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Static and Shared Libraries

An object library is a file containing the compiled object code for a (usually logically related) set of functions that may be called from application programs. Placing code for a set of functions in a single object library eases the tasks of program creation and maintenance. Modern UNIX systems provide two types of object libraries: static libraries and shared libraries.

Static libraries

Static libraries (sometimes also known as archives) were the only type of library on early UNIX systems. A static library is essentially a structured bundle of compiled object modules. To use functions from a static library, we specify that library in the link command used to build a program. After resolving the various function references from the main program to the modules in the static library, the linker extracts copies of the required object modules from the library and copies these into the resulting executable file. We say that such a program is statically linked.

The fact that each statically linked program includes its own copy of the object modules required from the library creates a number of disadvantages. One is that the duplication of object code in different executable files wastes disk space. A corresponding waste of memory occurs when statically linked programs using the same library function are executed at the same time; each program requires its own copy of the function to reside in memory. Additionally, if a library function requires modification, then, after recompiling that function and adding it to the static library, all applications that need to use the updated function must be relinked against the library.

Shared libraries

Shared libraries were designed to address the problems with static libraries.

If a program is linked against a shared library, then, instead of copying object modules from the library into the executable, the linker just writes a record into the executable to indicate that at run time the executable needs to use that shared library. When the executable is loaded into memory at run time, a program called the dynamic linker ensures that the shared libraries required by the executable are found and loaded into memory, and performs run-time linking to resolve the function calls in the executable to the corresponding definitions in the shared libraries. At run time, only a single copy of the code of the shared library needs to be resident in memory; all running programs can use that copy.

The fact that a shared library contains the sole compiled version of a function saves disk space. It also greatly eases the job of ensuring that programs employ the newest version of a function. Simply rebuilding the shared library with the new function definition causes existing programs to automatically use the new definition when they are next executed.

Published on Sat 12 March 2005 by Dave Wilson in Programming with tag(s): linux libraries