Runtime loading of dynamic libraries in Linux

In Linux, libraries which are linked during linking stage of the program, gets loaded during the startup of the program, unless a copy is already present in memory.

Another approach provided by Linux operating system is to load/unload the libraries at run time whenever it is needed.

API's are provided by Linux for

• Loading the Library -- dlopen()
• Looking up for symbols -- dlsym()
• Handling Errors -- dlerror()
• Unloading the Library -- dlclose()

Header File : #include <dlfcn.h>

Note: While building the program, you need to link 'ldl' library: -ldl

Flow:
1. Process begins with a call to dlopen API, providing the path of the shared library and the mode. Main two values for this mode are RTLD_NOW, RTLD_LAZY. This informs the dynamic linker when to perform relocations. RTLD_NOW will make all the necessary relocations at the dlopen call time, whereas RTLD_LAZY will perform relocations only when they're needed. dlopen function returns a handle which is used in all the subsequent API's
2. You then call dlsym() to get the addresses of the symbols (functions)
3. dlerror() function will returns a human readable error message if an API fails
4. When all the functions are accessed of the library, you can unload the shared library with a call to dlclose()

Example Code:

Let's create a shared library which we want to load at runtime using dlopen, dlsym calls

arithmetic.c:

	#include "arithmetic.h"
	int add(int a, int b)
	{
	return a+b;
	}
	int mul(int a, int b)
	{
	return a*b;
	}
	int divide(int a, int b)
	{
	if (b != 0)
	return a/b;
	return 0;
	}
	int sub(int a, int b)
	{
	return a-b;
	}
	int power(int a, int b)
	{
	int i;
	int result = 1;
	for (i=0; i < b; i++)
	{
	result = result * a;
	}
	return result;
	}

view raw arithmetic.c hosted with ❤ by GitHub

arithmetic.h:

	#ifndef __ARITHMETIC_H
	#define __ARITHMETIC_H
	int add(int a, int b);
	int sub(int a, int b);
	int mul(int a, int b);
	int divide(int a, int b);
	int power(int a, int b);
	#endif

view raw arithmetic.h hosted with ❤ by GitHub

Then write application code for loading this library at run time

main.c:

	#include <stdio.h>
	#include <stdlib.h>
	#include <dlfcn.h>
	#include "arithmetic.h"
	int main()
	{
	void *handle;
	int (*func_power)(int, int);
	char *error;
	handle = dlopen("/home/jamal/personal/gcc/dynamic_linking/libarith.so", RTLD_NOW);
	if (!handle) {
	fprintf(stderr, "%s\n", dlerror());
	exit(EXIT_FAILURE);
	}
	func_power = dlsym(handle, "power");
	if ((error = dlerror()) != NULL) {
	fprintf(stderr, "%s\n", error);
	exit(EXIT_FAILURE);
	}
	printf("Power of 2 and 4 is %d\n", func_power(2, 4));
	dlclose(handle);
	exit(EXIT_SUCCESS);
	}

view raw main.c hosted with ❤ by GitHub

Output: