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 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):
# ls /etc/ld.so.conf.d kernel-2.6.32-358.18.1.el6.x86_64.conf kernel-2.6.32-696.1.1.el6.x86_64.conf mariadb-x86_64.conf kernel-2.6.32-642.6.2.el6.x86_64.conf kernel-2.6.32-696.6.3.el6.x86_64.conf postgresql-pgdg-libs.conf
If you take a look at the mariadb-x86_64.conf, you will see an absolute path to package libraries.
# cat mariadb-x86_64.conf /usr/lib64/mysql
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).
# export LD_LIBRARY_PATH=/path/to/library/file
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:
library name => filename (some hexadecimal value) OR filename (some hexadecimal value) #this is shown when library name can’t be read
This command shows all shared library dependencies for the ls command.
# ldd /usr/bin/ls OR # ldd /bin/ls
Sample Output
linux-vdso.so.1 => (0x00007ffebf9c2000) libselinux.so.1 => /lib64/libselinux.so.1 (0x0000003b71e00000) librt.so.1 => /lib64/librt.so.1 (0x0000003b71600000) libcap.so.2 => /lib64/libcap.so.2 (0x0000003b76a00000) libacl.so.1 => /lib64/libacl.so.1 (0x0000003b75e00000) libc.so.6 => /lib64/libc.so.6 (0x0000003b70600000) libdl.so.2 => /lib64/libdl.so.2 (0x0000003b70a00000) /lib64/ld-linux-x86-64.so.2 (0x0000561abfc09000) libpthread.so.0 => /lib64/libpthread.so.0 (0x0000003b70e00000) libattr.so.1 => /lib64/libattr.so.1 (0x0000003b75600000)
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 is employed, performed by 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 the ldconfig command to effect the changes.
# ldconfig OR # ldconfig -v #shows files and directories it works with
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:
# ldconfig -n /path/to/your/shared/libraries
To get started with creating your own libraries, check out this guide from The Linux Documentation Project(TLDP).
That’s all for now! In this article, we gave you an introduction to libraries and 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.
Is /usr/local/lib searched for shared libraries?
Make sure your LD_LIBRARY_PATH is set up to include all directories you want to search and then test it again.
You can test this quickly with:
LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/lib ffmpeg which will set it only for that invocation.
Alternatively, you can edit /etc/ld.so.conf which contains the default directories searched. Some Linux distributions may not include /usr/local/lib in that file.
Note that you may also need to update the cache /etc/ld.so.cache by running ldconfig (as root, or with sudo ).
@paxdiablo Excellent solution! export LD_LIBRARY_PATH=/usr/local/lib/ I have just spent hours searching for this solution. Works fine on my Ubuntu 10.10 with X264 enabled.
The GNU standards recommend installing by default all libraries in /usr/local/lib when distributing source code (and all commands should go into /usr/local/bin).
The list of directories to be searched is stored in the file /etc/ld.so.conf. Many Red Hat-derived distributions don’t normally include /usr/local/lib in the file /etc/ld.so.conf. I consider this a bug, and adding /usr/local/lib to /etc/ld.so.conf is a common «fix» required to run many programs on Red Hat-derived systems.
On Debian /etc/ld.so.conf contains include /etc/ld.so.conf.d/*.conf , and /etc/ld.so.conf.d/libc.conf contains
# libc default configuration /usr/local/lib Yes and no
Programs have a compiled-in (ok, «linked-in») idea of where their libraries are to be found. If a program is expecting to find its lib in /usr/local/lib then it will.
There is also a program called ldconfig and a configuration file called /etc/ld.so.conf and most likely an /etc/ld.so.conf.d , and these are used to specify site-specific directories.
Read «man ld.so» which lists other knobs like the environment variable LD_LIBRARY_PATH .
LD.SO(8) Linux Programmer’s Manual LD.SO(8) NAME ld.so, ld-linux.so* - dynamic linker/loader DESCRIPTION The programs ld.so and ld-linux.so* find and load the shared libraries needed by a program, prepare the program to run, and then run it. . . . LDCONFIG(8) Linux Programmer’s Manual LDCONFIG(8) NAME /sbin/ldconfig - configure dynamic linker run time bindings SYNOPSIS /sbin/ldconfig [ -nNvXV ] [ -f conf ] [ -C cache ] [ -r root ] direc- tory . /sbin/ldconfig -l [ -v ] library . /sbin/ldconfig -p DESCRIPTION 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). The cache is used by the run-time linker, ld.so or ld- linux.so. ldconfig checks the header and filenames of the libraries it encounters when determining which versions should have their links updated. . . . Where does Ubuntu look for shared libraries?
When I run a process that links to a shared library at runtime (linked when the process starts, not linked later with dlload() ), where does it look for that shared library ( .so ) file other than LD_LIBRARY_PATH ? Background: I have some C++ code that I wrote that uses a particular third-party library. I have installed the library and compiled my code on two different platforms, both Ubuntu but different versions, and different versions of gcc as well. The library was compiled and installed from source, and is located in /usr/local/lib on both platforms. When I compile my code, I link with the pkg-config —libs parameters for the third-party library and I’ve verified that pkg-config —libs returns the exact same thing on both platforms. My code compiles successfully on both platforms, and LD_LIBRARY_PATH is not defined (or defined as empty: «» ) on both platforms. However, when I run it on one platoform it works fine, and on the other I get this error:
error while loading shared libraries: libthrift-0.9.0.so: cannot open shared object file: No such file or directory Funnily enough, the ones that doesn’t work is the newer version of Ubuntu and gcc. :/ So I’m trying to figure out how the working one is able to locate the library, so that I can make the broken one locate the library in the same way. (i.e., without setting LD_LIBRARY_PATH ) Update: Here’s my output from cat /etc/ld.so.conf.d/* . on the working (older) system:
/usr/lib/mesa /usr/lib32/mesa /usr/lib/alsa-lib # libc default configuration /usr/local/lib # Multiarch support /lib/x86_64-linux-gnu /usr/lib/x86_64-linux-gnu # libc default configuration /usr/local/lib # Multiarch support /lib/x86_64-linux-gnu /usr/lib/x86_64-linux-gnu /usr/lib/x86_64-linux-gnu/mesa