Address Binding - Learning Module

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Dynamic Loading

Loading Code at Runtime

Consider a web browser. It supports dozens of media formats: JPEG, PNG, WebP, MP4, WebM, PDF, and countless others. Each format requires specialized decoding code. But what if you never browse sites with PDF files? Why should your browser load PDF decoding code at startup?

Dynamic loading solves this problem. Instead of loading all code at program startup, programs can load code on demand, at runtime. The PDF decoder loads only when you first encounter a PDF. A camera app loads the portrait mode AI only when you select that feature. A game loads levels and assets only as needed.

Dynamic loading is the ultimate expression of flexible address binding. Not only can code load at any address (thanks to execution-time binding and PIC), but it can load at any time—or not at all. This flexibility enables plugin architectures, modular software design, and efficient resource utilization.

What You Will Learn

By the end of this page, you will understand the distinction between dynamic linking and dynamic loading, how runtime loading APIs work (dlopen, LoadLibrary), how symbols are resolved at runtime, plugin architecture design, and the tradeoffs of dynamic versus static loading.

Dynamic Linking vs Dynamic Loading

These terms are often confused, but they represent different concepts:

Dynamic Linking:

Shared libraries (.so, .dll) are specified at compile/link time
Libraries are loaded at program startup by the dynamic linker
All required libraries must be present when the program starts
Symbol references resolved at startup or on first call (lazy binding)

Dynamic Loading:

Libraries are loaded at runtime by the program itself
The program explicitly calls loading functions (dlopen, LoadLibrary)
Libraries don't need to exist at compile time
The program controls when and what to load

Dynamic Linking vs Dynamic Loading
Aspect	Dynamic Linking	Dynamic Loading
When libraries specified	Compile/link time	Runtime (code decides)
When libraries loaded	Program startup	When program requests
Loader responsible	Dynamic linker (ld-linux.so)	Application code (via dlopen)
Library must exist at startup	Yes (program fails otherwise)	No (load on demand)
Symbol resolution	Automatic (via PLT/GOT)	Manual (dlsym, GetProcAddress)
Use case	Standard shared libraries	Plugins, optional features

The spectrum of loading strategies:

Static Linking ──────▶ Dynamic Linking ──────▶ Dynamic Loading

    │                        │                        │
    │ All code embedded      │ Shared libs at         │ Load code
    │ in executable          │ startup                │ on demand
    │                        │                        │
    │ No runtime             │ Required libs          │ Full runtime
    │ dependencies           │ must exist             │ flexibility
    │                        │                        │
    │ Largest binary         │ Smaller binary         │ Smallest initial
    │                        │                        │ footprint

Most modern programs use a combination: critical libraries are dynamically linked (loaded at startup), while optional features use dynamic loading.

The Runtime Loading API

Operating systems provide APIs for programs to load libraries at runtime. The primary APIs are:

POSIX (Linux, macOS, Unix): dlopen, dlsym, dlclose, dlerror

Windows: LoadLibrary, GetProcAddress, FreeLibrary, GetLastError

These APIs follow a similar pattern:

Load the library into the process's address space
Lookup symbols (functions or variables) by name
Use the symbols (call functions, access data)
Unload the library when done (optional)

POSIX Dynamic Loading Example
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#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>  // Dynamic loading header
 
// Define the function signature we expect from the plugin
typedef int (*math_operation_t)(int, int);
 
int main(int argc, char *argv[]) {
    void *handle;
    math_operation_t operation;
    char *error;
    
    // ══════════════════════════════════════════════════════════════════
    // STEP 1: Load the shared library at runtime
    // ══════════════════════════════════════════════════════════════════
    
    // dlopen() loads a shared library into the process
    // RTLD_LAZY: Resolve symbols lazily (on first use)
    // RTLD_NOW:  Resolve all symbols immediately (catches errors early)
    
    handle = dlopen("./libmathplugin.so", RTLD_LAZY);
    
    if (!handle) {
        fprintf(stderr, "Failed to load library: %s\n", dlerror());
        return EXIT_FAILURE;
    }
    
    // ══════════════════════════════════════════════════════════════════
    // STEP 2: Look up a function by name
    // ══════════════════════════════════════════════════════════════════
    
