Ldd is a Linux command line utility that is used in case a user wants to know the shared library dependencies of an executable or even that of a shared library. You might have noticed many files starting with lib* in /lib and /usr/lib directories of your Linux machine. These files are called libraries. Library is a collection of resources such as subroutines/functions, classes, values or type specifications.
A library makes it possible for a program to use common routines without the administrative overhead of maintaining their source code, or the processing overhead of compiling them each time the program is compiled.
There are two types of libraries:
Static libraries: static libraries for complete programs that do not depend on external libraries to run. The feature of statically linked programs is that they work without installing any prerequisites. The static library is end with *.a extension and these libraries are included (a separate copy) into the programs that require its functions.
Dynamic libraries: dynamic libraries for tiny programs in size, These libraries end with .so extension, Another feature of using dynamic linking when many programs are running, It can share one copy of a library rather than occupying memory with many copies of the same code. So the recent programs use dynamic linking. In this article, we will go through the commands ldd which is used to manage the shared libraries.
Shared libraries
When we make a program, we need many pieces of code that someone else has already written to perform routine or specialized functions for us. These pieces of code are stored in shared libraries. To use them, we link them with our code, either when we build the program or when we run the program.
LDD Command
The ldd command prints shared object dependencies. The command's syntax is:
ldd [OPTIONS]... FILE...OPTIONS
- -v : Print all information.
- -d : process data relocation.
- -r : process data and function relocation.
- -u : print unused direct dependencies.
Please note down the following points before going through the command:
- The file, ld-linux.so is the dynamic linker or loader which checks for the desired link or library cache for the requested program and loads it.
- The cache file, /etc/ld.so.cache contains a list of libraries found in the directories specified in /etc/ld.so.conf. This helps to provide faster dynamic linking.
- The file /etc/ld.so.conf specifies the directories where to search for libraries.
Install ldd
ldd is installed by default in most Linux distributions. If you getting ldd command not found error, install it using:
Ubuntu / Debian
apt install libc-binRedhat / CentOS Stream
yum install glibc-commonFedora
dnf install glibc-commonArch
pacman -S glibcHow to use ldd command
Let's look into some useful usages of ldd command.
1. Display dependencies of the command
We will display the dependencies of cp command.
ldd /bin/cp Output:
linux-vdso.so.1 => (0x00007fffaf3ff000)
libselinux.so.1 => /lib64/libselinux.so.1 (0x0000003a06a00000)
librt.so.1 => /lib64/librt.so.1 (0x0000003a06200000)
libacl.so.1 => /lib64/libacl.so.1 (0x0000003a13000000)
libattr.so.1 => /lib64/libattr.so.1 (0x0000003a0ea00000)
libc.so.6 => /lib64/libc.so.6 (0x0000003a05200000)
libdl.so.2 => /lib64/libdl.so.2 (0x0000003a05a00000)
/lib64/ld-linux-x86-64.so.2 (0x0000003a04a00000)
libpthread.so.0 => /lib64/libpthread.so.0 (0x0000003a05600000)
2. Display dependencies of the command with details
We will display the dependencies of cp command with more details using -v option.
ldd -v /bin/cp Output:
linux-vdso.so.1 => (0x00007fff473ff000)
libselinux.so.1 => /lib64/libselinux.so.1 (0x0000003a06a00000)
librt.so.1 => /lib64/librt.so.1 (0x0000003a06200000)
libacl.so.1 => /lib64/libacl.so.1 (0x0000003a13000000)
libattr.so.1 => /lib64/libattr.so.1 (0x0000003a0ea00000)
libc.so.6 => /lib64/libc.so.6 (0x0000003a05200000)
libdl.so.2 => /lib64/libdl.so.2 (0x0000003a05a00000)
/lib64/ld-linux-x86-64.so.2 (0x0000003a04a00000)
libpthread.so.0 => /lib64/libpthread.so.0 (0x0000003a05600000)
Version information:
/bin/cp:
librt.so.1 (GLIBC_2.2.5) => /lib64/librt.so.1
libattr.so.1 (ATTR_1.1) => /lib64/libattr.so.1
libacl.so.1 (ACL_1.2) => /lib64/libacl.so.1
libacl.so.1 (ACL_1.0) => /lib64/libacl.so.1
libc.so.6 (GLIBC_2.6) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.4) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.3) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.3.4) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.2.5) => /lib64/libc.so.6
