C++ Dynamic Linking

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

Table of Contents

Dynamic Library Loading

Available Shared Libraries

Loading Process

Dynamic Linker Environment