    // Clear any existing error
    dlerror();
    
    // dlsym() finds a symbol (function or variable) in the loaded library
    operation = (math_operation_t)dlsym(handle, "add");
    
    // Check for errors (dlsym returns NULL for missing symbols,
    // but NULL might be a valid address, so check dlerror too)
    error = dlerror();
    if (error != NULL) {
        fprintf(stderr, "Failed to find symbol: %s\n", error);
        dlclose(handle);
        return EXIT_FAILURE;
    }
    
    // ══════════════════════════════════════════════════════════════════
    // STEP 3: Use the function!
    // ══════════════════════════════════════════════════════════════════
    
    int result = operation(10, 5);
    printf("Result: %d\n", result);  // Output: Result: 15
    
    // Can load other functions from the same library
    operation = (math_operation_t)dlsym(handle, "multiply");
    if ((error = dlerror()) == NULL) {
        printf("Multiply result: %d\n", operation(10, 5));  // Output: 50
    }
    
    // ══════════════════════════════════════════════════════════════════
    // STEP 4: Unload the library (optional, freed on process exit anyway)
    // ══════════════════════════════════════════════════════════════════
    
    dlclose(handle);
    
    return EXIT_SUCCESS;
}

How dlopen Works Internally

When you call dlopen(), a complex sequence of operations occurs to make the library's code available:

The dlopen process:

dlopen Internal Steps

•Search for the library — Check paths in order: explicit path, LD_LIBRARY_PATH, /etc/ld.so.cache, default paths (/lib, /usr/lib)
•Check if already loaded — If the library is already in the process, just increment its reference count and return the existing handle
•Open and map the file — Read the ELF header, map code and data segments into process memory using mmap()
•Allocate and fill GOT/PLT — Create the library's Global Offset Table and Procedure Linkage Table entries
•Load dependencies — Recursively load any libraries this library depends on (recorded in DT_NEEDED entries)
•Process relocations — Apply all relocation entries to resolve symbol references
•Run constructors — Execute functions marked with attribute((constructor)) or listed in .init_array
•Update symbol scope — Make the library's exported symbols available for lookup
•Return handle — Return an opaque handle the caller can use with dlsym() and dlclose()

dlopen Flags and Their Effects

Reference

DLOPEN FLAGS REFERENCE
═══════════════════════════════════════════════════════════════════════════════
 
SYMBOL RESOLUTION FLAGS (mutually exclusive):
 
RTLD_LAZY
  - Resolve symbols when they're actually used (lazy binding)
  - Faster library loading
  - Errors discovered when undefined symbol is called
  - Default behavior on many systems
 
RTLD_NOW
  - Resolve ALL symbols immediately when library loads
  - Slower loading, but catches missing symbols early
  - Required for RTLD_GLOBAL to work reliably
  - Use when: security-critical, need all-or-nothing loading
 
─────────────────────────────────────────────────────────────────────────────────
 
SYMBOL VISIBILITY FLAGS (can be combined with above):
 
RTLD_GLOBAL
  - Loaded library's symbols available for subsequently loaded libraries
  - Affects library dependency resolution
  - Example: Load A with RTLD_GLOBAL, then load B that uses A's symbols
 
RTLD_LOCAL (default)
  - Symbols in loaded library only visible to that library
  - Does NOT affect other dlopen calls
  - More encapsulated, prevents symbol pollution
 
─────────────────────────────────────────────────────────────────────────────────
 
SPECIAL FLAGS:
 
RTLD_NOLOAD
  - Don't actually load the library
  - Just check if it's already loaded
  - Returns existing handle or NULL
 
RTLD_NODELETE
  - Library won't be unloaded even when refcount hits 0
  - Use for libraries with complex cleanup issues
 
RTLD_DEEPBIND
  - Library prefers its own symbols over global ones
  - Prevents symbol interposition for this library
 
─────────────────────────────────────────────────────────────────────────────────
 
USAGE EXAMPLES:
 
// Plugin that loads quickly, tolerates lazy errors
void *h = dlopen("plugin.so", RTLD_LAZY);
 
// Critical library, verify all symbols exist
void *h = dlopen("security.so", RTLD_NOW);
 
// Library whose symbols other plugins need
void *h = dlopen("framework.so", RTLD_NOW | RTLD_GLOBAL);

Symbol Resolution at Runtime

The dlsym() function looks up symbols by name. This is fundamentally different from compile-time linking where the linker resolves symbols:

At compile time: The linker records that your code needs symbol "foo" and produces relocation entries. The dynamic linker resolves "foo" to an address at load time.

