Shared Libraries in Linux: Linking, Loading, Sonames and Versioning

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The Linux Programming Interface · TLPI

Fundamentals of Shared Libraries

Deep dive into shared library mechanics on Linux — linking, loading, sonames & versioning

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2 Tutorial Pages

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8 Code Examples

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16 Interview Q&As

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Sections 41.4.3–41.4.4

About This Chapter

This chapter series covers the fundamental mechanics of shared libraries (also called dynamic libraries or shared objects) on Linux. You will learn:

How a shared library name gets embedded inside an ELF executable at link time
How the dynamic linker (ld-linux.so.2) loads shared libraries at runtime
How LD_LIBRARY_PATH works and when NOT to use it
What a soname is and how it enables library versioning
How to create a shared library with a soname and manage the symbolic link chain
How to upgrade a library without recompiling any programs

Prerequisites: Basic C programming, familiarity with gcc, and understanding of what a compiled object file is.

Chapter Keywords

Shared Library (.so) Static Library (.a) Dynamic Linker ld-linux.so.2 ELF DT_NEEDED DT_SONAME LD_LIBRARY_PATH RPATH / $ORIGIN Soname Real Name Linker Name Symbolic Links ldconfig readelf objdump -fPIC -Wl,-soname ABI ldd

Tutorial Pages

Part 1 — Using a Shared Library Section 41.4.3

Learn how shared libraries are embedded in ELF executables, how the dynamic linker works, what LD_LIBRARY_PATH does, RPATH and $ORIGIN, and static vs dynamic linking compared.

DT_NEEDED Tag Dynamic Linker LD_LIBRARY_PATH RPATH / $ORIGIN Static vs Dynamic Linking ldd / readelf / objdump -fPIC 8 Interview Questions

Start Reading →

Part 2 — The Shared Library Soname Section 41.4.4

Understand what a soname is, how to create a shared library with a soname, the three names of a shared library, symbolic link chains, and how to manage library versioning with major/minor version bumps.

Soname (DT_SONAME) Three Library Names -Wl,-soname Symbolic Link Chain Library Versioning ABI Compatibility ldconfig 8 Interview Questions

Start Reading →

⚡ Quick Reference — Most Important Commands

# Create shared library with soname
gcc -g -shared -fPIC -Wl,-soname,libfoo.so.1 -o libfoo.so.1.2.3 *.o

# Create symbolic link chain
ln -s libfoo.so.1.2.3 libfoo.so.1    # soname link
ln -s libfoo.so.1     libfoo.so       # linker name link

# Link a program against a shared library
gcc -g -Wall -o prog prog.c -L. -lfoo

# Run with non-standard library directory
LD_LIBRARY_PATH=/path/to/libs ./prog

# Embed RPATH (production alternative)
gcc -g -Wall -o prog prog.c -L. -lfoo -Wl,-rpath,/path/to/libs

# Inspect embedded library names
readelf -d prog | grep NEEDED
ldd prog

# Check library soname
readelf -d libfoo.so.1.2.3 | grep SONAME

# Update ldconfig cache (after installing to /usr/lib etc.)
sudo ldconfig

Start Learning

Begin with Part 1 to understand the basics of using shared libraries

Part 1 → Using a Shared Library Part 2 → Sonames