Mastering ldd: How to List Shared Library Dependencies on Linux

1. Command Overview

The ldd command prints the list of shared libraries required by an executable or shared object file. It is not a compiled binary but a shell script that invokes the dynamic loader ( ld-linux.so) with specific environment variables.

2. Relevant Environment Variables

Key variables that affect ldd behavior are: LD_TRACE_LOADED_OBJECTS=1 (enabled by default)

LD_WARN

LD_BIND_NOW

LD_LIBRARY_VERSION

LD_VERBOSE

LD_DEBUG

Only LD_TRACE_LOADED_OBJECTS is required for normal operation; the others are optional.

3. Mapping Options to Variables

-d, --data-relocs

→

LD_WARN=yes

-r, --function-relocs

→ LD_WARN=yes and

LD_BIND_NOW=yes

-u, --unused

→

LD_DEBUG="unused"

-v, --verbose

→

LD_VERBOSE=yes

4. Using LD_TRACE_LOADED_OBJECTS Directly

Setting LD_TRACE_LOADED_OBJECTS to a non‑empty value forces the dynamic loader to list dependencies without actually running the target program. export LD_TRACE_LOADED_OBJECTS=1 Running a command such as ls after exporting this variable will display the libraries that ls depends on, e.g.:

linux-vdso.so.1 => (0x00007ffd1d3cf000)</code>
<code>/lib64/libc.so.6 => /lib64/libc.so.6 (0x00007fbbe665e000)</code>
<code>/lib64/ld-linux-x86-64.so.2 (0x00007fbbe7053000)

Unsetting the variable restores normal command behavior:

unset LD_TRACE_LOADED_OBJECTS

5. Command Syntax

ldd [OPTIONS] <FILES>

[OPTIONS]

are optional; <FILES> is a required list of executables or shared objects.

6. Common Options

--version

: show ldd version -v, --verbose: detailed mode, print all related information -u, --unused: list unused direct dependencies -d, --data-relocs: perform relocations and report missing objects -r, --function-relocs: perform data and function relocations (ELF only) --help: display help

7. Example Usage

Display the shared libraries required by /usr/bin/ls:

ldd /usr/bin/ls</code>
<code>linux-vdso.so.1 => (0x00007ffd90514000)</code>
<code>/lib64/libc.so.6 => /lib64/libc.so.6 (0x00007f294a2e4000)</code>
<code>/lib64/ld-linux-x86-64.so.2 (0x00007f294acd9000)

These examples demonstrate that ldd works by configuring the dynamic loader via environment variables, allowing safe inspection of binary dependencies.