At runtime: Your code executes dlsym(handle, "foo") and gets back a pointer you can call. No compiler/linker involvement—pure runtime resolution.

dlsym Symbol Search Rules

Diagram

DLSYM SYMBOL SEARCH BEHAVIOR
═══════════════════════════════════════════════════════════════════════════════
 
dlsym(handle, "symbol_name")
 
The behavior depends on the 'handle' value:
 
─────────────────────────────────────────────────────────────────────────────────
handle = dlopen() return value
─────────────────────────────────────────────────────────────────────────────────
  
  Search: Only in the specified library and its dependencies
  
  Example:
    void *h = dlopen("libA.so", RTLD_LAZY);  // libA depends on libB
    dlsym(h, "funcA")  → Found in libA ✓
    dlsym(h, "funcB")  → Found in libB (dependency of A) ✓
    dlsym(h, "funcC")  → NOT found (libC not a dependency) ✗
 
─────────────────────────────────────────────────────────────────────────────────
handle = RTLD_DEFAULT
─────────────────────────────────────────────────────────────────────────────────
 
  Search: All loaded objects, in load order (like normal symbol resolution)
  
  Example:
    dlsym(RTLD_DEFAULT, "printf")  → Found in libc ✓
    dlsym(RTLD_DEFAULT, "main")    → Found in main executable ✓
  
  Use case: Find a symbol without knowing which library provides it
 
─────────────────────────────────────────────────────────────────────────────────
handle = RTLD_NEXT
─────────────────────────────────────────────────────────────────────────────────
 
  Search: Start searching AFTER the current library
  
  Use case: Function interposition (wrapping functions)
  
  Example (in an interposition library):
  
    // Wrap malloc to add logging
    void *malloc(size_t size) {
        // Get the "real" malloc from the next library in search order
        static void *(*real_malloc)(size_t) = NULL;
        if (!real_malloc) {
            real_malloc = dlsym(RTLD_NEXT, "malloc");
        }
        
        printf("malloc called with size %zu\n", size);
        return real_malloc(size);  // Call the real malloc
    }

Symbol Versioning

Libraries often have multiple versions of the same function for backward compatibility (e.g., glibc's symbol versioning). dlsym() returns the default version, but dlvsym() can request a specific version: dlvsym(handle, "symbol", "VERSION_1.0"). This enables fine-grained control over which implementation you get.

Designing Plugin Architectures

Dynamic loading enables plugin architectures—systems where functionality can be added, removed, or updated without modifying or recompiling the main application. Well-designed plugins are the foundation of extensible software.

Common plugin architecture patterns:

Plugin Design Patterns

•Registry pattern — The host application defines a registry where plugins register themselves. Each plugin calls a registration function at load time.
•Factory pattern — Plugins export factory functions that create objects implementing host-defined interfaces. The host never sees concrete plugin classes.
•Hook pattern — The host defines extensibility points (hooks). Plugins register callbacks for hooks they want to handle.
•Service pattern — Plugins implement services with well-defined interfaces. The host queries plugins for services they provide.

Plugin Architecture Example
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/* ══════════════════════════════════════════════════════════════════
 * HOST APPLICATION - plugin_host.h
 * Defines the plugin interface that all plugins must implement
 * ══════════════════════════════════════════════════════════════════ */
 
#ifndef PLUGIN_HOST_H
#define PLUGIN_HOST_H
 
#define PLUGIN_API_VERSION 1
 
// Plugin metadata structure - every plugin must provide this
typedef struct {
    int api_version;        // Must match PLUGIN_API_VERSION
    const char *name;       // Human-readable plugin name
    const char *version;    // Plugin version string
    const char *author;     // Plugin author
} plugin_info_t;
 
// Plugin interface - functions every plugin must implement
typedef struct {
    // Required: Get plugin information
    plugin_info_t* (*get_info)(void);
    
