Libraries

Libraries represent a collection of compiled objects in a single file.

Their advantages are: reusability of components (using one shared library takes up less disk space), version management (old and new versions coexist simultaneously on one Linux system), component specialization (developers can focus their core competency in one library).

The types of libraries are: static (object code in a linked library, which becomes part of the application) and dynamic (shared objects, dynamically linked at runtime).

Structure of a library

The figure below shows a sample structure of a library:

08_library.png

Directory Structure

Use the following commands to create the directory structure:

mkdir libexample
mkdir libexample/src
mkdir libexample/src/addlib
mkdir libexample/bin
mkdir libexample/bin/static
mkdir libexample/bin/shared

Reference:Creating a shared and static library with the gnu compiler gcc

Library files

In the folder libexample/src/addlib/, create a file add.c with the following content:

#include <stdio.h>
int gSummand;
void setSummand(int summand) {
	gSummand = summand;
}
int add(int summand) {
	return gSummand + summand;
}
void __attribute__ ((constructor)) initLibrary(void) {
	printf("Library is initialized\n"); 
	gSummand = 0;
}
void __attribute__ ((destructor)) cleanUpLibrary(void) {
	printf("Library is exited\n"); 
}

In the folder libexample/src/addlib/, create a file add.h with the following content:

void setSummand(int summand);
int add(int summand);

In the folder libexample/src/addlib/, create a file answer.c with the following content:

#include "add.h"
int answer() {
	setSummand(20);
	return add(22); // 42 = 20 + 22
}

In the folder libexample/src/addlib/, create a file answer.h with the following content:

int answer();

In the folder libexample/src/, create a file main.c with the following content:

#include <stdio.h>
#include "addlib/add.h"
#include "addlib/answer.h"
int main(int argc, char* argv[])
{
	setSummand(5);
	printf("5 + 7 = %d\n", add(7));
	printf("And the answer is: %d\n", answer());
	return 0;
}