Shared library

A shared library or shared object is a file that is intended to be shared by executable files and further shared object files. Modules used by a program are loaded from individual shared objects into memory at load time or runtime, rather than being copied by a linker when it creates a single monolithic executable file for the program.

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.^{[citation needed]} But often linking of shared libraries is postponed until they are loaded.^{[dubious – discuss]}

Most modern operating systems[NB 1] can have shared library files of the same format as the executable files. This offers two main advantages: first, it requires making only one loader for both of them, rather than two (having the single loader is considered well worth its added complexity)^{[citation needed]}. Secondly, it allows the executables also to be used as shared libraries, if they have a symbol table. Typical combined executable and shared library formats are ELF and Mach-O (both in Unix) and PE (Windows).

In some older environments such as 16-bit Windows or MPE for the HP 3000, only stack-based data (local) was allowed in shared-library code, or other significant restrictions were placed on shared-library code.