dynamic-linking

Dynamic Linking

Purpose

Guide agents through Linux dynamic linking: shared library creation, RPATH/RUNPATH configuration, soname versioning, dlopen/dlsym plugin patterns, LD_PRELOAD interposition, and symbol visibility control.

Triggers

• "Cannot open shared object file: No such file or directory"
• "How do I set RPATH so my binary finds its shared library?"
• "How do I use dlopen/dlsym for a plugin system?"
• "What's the difference between RPATH and RUNPATH?"
• "How do I use LD_PRELOAD to intercept a function?"
• "How do I version my shared library with soname?"

Workflow

1. Creating a shared library

# Compile with -fPIC (position-independent code)
gcc -fPIC -c src/mylib.c -o mylib.o

# Link shared library with soname
gcc -shared -Wl,-soname,libmylib.so.1 \
    mylib.o -o libmylib.so.1.2.3

# Create symlinks (standard convention)
ln -s libmylib.so.1.2.3 libmylib.so.1   # soname link (used by ldconfig)
ln -s libmylib.so.1     libmylib.so      # link link (used at compile time)

# Register with ldconfig (system-wide)
sudo cp libmylib.so.1.2.3 /usr/local/lib/
sudo ldconfig

2. Soname versioning convention

libfoo.so.MAJOR.MINOR.PATCH
         │
         └── soname = libfoo.so.MAJOR

Version bump	When
PATCH	Bug fix, ABI unchanged
MINOR	New symbols added, backwards compatible
MAJOR	ABI break — existing binaries will break

Inspect soname:

readelf -d libmylib.so.1.2.3 | grep SONAME
objdump -p libmylib.so.1.2.3 | grep SONAME

3. RPATH vs RUNPATH

Both embed a library search path in the binary.

RPATH  → searched BEFORE LD_LIBRARY_PATH
RUNPATH → searched AFTER LD_LIBRARY_PATH (controllable at runtime)

Recommendation: prefer RUNPATH (-Wl,--enable-new-dtags)
                for deployment flexibility.

# Embed RPATH (old default)
gcc main.c -L./lib -lmylib \
    -Wl,-rpath,'$ORIGIN/../lib' -o myapp

# Embed RUNPATH (new default with --enable-new-dtags)
gcc main.c -L./lib -lmylib \
    -Wl,-rpath,'$ORIGIN/../lib' \
    -Wl,--enable-new-dtags -o myapp

# Inspect
readelf -d myapp | grep -E 'RPATH|RUNPATH'
chrpath -l myapp        # show
chrpath -r '/new/path' myapp  # modify existing