41.14. Summary

An object library is an aggregation of compiled object modules that can be employed by programs that are linked against the library. Like other UNIX implementations, Linux provides two types of object libraries: static libraries, which were the only type of library available under early UNIX systems, and the more modern shared libraries.

Because they provide several advantages over static libraries, shared libraries are the predominant type of library in use on contemporary UNIX systems. The advantages of shared libraries spring primarily from the fact that when a program is linked against the library, copies of the object modules required by the program are not included in the resulting executable. Instead, the (static) linker merely includes information in the executable file about the shared libraries that are required at run time. When the file is executed, the dynamic linker uses this information to load the required shared libraries. At run time, all programs using the same shared library share a single copy of that library in memory. Since shared libraries are not copied into executable files, and a single memory-resident copy of the shared library is employed by all programs at run time, shared libraries reduce the amount of disk space and memory required by the system.