Dynamic Linking at Runtime
Dynamic linking defers library loading until program execution, enabling code sharing and smaller executables.
Dynamic Library Loading
Available Shared Libraries
libc.so.6
v2.31 • 2.1 MB
printfmallocfree+2 more
libm.so.6
v2.31 • 1.5 MB
Depends on: libc.so.6
sincossqrt+2 more
libpthread.so.0
v2.31 • 156 KB
Depends on: libc.so.6
pthread_createpthread_joinpthread_mutex_lock
libstdc++.so.6
v10.2 • 1.9 MB
Depends on: libc.so.6, libm.so.6
std::coutstd::vectorstd::string+1 more
Loading Process
Click "Load" to simulate dynamic loading
Dynamic Linker Environment
LD_LIBRARY_PATH=/custom/lib:/usr/local/lib
LD_PRELOAD=./mylib.so
LD_DEBUG=libs,bindings
Static vs Dynamic
Static Linking
- Code copied into executable
- Larger file size
- No runtime dependencies
- Faster startup
Dynamic Linking
- References to shared libraries
- Smaller executables
- Runtime dependencies
- Memory sharing between processes
Creating Shared Libraries
# Compile with PIC (Position Independent Code) g++ -fPIC -c mylib.cpp # Create shared library g++ -shared -o libmylib.so mylib.o # Link against library g++ main.cpp -L. -lmylib -o program # Run with library path LD_LIBRARY_PATH=. ./program
Runtime Loading with dlopen
#include <dlfcn.h> // Load library at runtime void* handle = dlopen("libplugin.so", RTLD_LAZY); // Get function pointer typedef int (*func_t)(int); func_t func = (func_t)dlsym(handle, "function"); // Call function int result = func(42); // Unload library dlclose(handle);
GOT/PLT Mechanism
Dynamic symbols are resolved through:
- PLT (Procedure Linkage Table): Jump stubs
- GOT (Global Offset Table): Address storage
First call:
- Call goes to PLT stub
- PLT jumps to GOT entry
- GOT points back to resolver
- Resolver finds symbol
- Updates GOT with real address
Subsequent calls go directly through GOT.
Environment Variables
# Library search path LD_LIBRARY_PATH=/custom/lib:$LD_LIBRARY_PATH # Preload libraries LD_PRELOAD=./mylib.so ./program # Debug dynamic linker LD_DEBUG=libs ./program # Force immediate binding LD_BIND_NOW=1 ./program
Troubleshooting
Library Not Found
# Check dependencies ldd program # Find missing library ldconfig -p | grep libname # Add library path export LD_LIBRARY_PATH=/path/to/lib
Symbol Version Issues
# Check symbol versions objdump -T library.so # Force specific version patchelf --replace-needed old.so new.so program
Best Practices
- Version your libraries: Use SONAME
- Minimize dependencies: Reduce complexity
- Use RPATH carefully: Consider security
- Document dependencies: Help users
- Test with ldd: Verify before deployment
Next Steps
- Explore Memory Layout
- Learn about Symbol Resolution
- Understand Program Loading