ld_library_path Manual for Linux

Shared libraries enable programs to access common code during execution. This approach reduces memory usage and simplifies maintenance. The LD_LIBRARY_PATH environment variable controls where systems locate these libraries. Understanding LD_LIBRARY_PATH helps resolve library-related errors effectively.

What does LD_LIBRARY_PATH do?

The dynamic link loader searches for shared libraries when launching applications. LD_LIBRARY_PATH specifies directories to search before standard system locations. List multiple directories separated by colons. The loader examines these paths before checking /lib, /usr/lib, and other default directories.

This variable serves specific purposes:

• Testing new library versions against existing applications
• Relocating libraries to preserve older versions
• Creating self-contained application environments

Where can LD_LIBRARY_PATH cause problems?

The variable proves useful when applied correctly. However, developers often misuse it as a workaround for problems solvable through better methods. Global application of LD_LIBRARY_PATH creates dependencies that break when removed.

Three main issues emerge from improper use:

1. Security vulnerabilities: The loader searches LD_LIBRARY_PATH directories first. Malicious actors can inject harmful library versions. Setuid and setgid executables ignore this variable for this reason.

2. Performance degradation: The loader must search each specified directory for every required library. Failed open() calls with “ENOENT” errors accumulate quickly. Multiple directories magnify startup delays. Network-mounted directories compound this problem significantly.

3. Library inconsistency: Applications load incompatible library versions not intended during compilation. This creates crashes or produces incorrect results. Debugging becomes difficult when wrong library versions execute silently.

How do you check loaded libraries?

The ldd command displays required libraries for any executable:

$ ldd /usr/bin/file
        linux-vdso.so.1 =>  (0x00007fff9646c000)
        libmagic.so.1 => /usr/lib64/libmagic.so.1 (0x00000030f9a00000)
        libz.so.1 => /lib64/libz.so.1 (0x00000030f8e00000)
        libc.so.6 => /lib64/libc.so.6 (0x00000030f8200000)
        /lib64/ld-linux-x86-64.so.2 (0x00000030f7a00000)

This provides static dependency information. For runtime library analysis, examine a running process:

$ cat /proc/24362/maps

This reveals all libraries loaded during actual execution. Runtime dependencies often exceed those shown by ldd.

How can you avoid LD_LIBRARY_PATH problems?

Avoiding LD_LIBRARY_PATH completely produces the most reliable systems. When you must use it, minimize scope and duration. These methods provide better alternatives:

Compile with rpath: Specify library locations during compilation. The linker embeds paths directly into executables:

$ cc -o myprog obj1.o obj2.o -Wl,-rpath=/path/to/lib 
   -L/path/to/lib -lmylib

Alternatively, set LD_RUN_PATH before linking to specify multiple directories:

$ export LD_RUN_PATH=/path/to/lib1:/path/to/lib2:/path/to/lib3
$ cc -o myprog obj1.o obj2.o -L/path/to/lib1 -lmylib1

Verify correct paths using ldd after compilation. Fix any “not found” errors immediately.

Modify existing binaries: Tools like chrpath change rpath in compiled executables. Space limitations apply: you can only replace existing paths, not extend them. This method fails if no rpath exists initially.

$ chrpath -r /new/path/to/lib executable_name

Create wrapper scripts: When modifying executables proves impossible, write scripts that set variables for specific applications only:

#!/bin/bash
LD_LIBRARY_PATH=/path/to/lib1:/path/to/lib2:/path/to/lib3
export LD_LIBRARY_PATH
exec /path/to/bin/myprog "$@"

This isolates library paths to single applications. Mark scripts executable and launch programs through them.

Test temporarily: For quick verification, set the variable for one command only:

$ env LD_LIBRARY_PATH=/path/to/lib1:/path/to/lib2 ./myprog

Never export variables in your shell and then run programs. This pollutes your environment for subsequent commands.

Use ldconfig for system libraries: Add directories to /etc/ld.so.conf for system-wide library management:

# echo "/path/to/custom/lib" >> /etc/ld.so.conf
# ldconfig

This updates the system library cache properly. The approach works best for permanent system libraries.

Summary

The LD_LIBRARY_PATH variable affects how Linux systems locate shared libraries. Proper use solves specific library management problems. Improper use creates security risks, performance issues, and system instability. Compile programs with rpath whenever possible. Use wrapper scripts for third-party applications. Apply ldconfig for system-wide library management. Avoid setting LD_LIBRARY_PATH in login profiles. These practices ensure stable, secure application behavior.

If you’re new to Linux on your Chromebook, understanding environment variables forms an essential foundation. The same principles apply whether you’re using Linux development tools or managing server applications.