102.3 Lesson 1

Certificate:

LPIC-1

Version:

5.0

Topic:

102 Linux Installation and Package Management

Objective:

102.3 Manage shared libraries

Lesson:

1 of 1

Introduction

In this lesson we will be discussing shared libraries, also known as shared objects: pieces of compiled, reusable code like functions or classes, that are recurrently used by various programs.

To start with, we will explain what shared libraries are, how to identify them and where they are found. Next, we will go into how to configure their storage locations. Finally, we will show how to search for the shared libraries on which a particular program depends.

Concept of Shared Libraries

Similar to their physical counterparts, software libraries are collections of code that are intended to be used by many different programs; just as physical libraries keep books and other resources to be used by many different people.

To build an executable file from a program’s source code, two important steps are necessary. First, the compiler turns the source code into machine code that is stored in so-called object files. Secondly, the linker combines the object files and links them to libraries in order to generate the final executable file. This linking can be done statically or dynamically. Depending on which method we go for, we will be talking about static libraries or, in case of dynamic linking, about shared libraries. Let us explain their differences.

Static libraries: A shared library is merged with the program at link time. A copy of the library code is embedded into the program and becomes part of it. Thus, the program has no dependencies on the library at run time because the program already contains the libraries code. Having no dependencies can be seen as an advantage since you do not have to worry about making sure the used libraries are always available. On the downside, statically linked programs are heavier.
Shared (or dynamic) libraries: In the case of shared libraries, the linker simply takes care that the program references libraries correctly. The linker does, however, not copy any library code into the program file. At run time, though, the shared library must be available to satisfy the program’s dependencies. This is an economical approach to managing system resources as it helps reduce the size of program files and only one copy of the library is loaded in memory, even when it is used by multiple programs.

Shared Object File Naming Conventions

The name of a shared library, also known as soname, follows a pattern which is made up of three elements:

Library name (normally prefixed by lib)
so (which stands for “shared object”)
Version number of the library

Here an example: libpthread.so.0

By contrast, static library names end in .a, e.g. libpthread.a.

Note	Because the files containing shared libraries must be available when the program is executed, most Linux systems contain shared libraries. Since static libraries are only required in a dedicated file when a program is linked, they might not be present on an end user system.

glibc (GNU C library) is a good example of a shared library. On a Debian GNU/Linux 9.9 system, its file is named libc.so.6. Such rather general file names are normally symbolic links that point to the actual file containing a library, whose name contains the exact version number. In case of glibc, this symbolic link looks like this:

$ ls -l /lib/x86_64-linux-gnu/libc.so.6
lrwxrwxrwx 1 root root 12 feb  6 22:17 /lib/x86_64-linux-gnu/libc.so.6 -> libc-2.24.so

This pattern of referencing shared library files named by a specific version by more general file names is common practice.

Other examples of shared libraries include libreadline (which allows users to edit command lines as they are typed in and includes support for both Emacs and vi editing modes), libcrypt (which contains functions related to encryption, hashing, and encoding), or libcurl (which is a multiprotocol file transfer library).

Common locations for shared libraries in a Linux system are:

/lib
/lib32
/lib64
/usr/lib
/usr/local/lib

Note	The concept of shared libraries is not exclusive to Linux. In Windows, for example, they are called DLL which stands for dynamic linked libraries.

Configuration of Shared Library Paths

The references contained in dynamically linked programs are resolved by the dynamic linker (ld.so or ld-linux.so) when the program is run. The dynamic linker searches for libraries in a number of directories. These directories are specified by the library path. The library path is configured in the /etc directory, namely in the file /etc/ld.so.conf and, more common nowadays, in files residing in the /etc/ld.so.conf.d directory. Normally, the former includes just a single include line for *.conf files in the latter:

$ cat /etc/ld.so.conf
include /etc/ld.so.conf.d/*.conf

The /etc/ld.so.conf.d directory contains *.conf files:

$ ls /etc/ld.so.conf.d/
libc.conf  x86_64-linux-gnu.conf

These *.conf files must include the absolute paths to the shared library directories:

$ cat /etc/ld.so.conf.d/x86_64-linux-gnu.conf
# Multiarch support
/lib/x86_64-linux-gnu
/usr/lib/x86_64-linux-gnu

The ldconfig command takes care of reading these config files, creating the aforementioned set of symbolic links that help to locate the individual libraries and finally of updating the cache file /etc/ld.so.cache. Thus, ldconfig must be run every time configuration files are added or updated.

