Dynamic Linking

# 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

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

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

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

1. DT_RPATH (if no DT_RUNPATH present)
2. LD_LIBRARY_PATH (env var, ignored for suid binaries)
3. DT_RUNPATH
4. /etc/ld.so.cache  (populated by ldconfig from /etc/ld.so.conf)
5. /lib, /usr/lib

LD_DEBUG=libs ./myapp      # trace library loading decisions
ldd myapp                  # show resolved libraries
ldd -v myapp               # verbose with version requirements

#include <dlfcn.h>

typedef int (*plugin_fn_t)(const char *input);

void load_plugin(const char *path) {
    // RTLD_NOW: resolve all symbols immediately (fail fast)
    // RTLD_LAZY: resolve on first call (default)
    // RTLD_LOCAL: symbols not visible to other loaded libs
    // RTLD_GLOBAL: symbols visible globally
    void *handle = dlopen(path, RTLD_NOW | RTLD_LOCAL);
    if (!handle) {
        fprintf(stderr, "dlopen: %s\n", dlerror());
        return;
    }

    // Clear previous errors
    dlerror();

    plugin_fn_t fn = (plugin_fn_t)dlsym(handle, "plugin_run");
    const char *err = dlerror();
    if (err) {
        fprintf(stderr, "dlsym: %s\n", err);
        dlclose(handle);
        return;
    }

    fn("hello");
    dlclose(handle);
}

gcc main.c -ldl -o myapp

// myinterpose.c — intercept malloc
#define _GNU_SOURCE
#include <stdio.h>
#include <dlfcn.h>

void *malloc(size_t size) {
    static void *(*real_malloc)(size_t) = NULL;
    if (!real_malloc)
        real_malloc = dlsym(RTLD_NEXT, "malloc");  // find next malloc in chain

    void *ptr = real_malloc(size);
    fprintf(stderr, "malloc(%zu) = %p\n", size, ptr);
    return ptr;
}

gcc -shared -fPIC -o myinterpose.so myinterpose.c -ldl

# Apply to any binary
LD_PRELOAD=./myinterpose.so ./myapp
LD_PRELOAD=/path/to/libfaketime.so ./myapp  # time manipulation

// Mark default: visible to linker
__attribute__((visibility("default")))
int public_api(void) { return 42; }

// Hidden: internal, not exported
__attribute__((visibility("hidden")))
static int internal_helper(void) { return 0; }

# mylib.map
MYLIB_1.0 {
    global:
        mylib_init;
        mylib_process;
    local:
        *;          # hide everything else
};

gcc -shared -fPIC -Wl,--version-script=mylib.map \
    -o libmylib.so mylib.c

# Check exported symbols
nm -D --defined-only libmylib.so
objdump -T libmylib.so

gcc -shared -fPIC -fvisibility=hidden \
    mylib.c -o libmylib.so