    // Required: Initialize the plugin
    int (*init)(void);
    
    // Required: Cleanup and shutdown
    void (*cleanup)(void);
    
    // Optional: Process data (or whatever the host needs)
    int (*process)(const void *input, void *output);
} plugin_interface_t;
 
// The function name every plugin must export
#define PLUGIN_ENTRY_SYMBOL "plugin_entry"
 
// Plugin entry function type - returns the interface
typedef plugin_interface_t* (*plugin_entry_fn)(void);
 
#endif /* PLUGIN_HOST_H */
 
/* ══════════════════════════════════════════════════════════════════
 * HOST APPLICATION - plugin_loader.c
 * Loads and manages plugins
 * ══════════════════════════════════════════════════════════════════ */
 
#include <stdio.h>
#include <dirent.h>
#include <dlfcn.h>
#include "plugin_host.h"
 
#define MAX_PLUGINS 100
 
typedef struct {
    void *handle;
    plugin_interface_t *interface;
} loaded_plugin_t;
 
loaded_plugin_t plugins[MAX_PLUGINS];
int plugin_count = 0;
 
int load_plugin(const char *path) {
    // 1. Load the shared library
    void *handle = dlopen(path, RTLD_NOW);
    if (!handle) {
        fprintf(stderr, "Failed to load %s: %s\n", path, dlerror());
        return -1;
    }
    
    // 2. Find the entry point
    plugin_entry_fn entry = dlsym(handle, PLUGIN_ENTRY_SYMBOL);
    if (!entry) {
        fprintf(stderr, "No entry symbol in %s\n", path);
        dlclose(handle);
        return -1;
    }
    
    // 3. Get the plugin interface
    plugin_interface_t *iface = entry();
    if (!iface) {
        fprintf(stderr, "Plugin entry returned NULL\n");
        dlclose(handle);
        return -1;
    }
    
    // 4. Verify API version compatibility
    plugin_info_t *info = iface->get_info();
    if (info->api_version != PLUGIN_API_VERSION) {
        fprintf(stderr, "Plugin API version mismatch\n");
        dlclose(handle);
        return -1;
    }
    
    // 5. Initialize the plugin
    if (iface->init() != 0) {
        fprintf(stderr, "Plugin init failed\n");
        dlclose(handle);
        return -1;
    }
    
    // 6. Store the plugin
    plugins[plugin_count].handle = handle;
    plugins[plugin_count].interface = iface;
    plugin_count++;
    
    printf("Loaded plugin: %s v%s by %s\n", 
           info->name, info->version, info->author);
    
    return 0;
}

Library Constructors and Destructors

Dynamically loaded libraries can execute code automatically when loaded or unloaded. These are called constructors and destructors (not to be confused with C++ class constructors/destructors).

Constructors: Run when the library is loaded (dlopen or program startup) Destructors: Run when the library is unloaded (dlclose or program exit)

Library Constructors and Destructors
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/* Library with constructor and destructor */
 
#include <stdio.h>
 
// GCC/Clang constructor attribute
__attribute__((constructor))
void library_init(void) {
    printf("Library loaded! Performing initialization...\n");
    // Initialize resources, register handlers, etc.
}
 
// Multiple constructors with priorities (lower = earlier)
__attribute__((constructor(101)))
void early_init(void) {
    printf("Early initialization (priority 101)\n");
}
 
__attribute__((constructor(102)))
void late_init(void) {
    printf("Late initialization (priority 102)\n");
}
 
// GCC/Clang destructor attribute
__attribute__((destructor))
void library_cleanup(void) {
    printf("Library unloading! Cleaning up...\n");
    // Free resources, unregister handlers, etc.
}
 
/* Execution order:
 * 
 * On dlopen("libexample.so") or program start:
 *   1. early_init()  (priority 101)
 *   2. late_init()   (priority 102)
 *   3. library_init() (default priority 65535)
 * 
 * On dlclose() or program exit:
 *   1. library_cleanup()
 *   2. late_init (destructor with priority 102)
 *   3. early_init (destructor with priority 101)
 *   (reversed order from constructors)
 */
 