/lib64/libselinux.so.1:
libdl.so.2 (GLIBC_2.2.5) => /lib64/libdl.so.2
ld-linux-x86-64.so.2 (GLIBC_2.3) => /lib64/ld-linux-x86-64.so.2
libc.so.6 (GLIBC_2.4) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.8) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.3) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.3.4) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.2.5) => /lib64/libc.so.6
/lib64/librt.so.1:
libpthread.so.0 (GLIBC_2.2.5) => /lib64/libpthread.so.0
libpthread.so.0 (GLIBC_PRIVATE) => /lib64/libpthread.so.0
libc.so.6 (GLIBC_2.3.2) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.2.5) => /lib64/libc.so.6
libc.so.6 (GLIBC_PRIVATE) => /lib64/libc.so.6
/lib64/libacl.so.1:
libattr.so.1 (ATTR_1.0) => /lib64/libattr.so.1
libc.so.6 (GLIBC_2.4) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.3.4) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.2.5) => /lib64/libc.so.6
/lib64/libattr.so.1:
libc.so.6 (GLIBC_2.4) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.2.5) => /lib64/libc.so.6
/lib64/libc.so.6:
ld-linux-x86-64.so.2 (GLIBC_PRIVATE) => /lib64/ld-linux-x86-64.so.2
ld-linux-x86-64.so.2 (GLIBC_2.3) => /lib64/ld-linux-x86-64.so.2
/lib64/libdl.so.2:
ld-linux-x86-64.so.2 (GLIBC_PRIVATE) => /lib64/ld-linux-x86-64.so.2
libc.so.6 (GLIBC_PRIVATE) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.2.5) => /lib64/libc.so.6
/lib64/libpthread.so.0:
ld-linux-x86-64.so.2 (GLIBC_2.3) => /lib64/ld-linux-x86-64.so.2
ld-linux-x86-64.so.2 (GLIBC_2.2.5) => /lib64/ld-linux-x86-64.so.2
ld-linux-x86-64.so.2 (GLIBC_PRIVATE) => /lib64/ld-linux-x86-64.so.2
libc.so.6 (GLIBC_2.3.2) => /lib64/libc.so.6
libc.so.6 (GLIBC_PRIVATE) => /lib64/libc.so.6
libc.so.6 (GLIBC_2.2.5) => /lib64/libc.so.6
3. Display unused direct dependencies of the command
We can display unused direct dependencies of cp command using -u option.
ldd -u /bin/cp Output:
Unused direct dependencies:
/lib64/libselinux.so.1
/lib64/librt.so.1
/lib64/libacl.so.1
/lib64/libattr.so.14. Display ldd works only on dynamic executables
We will display ldd works only on dynamic executables using -r option.
ldd -r /smart/pycharm-community-2017.3.3/bin/pycharm.sh Output:
not a dynamic executable
The output displayed a clear message state that the supplied file is not a dynamic executable.
5. ldd with standard command line executable
When we try ldd on a standard command line executable like ls, We need full path to the dynamic executable.
ldd ls Output:
ldd: ./ls: No such file or directoryWe see that ldd states that it cannot find ls.
ldd /bin/ls Output:
linux-vdso.so.1 => (0x00007fff5cbea000)
libselinux.so.1 => /lib64/libselinux.so.1 (0x0000003a06a00000)
librt.so.1 => /lib64/librt.so.1 (0x0000003a06200000)
libcap.so.2 => /lib64/libcap.so.2 (0x0000003a07600000)
libacl.so.1 => /lib64/libacl.so.1 (0x0000003a13000000)
libc.so.6 => /lib64/libc.so.6 (0x0000003a05200000)
libdl.so.2 => /lib64/libdl.so.2 (0x0000003a05a00000)
/lib64/ld-linux-x86-64.so.2 (0x0000003a04a00000)
libpthread.so.0 => /lib64/libpthread.so.0 (0x0000003a05600000)
libattr.so.1 => /lib64/libattr.so.1 (0x0000003a0ea00000)
But with the absolute path, ldd worked fine.
6. Know a given executable daemon supports TCP Wrapper
To determine whether a given executable daemon supports TCP Wrapper or not, run the following command.
sudo ldd /usr/sbin/sshd | grep libwrap Output:
libwrap.so.0 => /lib64/libwrap.so.0 (0x00007f1cc2ac6000)
The output indicates that the OpenSSH (sshd) daemon supports TCP Wrapper.
7. ldd with missing dependency
We can use the ldd command when an executable is failing because of a missing dependency. Once we found a missing dependency, we can install it or update the cache with the ldconfig command.
sudo ldd /bin/mvlibacl.so.1 => /lib/libacl.so.1 (0×40016000)
libc.so.6 => /lib/libc.so.6 (0x4001c000)
libattr.so.1 => /lib/libattr.so.1 (0×40141000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0×40000000)We will perform relocations and report any missing objects (ELF only) by typing the command.
sudo ldd -d path/to/executable_fileWe will perform relocations for both data objects and functions, and report any missing objects or functions (ELF only) by typing the following command.
sudo ldd -r path/to/executable_fileConclusion
In this tutorial, we learned how to use ldd command with examples.
Thanks for reading, please provide your feedback and suggestions in the below comment section.
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