The Linux Programming Interface

846 Chapter 41

Typically, the linker name is created in the same directory as the file to which it

refers. It can be linked either to the real name or to the soname of the most recent

major version of the library. Usually, a link to the soname is preferable, so that changes

to the soname are automatically reflected in the linker name. (In Section 41.7, we’ll see

that the ldconfig program automates the task of keeping sonames up to date, and

thus implicitly maintains linker names if we use the convention just described.)

If we want to link a program against an older major version of a shared library,

we can’t use the linker name. Instead, as part of the link command, we would

need to indicate the required (major) version by specifying a particular real

name or soname.

The following are some examples of linker names:

libdemo.so -> libdemo.so.2

libreadline.so -> libreadline.so.5

Table 41-1 summarizes information about the shared library real name, soname,

and linker name, and Figure 41-3 portrays the relationship between these names.

Figure 41-3: Conventional arrangement of shared library names

Creating a shared library using standard conventions

Putting all of the above information together, we now show how to build our demon-

stration library following the standard conventions. First, we create the object files:

$ gcc -g -c -fPIC -Wall mod1.c mod2.c mod3.c

Then we create the shared library with the real name libdemo.so.1.0.1 and the

soname libdemo.so.1.

Table 41-1: Summary of shared library names

Name Format Description

real name libname.so.maj.min File holding library code; one instance per major-

plus-minor version of the library.

soname libname.so.maj One instance per major version of library;

embedded in executable at link time; used at run

time to find library via a symbolic link with same

name that points to corresponding (most up-to-

date) real name.

linker name libname.so Symbolic link to latest real name or (more usually)

latest soname; single instance; allows construction

of version-independent link commands.

libname.so.maj.min

real name

(regular file)

Object code for

library modules

(symbolic link)

libname.so.maj.min

(symbolic link)

libname.so.maj

libname.so.maj

soname

libname.so

linker name