Linux list all shared libraries

Linux Commands For Shared Library Management & Debugging Problem

I f you are a developer, you will re-use code provided by others. Usually /lib, /lib64, /usr/local/lib, and other directories stores various shared libraries. You can write your own program using these shared libraries. As a sys admin you need to manage and install these shared libraries. Use the following commands for shared libraries management, security, and debugging problems.

What is a Library In Linux or UNIX?

In Linux or UNIX like operating system, a library is noting but a collection of resources such as subroutines / functions, classes, values or type specifications. There are two types of libraries:

1. Static libraries – All lib*.a fills are included into executables that use their functions. For example you can run a sendmail binary in chrooted jail using statically liked libs.
2. Dynamic libraries or linking [ also known as DSO (dynamic shared object)] – All lib*.so* files are not copied into executables. The executable will automatically load the libraries using ld.so or ld-linux.so.

Linux Library Management Commands

1. ldconfig : Updates the necessary links for the run time link bindings.
2. ldd : Tells what libraries a given program needs to run.
3. ltrace : A library call tracer.
4. ld.so/ld-linux.so: Dynamic linker/loader.

Important Files

As a sys admin you should be aware of important files related to shared libraries:

1. /lib/ld-linux.so.* : Execution time linker/loader.
2. /etc/ld.so.conf : File containing a list of colon, space, tab, newline, or comma separated directories in which to search for libraries.
3. /etc/ld.so.cache : File containing an ordered list of libraries found in the directories specified in /etc/ld.so.conf. This file is not in human readable format, and is not intended to be edited. This file is created by ldconfig command.
4. lib*.so.version : Shared libraries stores in /lib, /usr/lib, /usr/lib64, /lib64, /usr/local/lib directories.

#1: ldconfig command

You need to use the ldconfig command to create, update, and remove the necessary links and cache (for use by the run-time linker, ld.so) to the most recent shared libraries found in the directories specified on the command line, in the file /etc/ld.so.conf, and in the trusted directories (/usr/lib, /lib64 and /lib). The ldconfig command checks the header and file names of the libraries it encounters when determining which versions should have their links updated. This command also creates a file called /etc/ld.so.cache which is used to speed up linking.

Examples

In this example, you’ve installed a new set of shared libraries at /usr/local/lib/:
$ ls -l /usr/local/lib/
Sample outputs:

Now when you run an app related to libGeoIP.so, you will get an error about missing library. You need to run ldconfig command manually to link libraries by passing them as command line arguments with the -l switch:
# ldconfig -l /path/to/lib/our.new.lib.so
Another recommended options for sys admin is to create a file called /etc/ld.so.conf.d/geoip.conf as follows:

Now just run ldconfig to update the cache:
# ldconfig
To verify new libs or to look for a linked library, enter:
# ldconfig -v
# ldconfig -v | grep -i geoip
Sample outputs:

Troubleshooting Chrooted Jails

You can print the current cache with the -p option:
# ldconfig -p
Putting web server such as Apache / Nginx / Lighttpd in a chroot jail minimizes the damage done by a potential break-in by isolating the web server to a small section of the filesystem. It is also necessary to copy all files required by Apache inside the filesystem rooted at /jail/ directory , including web server binaries, shared Libraries, modules, configuration files, and php/perl/html web pages. You need to also copy /etc/ files and /etc/ld.so.conf.d/ directory to /jail/etc/ directory. Use the ldconfig command to update, print and troubleshoot chrooted jail problems:

Rootkits

A rootkit is a program (or combination of several programs) designed to take fundamental control of a computer system, without authorization by the system’s owners and legitimate managers. Usually, rootkit use /lib, /lib64, /usr/local/lib directories to hide itself from real root users. You can use ldconfig command to view all the cache of all shared libraries and unwanted programs:
# /sbin/ldconfig -p | less
You can also use various tools to detect rootkits under Linux.

Common errors

You may see the errors as follows:

Dynamic linker error in foo
Can’t map cache file cache-file
Cache file cache-file foo

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All of the above errors means the linker cache file /etc/ld.so.cache is corrupt or does not exists. To fix these errors simply run the ldconfig command as follows:
# ldconfig

Can’t find library xyz Error

The executable required a dynamically linked library that ld.so or ld-linux.so cannot find. It means a library called xyz needed by the program called foo not installed or path is not set. To fix this problem install xyz library and set path in /etc/ld.so.conf file or create a file in /etc/ld.so.conf.d/ directory.

#2: ldd command

ldd (List Dynamic Dependencies) is a Unix and Linux program to display the shared libraries required by each program. This tools is required to build and run various server programs in a chroot jail. A typical example is as follows to list the Apache server shared libraries, enter:
# ldd /usr/sbin/httpd
Sample outputs:

Now, you can copy all those libs one by one to /jail directory

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Understanding Shared Libraries in Linux

In programming, a library is an assortment of pre-compiled pieces of code that can be reused in a program. Libraries simplify life for programmers, in that they provide reusable functions, routines, classes, data structures and so on (written by a another programmer), which they can use in their programs.

For instance, if you are building an application that needs to perform math operations, you don’t have to create a new math function for that, you can simply use existing functions in libraries for that programming language.

