101 2.3 manage shared libraries

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Linux Essenciais and System Administration

Key Knowledge Areas
Identify shared libraries.
Identify the typical locations of system libraries.
Load shared libraries.
Linux Installation and Package Management
Manage shared libraries
Terms and Utilities
ldd
ldconfig
/etc/ld.so.conf
LD_LIBRARY_PATH
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When source code is written, programmers do not rewrite code for common operations.
These operations are written once and used again by many other programmers.
These common operations are stored in what are called libraries.
For an application to successfully compile and run, it needs access to the libraries it was
written to use.
Dependencies of executable programs on shared libraries
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Statically compiled applications.
Some source code compiles libraries into final executable - Dont need the libraries to run.
dynamically compiled applications.
Applications linked to the libraries during compile and must access to the library files later.
statically compiling makes the application larger than a dynamically compiled version.

Ex. on many Linux systems is ln cmd (/bin/ln), which creates links between files, either hard
links or soft (or symbolic) links.
This cmd uses shared libraries.
Shared libraries involve symbolic links between a generic name for the library and a specific
level of the library, if not present or broken, the ln command itself might be inoperative.
Progs with dynamically shared libraries and statically linked versions
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To protect against this possibility, some Linux systems include a statically linked version of the
ln program as the sln program (/sbin/sln).
Example: Fedora12 64-bit - difference in size between dynamically linked ln and statically linked sln.
yourname@yourcomp~$ ls -l /sbin/sln /bin/ln
-rwxr-xr-x. 1 root root 47384 2010-01-12 09:35 /bin/ln
-rwxr-xr-x. 1 root root 603680 2010-01-04 09:07 /sbin/sln
Ex:
Statically linked programs are likely to be larger

Linux’s shared libraries are stored in several paths:
/lib Main shared libraries
/usr/lib Supplemental libraries
/usr/X11R6/lib Shared libraries for X Window
Path and Naming convention
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Shared libraries naming convention:
libraryname-major-minor-patch.so (so extension stands for shared object)
Example: libcrypt-2-1-3.so
name of library: libcrypt. major version: 2 minor version: 1 patch level: 3
In many cases there’s symbolic links to the shared library. These are named as:
libraryname.so libraryname.so.major
Example: libcrypt.so libcrypt.so.2
These links allow software to point to them instead of the actual file. This way the minor version and patch
level can change without breaking the association.

Linux systems run on hardware that supports 32-bit and 64-bit executables.
Many libraries are thus compiled in 32-bit and 64-bit versions.
ldd Output: Viewing required shared libraries
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64-bit versions are stored under /lib64; 32-bit versions live in the traditional /lib tree.
Libraries /lib64/libc-2.11.1.so and /lib/libc-2.11.1.so allow C Progs of 64-bit and 32-bit to run on Linux 64-bit System.
To see which libraries an application uses, use command: ldd file_name
~/usr/bin# ldd wget
libc.so.6 => /lib/libc.so.6 (0x40020000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
Ex:
wget uses libraries libc.so version 6 and ld-linux.so version 2.

ldd Output: Dependencies between libraries
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Left hand column indicates what the program would like to be linked to (no path name given),
Right hand column shows what the program will actually be linked to (full path and often a symbolic link)
~/usr/bin# ldd /bin/bash
libtermcap.so.2 => /lib/libtermcap.so.2 (0x40024000)
libdl.so.2 => /lib/libdl.so.2 (0x40028000)
libc.so.6 => /lib/libc.so.6 (0x4002b000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
Ex:
When programs are loaded into memory, part of loading process is to make sure that relevant shared
libraries are loaded when the program loads.
A library can require other libraries. Dependencies are normal for large applications.
A program may use a graphics library, which can use the X11 library, which in turn uses the C library.
~$ ls -lg /lib/libc.so.6
lrwxrwxrwx 1 root 14 Oct 14 10:47 /lib/libc.so.6 -> libc-2.2.93.so
~$ ls -lg /lib/libc-2.2.93.so
-rwxr-xr-x 1 root 1327065 Sep 6 2002 /lib/libc-2.2.93.so
bash asked to link to libc.so.6, and it ended up linking to /lib/libc2.2.93.so.

