Shared libraries

are modern innovations that address the disadvantages of

static libraries. A shared library is an object module that,

at run time

, can be

loaded at an arbitrary memory address and linked with a program in memory.

This process is known as

dynamic linking

and is performed by a program called a

dynamic linker

.

Shared libraries are also referred to as

shared objects

, and on Unix systems

are typically denoted by the

.so

suffix. Microsoft operating systems make heavy

use of shared libraries, which they refer to as DLLs (dynamic link libraries).

Shared libraries are “shared” in two different ways. First, in any given file

system, there is exactly one

.so

file for a particular library. The code and data in

this

.so

file are shared by all of the executable object files that reference the library,

as opposed to the contents of static libraries, which are copied and embedded in

the executables that reference them. Second, a single copy of the

.text

section of

a shared library in memory can be shared by different running processes. We will

explore this in more detail when we study virtual memory in Chapter 9.