Examples of libraries in Linux include libc (the standard C library) or glibc (GNU version of the standard C library), libcurl (multiprotocol file transfer library), libcrypt (library used for encryption, hashing, and encoding in C) and many more.

Linux supports two classes of libraries, namely:

• Static libraries – are bound to a program statically at compile time.
• Dynamic or shared libraries – are loaded when a program is launched and loaded into memory and binding occurs at run time.

Dynamic or shared libraries can further be categorized into:

• Dynamically linked libraries – here a program is linked with the shared library and the kernel loads the library (in case it’s not in memory) upon execution.
• Dynamically loaded libraries – the program takes full control by calling functions with the library.

Shared Library Naming Conventions

Shared libraries are named in two ways: the library name (a.k.a soname) and a “filename” (absolute path to file which stores library code).

For example, the soname for libc is libc.so.6: where lib is the prefix, c is a descriptive name, so means shared object, and 6 is the version. And its filename is: /lib64/libc.so.6. Note that the soname is actually a symbolic link to the filename.

Locating Shared Libraries in Linux

Shared libraries are loaded by ld.so (or ld.so.x) and ld-linux.so (or ld-linux.so.x) programs, where x is the version. In Linux, /lib/ld-linux.so.x searches and loads all shared libraries used by a program.

A program can call a library using its library name or filename, and a library path stores directories where libraries can be found in the filesystem. By default, libraries are located in /usr/local/lib, /usr/local/lib64, /usr/lib and /usr/lib64; system startup libraries are in /lib and /lib64. Programmers can, however, install libraries in custom locations.

The library path can be defined in /etc/ld.so.conf file which you can edit with a command line editor.

The line(s) in this file instruct the kernel to load file in /etc/ld.so.conf.d. This way, package maintainers or programmers can add their custom library directories to the search list.

If you look into the /etc/ld.so.conf.d directory, you’ll see .conf files for some common packages (kernel, mysql and postgresql in this case):

If you take a look at the mariadb-x86_64.conf, you will see an absolute path to package’s libraries.

The method above sets the library path permanently. To set it temporarily, use the LD_LIBRARY_PATH environment variable on the command line. If you want to keep the changes permanent, then add this line in the shell initialization file /etc/profile (global) or

/.profile (user specific).

Managing Shared Libraries in Linux

Let us now look at how to deal with shared libraries. To get a list of all shared library dependencies for a binary file, you can use the ldd utility. The output of ldd is in the form:

This command shows all shared library dependencies for the ls command.

Sample Output

Because shared libraries can exist in many different directories, searching through all of these directories when a program is launched would be greatly inefficient: which is one of the likely disadvantages of dynamic libraries. Therefore a mechanism of caching employed, performed by a the program ldconfig.

By default, ldconfig reads the content of /etc/ld.so.conf, creates the appropriate symbolic links in the dynamic link directories, and then writes a cache to /etc/ld.so.cache which is then easily used by other programs.

This is very important especially when you have just installed new shared libraries or created your own, or created new library directories. You need to run ldconfig command to effect the changes.

After creating your shared library, you need to install it. You can either move it into any of the standard directories mentioned above, and run the ldconfig command.

Alternatively, run the following command to create symbolic links from the soname to the filename:

To get started with creating your own libraries, check out this guide from The Linux Documentation Project(TLDP).

Thats all for now! In this article, we gave you an introduction to libraries, explained shared libraries and how to manage them in Linux. If you have any queries or additional ideas to share, use the comment form below.

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Command to list the directories in which to look for shared libraries?

I’ve read which equivalent for shared libraries and Where do executables look for shared objects at runtime, but is there a command to list all the directories in which the shared libraries will be searched?

Something like a command that auto-computes all the list explained in the second question.

4 Answers 4

long: The related environment variables are system- and configuration-dependent. For a given system/configuration, you could write a script which does this.

Where do executables look for shared objects at runtime gives some insight, but is incomplete. It mentions OSX and Solaris, but focuses on Linux, pointing to two resources:

• Program Library HOWTO: 3. Shared Libraries (which mentions LD_LIBRARY_PATH without mentioning the less-often LD_LIBRARY_PATH_64 , and also gives a one-line mention of rpath).
• ld.so, ld-linux.so* — dynamic linker/loader, which again mentions some of the environment variables and rpath.

You would also find these useful:

ldconfig creates the necessary links and cache to the most recent shared libraries found in the directories specified on the command line, in the file /etc/ld.so.conf , and in the trusted directories, /lib and /usr/lib (on some 64-bit architectures such as x86-64, lib and /usr/lib are the trusted directories for 32-bit libraries, while /lib64 and /usr/lib64 are used for 64-bit libraries).

In particular, » sudo ldconfig -v «

-v , —verbose
Verbose mode. Print current version number, the name of each directory as it is scanned, and any links that are created. Overrides quiet mode.

which is close to what was asked, but gives lots of extraneous information. (And it is largely Linux-specific, though BSDs use it — but different, see manual page). If you make some assumptions about its output format, you could get directories from this using

which gives (on one system)

To recap: there is no command, but you can make a script, which is system-dependent.

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