Linux lib directory path

Ubuntu Linux Library Path

How do I determine the Ubuntu Linux Library Path? That is, how does the linker know where to got to grab the object files when linking my program?

5 Answers 5

Look at /etc/ld.so.conf and the files in the /etc/ld.so.conf.d/ directory — that’s where it is set.

The file paths can be set explicitly when linking using the -L parameter, as well as the environment variable LD_LIBRARY_PATH .

There are also some paths hard-coded into the linker, using the -L param. You can see these with the command:

If it’s not a standard path ( /lib , /usr/lib ), you can specify the location with the compiler flag. For g++ , it’s -L/some/path/lib . If you use autotools, you can just configure with LDFLAGS=-L/some/path/lib if you need a specific path. If configure has been properly designed for the project, it should have a —with-some-library=PATH option, where you can also specify a path for that library only.

When linking, you need to specify the -L flag to indicate where the library is located. At runtime, the dynamic linker uses the paths given in «/etc/ld.so.conf», «/etc/ld.so.conf.d/*» and the value of LD_LIBRARY_PATH.

«sudo ldconfig» updates the system’s cache if you’ve just installed something new.

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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 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.

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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 

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Setting Library path in Linux

You need to use ldconfig config file and ldconfig command which 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 (for Debian distributions), and in the trusted directories such as /lib64 or /usr/lib64 (/lib or /usr/lib on 32 bit systems). The /etc/ld.so.conf contains lib settings which can be used to add or delete paths.

However, you need to simply drop your config file in /etc/ld.so.conf.d/ directory and it will be used by /sbin/ldconfig to configure dynamic linker run time bindings.

Add Your Path

Create a file called /etc/ld.so.conf.d/myapp.conf:

Activate Your Library Path

You must run the following command to activate path:

Verify Your New Library Path

# ldconfig -v | less 
# ldconfig -v | grep /usr/local/lib

/usr/local/lib: libGeoIP.so.1 -> libGeoIP.so.1.4.6 libGeoIPUpdate.so.0 -> libGeoIPUpdate.so.0.0.0 /usr/lib64/mysql: libmysqlclient_r.so.15 -> libmysqlclient_r.so.15.0.0 libmysqlclient.so.15 -> libmysqlclient.so.15.0.0 /lib: libutil.so.1 -> libutil-2.5.so

How Do I Delete The Library Path?

# rm /etc/ld.so.conf.d/myapp.conf # ldconfig

How Do I Edit The Library Path?

Simply edit the file and reload the changes:

# vi /etc/ld.so.conf.d/myapp.conf # ldconfig

How Do I Compile Program With Shared Libs And GNU GCC?

You can use the following gcc:

$ gcc -Wl,-R/path/to/lib -I/path/to/include -L/path/to/lib -o myAppName mycode.c -llibapp2

Add to LD_LIBRARY_PATH

Add the library path you need and set this at shell for temporary use or add to the shell initialization file for permanent effect:

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/opt/foobar/lib

Check shared libraries used by program

You can see the list of the shared libraries used by a program using ldd. So, for example, you can see the shared libraries used by ls by typing:

Generally you’ll see a list of the sonames being depended on, along with the directory that those names resolve to. In practically all cases you’ll have at least two dependencies:

• /lib/ld-linux.so.N (where N is 1 or more, usually at least 2). This is the library that loads all other libraries.
• libc.so.N (where N is 6 or more). This is the C library. Even other languages tend to use the C library (at least to implement their own libraries), so most programs at least include this one.

Beware: do not run ldd on a program you don’t trust. As is clearly stated in the ldd(1) manual, ldd works by (in certain cases) by setting a special environment variable (for ELF objects, LD_TRACE_LOADED_OBJECTS) and then executing the program. It may be possible for an untrusted program to force the ldd user to run arbitrary code (instead of simply showing the ldd information). So, for safety’s sake, don’t use ldd on programs you don’t trust to execute.

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