ldd Output: types of information
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Ex: In Fedora 12 64-bit system (echidna). ln cmd needs 3 shared libraries:
~$ ldd /bin/ln
linux-vdso.so.1 => (0x00007fff644af000)
libc.so.6 => /lib64/libc.so.6 (0x00000037eb800000)
/lib64/ld-linux-x86-64.so.2 (0x00000037eb400000)
1.(linux-vdso.so.1) is the Linux Virtual Dynamic Shared Object.
Necessary logic to allow user programs to access system functions through fastest means available on the processor.
Either by interrupt, or for most newer CPUs by fast system call.
Types of ldd info: (1) Linux Virtual Dynamic Shared Objects; (2) Pointers; (3) Absolute paths.
2.(libc.so.6) has a pointer to /lib64/libc.so.6.
3.(/lib64/ld-linux-x86-64.so.2) is the absolute path to another library.
~$ ls -l /lib64/libc.so.6 /lib64/ld-linux-x86-64.so.2
lrwxrwxrwx. 1 root root 12 2010-01-14 14:24 /lib64/ld-linux-x86-64.so.2 -> ld-2.11.1.so
lrwxrwxrwx. 1 root root 14 2010-01-14 14:24 /lib64/libc.so.6 -> libc-2.11.1.so
ls -l cmd shows that last 2 libraries are, in turn, symbolic links to specific versions of the libraries.

Configuring shared libraries
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If a shared library is installed manually by editing the LIBRARY CONFIGURATION FILE , you must
manually inform the system to use the new libraries.
(with ldconfig)
The LIBRARY CONFIGURATION FILE is in:
/etc/ld.so.conf and /etc/ld.so.cache - contains the directories to be searched for shared libraries.
/etc/ld.so.conf specifies that all the .conf files from subdirectory ld.so.conf.d should be included.
Older systems may have all entries in /etc/ld/so/conf and not include entries from /etc/ld.so.conf.d
Ex: ~$ cat /etc/ld.so.conf
include ld.so.conf.d/*.conf
~$ ls /etc/ld.so.conf.d/*.conf
/etc/ld.so.conf.d/kernel-2.6.31.12-174.2.19.fc12.x86_64.conf
/etc/ld.so.conf.d/mysql-x86_64.conf
/etc/ld.so.conf.d/qt-x86_64.conf
/etc/ld.so.conf.d/tix-x86_64.conf
/etc/ld.so.conf.d/xulrunner-64.conf

Configuring shared libraries
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Command ldconfig Processes ld.so.conf – including the file libraries from the file, and from
the trusted directories: /lib and /usr/lib, and creates the ld.so.cache.
Ex: ~$
In summary - The ld.so.cache file is created from the ld.so.conf file by using ldconfig.
Any time that ld.so.conf file is changed, the cache file needs to be updated - Using ldconfig

Loading and setting specific libraries paths
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Applications can be made to load libraries from directories not listed in /etc/ld.so.conf
older apps that need specific library, developing versions of shared libraries, running scripts with product-specific shared libraries installed in /opt.
If the library is not in the standard path, you can search and add the nonstandard path with:
LD_LIBRARY_PATH environment variable.
export LD_LIBRARY_PATH=/usr/lib/oldstuff:/opt/IBM/AgentController/lib
If there’s more than 1 list of directories to search, separate them by colon.
Export LD_LIBRARY_PATH=/usr/newpath
LD_PRELOAD: Specifies libraries containing symbols that override symbols in the real
libraries.
LD_PRELOAD is seldom used except for debugging and compatibility applications (SOCKS proxy support).
Add the /usr/newpath to the list of searched directories for libraries.
Ex:
Ex:

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101 2.3 manage shared libraries

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