LPI 101 Ch08 Managing Shared Libraries

8/9/2019 LPI 101 Ch08 Managing Shared Libraries

1/29

SAIGONLAB 83 Nguyn Th Nh, P9, Q.Tn Bnh, Tp. HCM LPI 102

Chapter 8

Managing Shared Libraries


2/29


Objectives

Manage shared libraries

Understand and identify shared libraries and their

locations

Define and implement commands associated with

system libraries

Install and support Multiple Versions of a Shared

Library


3/29


MANAGING SHARED LIBRARIES

Programs are built using libraries of functions. There are

two ways to link any library into a program: dynamically

(using sharedlibraries), and statically

Static link: the contents of that library is copied into the

binary executable program file. Thus a statically linked

binary should be able to run on any Linux system without

any supporting files

Dynamic link : executable program contains references tothe libraries and functions. When the program is executed

these linkages are resolved: partially through code that is

linked into program. Program binaries are much smaller


4/29


5/29


The libc and ld.so/ld-linux.so Libraries

The most important library on a Linux system is libc. Almost

all programs are linked against libc

Also all Linux dynamically linked programs depend on the

ld.so and ld-linux.so libraries If any of these files are damaged or missing then most

commands on the system will NOT work.This includes the

user shells, most shell commands like ls, mv, cp,


6/29


Updating libc

Recommended way to replace a shared library:

1. Create the new files:

On the same file system as the older versions which you intend to

replace, in a subdirectory or under different names

# cp libc-2.2.4.so /lib

2. Issue the ln -f command :

# cd /lib

# ln f libc-2.2.4.so libc.so.6 You must NOT remove ("rm") the files in one command, intending

to copy or move (cp or mv) the new versions in another step

because the cp and mv commands depend on these libraries


7/29


Inode Numbers

Inode numbers are identifiers of a file or directory on a filesystem

Inode number is unique for each file or directory in a filesystem

Inode numbers are also called index numbers


8/29


Inode Numbers

Inode numbers can be displayed using ls command withioption

$ ls i ~

12110 dante 68349 dir3 12118 file3

12115 dante_1 68451 dir4 12119 file4

67773 dir1 12169 file1 68552 practice


9/29


Linking Files and Directories

Links are used to create alternate names or aliases for files anddirectories on a system

There are two kinds of links:

hard link

symbolic link (orsoftlink)


10/29


Hard link

Used to link files on the same file system

Files that are hard linked share the same inode number (referto the same data on disk)

Hard links are not used to link directories and cannot cross filesystems.


11/29


12/29


Hard link


13/29


Symbolic Link

Used to link files ordirectories.

Symbolically linked files do not share a single inode, these links

can cross file system .

A file with symbolically link:

$ls -l Test

-rw-r--r-- 1 torey staff 35 May 8 linktest

lrwxrwxrwx 1 torey staff 8 May 12 symlink--> linktest

$ls -F

linktest

symlink@


14/29


Symbolic Link

The structure of a symbolic link is as follows:


15/29


Creating Link

Command Format :ln [-s]


16/29


Examle Creating Hard Link

$ln /export/home/user2/dante essay

$ls -i /export/home/user2/dante

89532 dante

$ls -i essay

89532 essay


17/29


Examle Creating Symbolic Link

$ln -s tutor.vi symlink

$ls -l symlink

lrwxrwxrwx 1 torey staff 8 May 9 symlink--->tutor.vi


18/29


Files Related to Shared Libraries

In the /etc/ directory there are critical files which are neededby the dynamic linking libraries: /etc/ld.so.cache and

/etc/ld.so.conf

ld.so.cache file is an indexof all the shared libraries. This fileis built and updated using the ldconfig command

ld.so.conf file is a list of all the directories on the system where

the default shared libraries are to be found


19/29


Commands Related to System Libraries

ldconfig maintain the /etc/ld.so.cache

file

ldd view the list of libraries againstwhich a binary is linked

ltrace trace each library function that

is called from a program


20/29


ldconfig Command

Syntax : ldconfig [options] lib_dirs Description: Update the ld.so cache file with shared libraries

specified on the command line in lib_dirs

Options :

-p Display the contents of the current cache insteadof recreating it.

-v Verbose mode. Display progress duringexecution


21/29


ldd Command

Syntax : ldd programs Example :

Thebash shell requires four shared libraries:

# ldd /bin/bash

libtermcap.so.2 => /lib/libtermcap.so.2 (0x4002c000)

libdl.so.2 => /lib/libdl.so.2 (0x40030000)

libc.so.6 => /lib/i686/libc.so.6 (0x40034000)

/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)


22/29


Installing and Supporting Multiple Versions of a Shared Library

Generally programs will use the appropriate library againstwhich they were linked

It is also possible for programs to be linked to more specific

versions of a given library , in cases where they must depend

on some specific behavior that is missingfrom future versions

Finally it's possible to override the normal shared library

loading mechanism using LD_* environment variables


23/29


Installing and Supporting Multiple Versions of a Shared

Library

LD_LIBRARY_PATH

A colon-separated list of directories in which to search for

ELF libraries at execution time. This is similar to the PATH

environment variable.

LD_PRELOAD

A whitespace-separated list of additional, user-specified, ELF

shared libraries to be loaded before all others.


24/29


Errors Related to Missing or Damaged Shared Libraries

Errors :

- File not found

- error in loading shared libraries: libprintit.so: cannot

open shared object file: No such file or directory


25/29



Check to see if the command is a dangling symlink ( use ls -iand follow symlinks until you find a regular file )

Check the file type ( using the file command )

If the file is a script, check the

"hash bang

"

line ( the firstline should be of the form: # !... and should contain a full

path to the appropriate script interpreter's executable)


26/29



If the file is dynamically linked, check for missing sharedlibraries, or permissions problems on those libraries or their

parent directories (using the ldd command)

Check your environment for any variables with names like

LD_* (using the printenv or set commands)


27/29



These error messages can result from missing shared library

(. so) files. Shared libraries might be linked against other

shared libraries

So, when troubleshooting problems that might involve a

missing shared library: run the ldd command on the

troublesome binary, AND ON EACH OF THE LIBRARIES

TO WHICH THAT BINARY ISLINKED


28/29



Errors may be due topermissions problems on any of thelibraries

Errors can also be the result of a corrupted or out-of-date

ld.so.cache file, which can make the entire system unusable.

Remove it and build a new one with ldconfig

In addition, there may be a missing line in the /etc/ld.so.conf

file: check and repair it with a text editor and run the ldconfig

command if necessary


29/29


Summary

Manage shared libraries

Understand and identify shared libraries and their locations

Define and implement commands associated with system

libraries Install and support Multiple Versions of a Shared Library

Documents

LPI 101 Ch08 Managing Shared Libraries