/* Alternative: .init_array and .fini_array sections
 * 
 * The compiler generates these from constructor/destructor attributes,
 * but you can also add pointers directly:
 */
 
void my_init_func(void) { printf("Init via .init_array\n"); }
void my_fini_func(void) { printf("Fini via .fini_array\n"); }
 
// Place pointers in the appropriate sections
__attribute__((section(".init_array")))
static void (*init_ptr)(void) = my_init_func;
 
__attribute__((section(".fini_array")))
static void (*fini_ptr)(void) = my_fini_func;

Constructor Pitfalls

Library constructors run before main() and before the program can handle errors. Keep constructor code minimal and robust. Avoid:

Allocating large resources (may not be needed if library unused)
Calling functions that may not be initialized yet
Any operation that can block or fail hard
Dependencies on other libraries that may not be loaded yet

Real-World Applications of Dynamic Loading

Dynamic loading is everywhere in modern software. Understanding these use cases helps you recognize when dynamic loading is the right solution.

Real-World Dynamic Loading Applications

•Browser plugins and extensions — Chrome/Firefox load extensions as separate modules. Each extension is isolated and can be enabled/disabled without browser restart.
•Database drivers — Applications connect to multiple databases (MySQL, PostgreSQL, SQLite) by loading the appropriate driver at runtime. A config file specifies which driver to use.
•Game engines — Engine cores are small; levels, assets, and game logic load dynamically. Modding support uses dynamic loading for user-created content.
•Image/media codecs — Photo editors load codec plugins for each format. Only codecs for images you actually open are loaded.
•IDE plugins — VSCode, IntelliJ, Eclipse—all use dynamic loading for extensions. Thousands of plugins available, users install only what they need.
•Audio processing (VST/AU) — Digital audio workstations load instrument and effect plugins dynamically. Musicians mix plugins from different vendors.
•Scientific computing — MATLAB, Python (NumPy/SciPy) load optimized math libraries (MKL, OpenBLAS) at runtime based on hardware detection.
•Hardware abstraction — Graphics drivers (Direct3D, Vulkan, OpenGL implementations) are loaded based on available hardware.

Case study: Python's extension modules

Python uses dynamic loading extensively. When you import numpy, Python:

Searches for numpy in sys.path
Finds numpy/init.py and associated .so files
Calls dlopen on numpy.core._multiarray_umath.cpython-39-x86_64-linux-gnu.so
Calls dlsym to find PyInit__multiarray_umath
Calls the init function to register the module

This is why C extensions can crash your Python interpreter—they run native code loaded via dlopen.

When to Use Dynamic Loading
Use Dynamic Loading When	Use Static/Startup Linking When
Functionality is optional/rarely used	Functionality is always needed
Third-party plugins/extensions expected	Closed, self-contained application
Memory efficiency is critical	Startup time is critical (no runtime loading)
Hot-swapping code at runtime needed	Reliability/stability paramount
Many possible implementations (drivers)	Single, known implementation

Pitfalls and Best Practices

Dynamic loading is powerful but fraught with potential issues. These best practices help you avoid common problems:

Common Pitfalls

•ABI incompatibility — If the host and plugin are compiled with different compilers or flags, their ABIs may differ. C++ is especially dangerous (name mangling, vtable layout). Use a C ABI for plugin interfaces.
•Memory ownership confusion — If a plugin allocates memory and the host frees it (or vice versa), crashes result because they may use different allocators. Establish clear ownership conventions.
•Unloading with active references — Calling dlclose on a library while code is still using its functions or data causes crashes. Ensure all references are released before unloading.
•Global state conflicts — Two plugins may have identically-named globals that collide. Use static (file-local) globals and proper namespacing.
•Thread safety — dlopen/dlclose are not thread-safe in all implementations. Serialize loading operations or use thread-safe alternatives.
•Security: loading untrusted code — Loading a malicious .so gives it full process privileges. Validate plugin sources, use code signing, sandbox if possible.