Useful options for ldconfig are:

-v, --verbose

Display the library version numbers, the name of each directory, and the links that are created:

$ sudo ldconfig -v
/usr/local/lib:
/lib/x86_64-linux-gnu:
    libnss_myhostname.so.2 -> libnss_myhostname.so.2
	libfuse.so.2 -> libfuse.so.2.9.7
	libidn.so.11 -> libidn.so.11.6.16
	libnss_mdns4.so.2 -> libnss_mdns4.so.2
	libparted.so.2 -> libparted.so.2.0.1
	(...)

So we can see, for example, how libfuse.so.2 is linked to the actual shared object file libfuse.so.2.9.7.

-p, --print-cache

Print the lists of directories and candidate libraries stored in the current cache:

$ sudo ldconfig -p
1094 libs found in the cache `/etc/ld.so.cache'
    libzvbi.so.0 (libc6,x86-64) => /usr/lib/x86_64-linux-gnu/libzvbi.so.0
	libzvbi-chains.so.0 (libc6,x86-64) => /usr/lib/x86_64-linux-gnu/libzvbi-chains.so.0
	libzmq.so.5 (libc6,x86-64) => /usr/lib/x86_64-linux-gnu/libzmq.so.5
	libzeitgeist-2.0.so.0 (libc6,x86-64) => /usr/lib/x86_64-linux-gnu/libzeitgeist-2.0.so.0
	(...)

Note how the cache uses the fully qualified soname of the links:

$ sudo ldconfig -p |grep libfuse
	  libfuse.so.2 (libc6,x86-64) => /lib/x86_64-linux-gnu/libfuse.so.2

If we long list /lib/x86_64-linux-gnu/libfuse.so.2, we will find the reference to the actual shared object file libfuse.so.2.9.7 which is stored in the same directory:

$ ls -l /lib/x86_64-linux-gnu/libfuse.so.2
lrwxrwxrwx 1 root root 16 Aug 21  2018 /lib/x86_64-linux-gnu/libfuse.so.2 -> libfuse.so.2.9.7

Note	Since it requires write access to `/etc/ld.so.cache` (owned by root), you must either be root or use `sudo` to invoke `ldconfig`. For more information about `ldconfig` switches, refer to its manual page.

In addition to the configuration files described above, the LD_LIBRARY_PATH environment variable can be used to add new paths for shared libraries temporarily. It is made up of a colon-separated (:) set of directories where libraries are looked up. To add, for example, /usr/local/mylib to the library path in the current shell session, you could type:

$ LD_LIBRARY_PATH=/usr/local/mylib

Now you can check its value:

$ echo $LD_LIBRARY_PATH
/usr/local/mylib

To add /usr/local/mylib to the library path in the current shell session and have it exported to all child processes spawned from that shell, you would type:

$ export LD_LIBRARY_PATH=/usr/local/mylib

To remove the LD_LIBRARY_PATH environment variable, just type:

$ unset LD_LIBRARY_PATH

To make the changes permanent, you can write the line

export LD_LIBRARY_PATH=/usr/local/mylib

in one of Bash’s initialization scripts such as /etc/bash.bashrc or ~/.bashrc.

Note	`LD_LIBRARY_PATH` is to shared libraries what `PATH` is to executables. For more information about environment variables and shell configuration, refer to the respective lessons.

Searching for the Dependencies of a Particular Executable

To look up the shared libraries required by a specific program, use the ldd command followed by the absolute path to the program. The output shows the path of the shared library file as well as the hexadecimal memory address at which it is loaded:

$ ldd /usr/bin/git
    linux-vdso.so.1 =>  (0x00007ffcbb310000)
	libpcre.so.3 => /lib/x86_64-linux-gnu/libpcre.so.3 (0x00007f18241eb000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f1823fd1000)
	libresolv.so.2 => /lib/x86_64-linux-gnu/libresolv.so.2 (0x00007f1823db6000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f1823b99000)
	librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f1823991000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f18235c7000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f182445b000)

