local variable is a variable defined inside a function block.- Definition, declaration and the extern keyword :...
- Library (computer science)Library (computer science)In computer science, a library is a collection of resources used to develop software. These may include pre-written code and subroutines, classes, values or type specifications....
- Linker (computing)
- Loader (computing)Loader (computing)In computing, a loader is the part of an operating system that is responsible for loading programs. It is one of the essential stages in the process of starting a program, as it places programs into memory and prepares them for execution...
- Object fileObject fileAn object file is a file containing relocatable format machine code that is usually not directly executable. Object files are produced by an assembler, compiler, or other language translator, and used as input to the linker....
- PrebindingPrebindingPrebinding is a method for reducing the time it takes to launch executables in the Mach-O file format. For example, this is what Mac OS X is doing when in the "Optimizing" stage of installing system software or certain applications....
- Shared library