Best Practices

•Use a C ABI for plugin interfaces — Avoid C++ classes, templates, or exceptions across the plugin boundary. Expose a pure C interface.
•Version your plugin API — Include a version number plugins must check. Reject incompatible plugins gracefully.
•Define clear allocation ownership — If the host provides memory, the host frees it. If the plugin allocates, provide a plugin-side free function.
•Use RTLD_NOW during development — Catch missing symbols immediately rather than at first call.
•Handle dlopen failures gracefully — Always check return values. Provide meaningful error messages using dlerror().
•Consider RTLD_NODELETE for debugging — Prevent unloading to keep debug symbols available in stack traces.
•Test on all target platforms — Dynamic loading behavior varies between Linux, macOS, and Windows. Test portability.

Robust Plugin Loading Pattern
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/* Robust plugin loading with proper error handling */
 
#include <dlfcn.h>
#include <stdio.h>
#include <stdlib.h>
 
typedef struct plugin_handle {
    void *dl_handle;
    void *interface;
    int ref_count;
} plugin_handle_t;
 
plugin_handle_t* load_plugin_robust(const char *path) {
    // Clear any previous errors
    dlerror();
    
    // Load with RTLD_NOW to catch all errors immediately
    void *dl_handle = dlopen(path, RTLD_NOW | RTLD_LOCAL);
    if (!dl_handle) {
        char *err = dlerror();
        fprintf(stderr, "dlopen failed for '%s': %s\n", 
                path, err ? err : "unknown error");
        return NULL;
    }
    
    // Look for entry point
    dlerror();  // Clear errors
    void *(*get_interface)(void) = dlsym(dl_handle, "get_interface");
    char *err = dlerror();
    
    if (err) {
        fprintf(stderr, "dlsym failed: %s\n", err);
        dlclose(dl_handle);
        return NULL;
    }
    
    // Get the interface
    void *interface = get_interface();
    if (!interface) {
        fprintf(stderr, "Plugin returned NULL interface\n");
        dlclose(dl_handle);
        return NULL;
    }
    
    // Verify version (assuming interface has version field)
    // ... version check code ...
    
    // Create our handle wrapper
    plugin_handle_t *handle = malloc(sizeof(plugin_handle_t));
    if (!handle) {
        dlclose(dl_handle);
        return NULL;
    }
    
    handle->dl_handle = dl_handle;
    handle->interface = interface;
    handle->ref_count = 1;
    
    return handle;
}
 
void release_plugin(plugin_handle_t *handle) {
    if (!handle) return;
    
    handle->ref_count--;
    if (handle->ref_count == 0) {
        // Call cleanup if defined
        void (*cleanup)(void) = dlsym(handle->dl_handle, "cleanup");
        if (cleanup) cleanup();
        
        dlclose(handle->dl_handle);
        free(handle);
    }
}

Summary: Dynamic Loading

We've explored dynamic loading—the ability to load code at runtime, on demand. Let's consolidate the key concepts:

Key Takeaways

•Dynamic loading differs from dynamic linking — Linking specifies libraries at compile time; loading occurs whenever the program chooses at runtime.
•dlopen/LoadLibrary load libraries into the process — They map code/data, process relocations, run constructors, and return a handle.
•dlsym/GetProcAddress resolve symbols by name at runtime — No compiler involvement—pure runtime lookup by string name.
•Plugin architectures leverage dynamic loading — Define interfaces in headers, load implementations at runtime for extensibility.
•Library constructors and destructors run automatically — Use for initialization and cleanup, but keep them lightweight.
•Many real-world systems use dynamic loading — Browsers, databases, games, IDEs, and more rely on runtime code loading.
•Best practices prevent common pitfalls — Use C ABI, version APIs, handle errors, manage memory ownership carefully.

Module Complete:

Congratulations! You've completed the Address Binding module. You now understand the complete journey of address resolution—from compile-time binding's rigid simplicity, through load-time binding's multiprogramming enablement, to execution-time binding's virtual memory foundation, and finally to dynamic loading's runtime flexibility.

These concepts underpin memory management, security (ASLR, DEP), shared libraries, plugin systems, and operating system architecture. Whether you're debugging a loader error, designing a plugin system, or optimizing memory usage, this knowledge forms the foundation of your understanding.

Module Complete

You now understand the complete spectrum of address binding—from source code to execution, from absolute addresses to dynamic loading. You can design plugin architectures, debug loading issues, and appreciate the sophisticated machinery that makes modern software possible. This knowledge is fundamental to systems programming, operating system development, and secure software design.