Likewise, we use ldd to search for the dependencies of a shared object:

$ ldd /lib/x86_64-linux-gnu/libc.so.6
	/lib64/ld-linux-x86-64.so.2 (0x00007fbfed578000)
	linux-vdso.so.1 (0x00007fffb7bf5000)

With the -u (or --unused) option ldd prints the unused direct dependencies (if they exist):

$ ldd -u /usr/bin/git
Unused direct dependencies:
	/lib/x86_64-linux-gnu/libz.so.1
	/lib/x86_64-linux-gnu/libpthread.so.0
	/lib/x86_64-linux-gnu/librt.so.1

The reason for unused dependencies is related to the options used by the linker when building the binary. Although the program does not need an unused library, it was still linked and labelled as NEEDED in the information about the object file. You can investigate this using commands such as readelf or objdump, which you will soon use in the explorational exercise.

Guided Excercises

Divide the following shared library names into their parts:

Complete file name Library name so suffix Version number

linux-vdso.so.1

libprocps.so.6

libdl.so.2

libc.so.6

libsystemd.so.0

ld-linux-x86-64.so.2
You have developed a piece of software and want to add a new shared library directory to your system (/opt/lib/mylib). You write its absolute path in a file called mylib.conf.
- In what directory should you put this file?
- What command should you run to make the changes fully effective?
What command would you use to list the shared libraries required by kill?

Explorational Exercises

objdump is a command line utility that displays information from object files. Check if it is installed in your system with which objdump. If it is not, please, install it.
- Use objdump with the -p (or --private-headers) and grep to print the dependencies of glibc:
- Use objdump with the -p (or --private-headers) and grep to print the soname of glibc:
- Use objdump with the -p (or --private-headers) and grep to print the dependencies of Bash:

Summary

In this lesson you have learned:

What a shared (or dynamic) library is.
The differences between shared and static libraries.
The names of shared libraries (sonames).
The preferred locations for shared libraries in a Linux system such as /lib or /usr/lib.
The purpose of the dynamic linker ld.so (or ld-linux.so).
How to configure shared library paths by means of files in /etc/ such as ld.so.conf or the ones in the ld.so.conf.d directory.
How to configure shared library paths by means of the LD_LIBRARY_PATH environment variable.
How to look up for executable and shared library dependencies.

Commands used in this lesson:

ls: List directory contents.
cat: Concatenate files and print on the standard output.
sudo: Have the superuser executing the command with administrative privileges.
ldconfig: Configure dynamic linker run-time bindings.
echo: Display value of environment variable.
export: Export value of environment variable to child shells.
unset: Remove environment variable.
ldd: Print shared objects dependencies of a program.
readelf: Display information about ELF files (ELF stands for executable and linkable format).
objdump: Print information from object files.

Answers to Guided Exercises

Divide the following shared library names into their parts:

Complete file name Library name so suffix Version number

linux-vdso.so.1

linux-vdso

so

1

libprocps.so.6

libprocps

so

6

libdl.so.2

libdl

so

2

libc.so.6

libc

so

6

libsystemd.so.0

libsystemd

so

0

ld-linux-x86-64.so.2

ld-linux-x86-64

so

2

You have developed a piece of software and want to add a new shared library directory to your system (/opt/lib/mylib). You write its absolute path in a file called mylib.conf.
- In what directory should you put this file?
  
  /etc/ld.so.conf.d
- What command should you run to make the changes fully effective?
  
  ldconfig
What command would you use to enumerate the shared libraries required by kill?

ldd /bin/kill

Answers to Explorational Exercises

objdump is a command line utility that displays information from object files. Check if it is installed in your system with which objdump. If it is not, please, install it.
- Use objdump with the -p (or --private-headers) and grep to print the dependencies of glibc:
  
  objdump -p /lib/x86_64-linux-gnu/libc.so.6 | grep NEEDED
- Use objdump with the -p (or --private-headers) and grep to print the soname of glibc:
  
  objdump -p /lib/x86_64-linux-gnu/libc.so.6 | grep SONAME
- Use objdump with the -p (or --private-headers) and grep to print the dependencies of Bash:
  
  objdump -p /bin/bash | grep NEEDED