Datasheet

In this way, the system can arrange for a single copy of a shared library to be used by many applications

at once and stored just once on the disk. An additional benefit is that the shared library can be updated

independently of the applications that rely on it. Symbolic links from the

/lib/libm.so file to the actual

library revision (

/lib/libm.so.N where N represents a major version number — 6 at the time of writing)

are used. When Linux starts an application, it can take into account the version of a library required by the

application to prevent major new versions of a library from breaking older applications.

For Linux systems, the program (the dynamic loader) that takes care of loading shared libraries and resolv-

ing client program function references is called

ld.so and may be made available as ld-linux.so.2 or

ld-lsb.so.2 or ld-lsb.so.3. The additional locations searched for shared libraries are configured in the

file

/etc/ld.so.conf, which needs to be processed by ldconfig if changed (for example, if X11 shared

libraries are added when the X Window System is installed).

You can see which shared libraries are required by a program by running the utility

ldd. For example, if

you try running it on your example application, you get the following:

$ ldd program

linux-gate.so.1 => (0xffffe000)

libc.so.6 => /lib/libc.so.6 (0xb7db4000)

/lib/ld-linux.so.2 (0xb7efc000)

In this case, you see that the standard C library (libc) is shared (.so). The program requires major

Version 6. Other UNIX systems will make similar arrangements for access to shared libraries. Refer to

your system documentation for details.

In many ways, shared libraries are similar to dynamic-link libraries used under Windows. The

.so

libraries correspond to .DLL files and are required at run time, and the .a libraries are similar to .LIB

files included in the program executable.

Getting Help

The vast majority of Linux systems are reasonably well documented with respect to the system program-

ming interfaces and standard utilities. This is true because, since the earliest UNIX systems, programmers

have been encouraged to supply a manual page with their applications. These manual pages, which are

sometimes provided in a printed form, are invariably available electronically.

The

man command provides access to the online manual pages. The pages vary considerably in quality

and detail. Some may simply refer the reader to other, more thorough documentation, whereas others

give a complete list of all options and commands that a utility supports. In either case, the manual page

is a good place to start.

The GNU software suite and some other free software use an online documentation system called

info.

You can browse full documentation online using a special program,

info, or via the info command of

The following example outputs are taken from a SUSE 10.3 distribution. Your output

may differ slightly if you are not using this distribution.

Chapter 1: Getting Started

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