STATIC AND SHARED C LIBRARIES WITH GCC

Intro

Creating a shared or a static library might seem like a daunting task at first since finding good documentation is unnecessarily difficult. I thought I would change that by making a simple blog post alongside a git repo.

Required knowledge

I am going to skip over explaining what an object file, a static and a shared library is. If these terms are foreign to you, I frankly do not know how or why you managed to end up here.

The good stuff

First things first, let's create some source files.

// src/life/life.h
#ifndef LIFE_H
#define LIFE_H
int meaningOfLife();
#endif

// src/life/life.c
#include <stdio.h>
#include "life.h"

int meaningOfLife() {
  return 42;
}

// Function that is called when the library is loaded
void __attribute__ ((constructor)) initLibrary(void) {
    printf("Library is initialized\n"); 
}

// Function that is called when the library is closed
void __attribute__ ((destructor)) cleanUpLibrary(void) {
    printf("Library is exited\n"); 
}

// src/main.c
#include <stdio.h>
#include "life/life.h"

int main(int argc, char** argv) {
  printf("The meaning of life is %d\n", meaningOfLife());
  return 0;
}

Now that all code has been written, let's run some scripts:

# Create a static library
mkdir -p bin/static
gcc -c src/life/life.c -o bin/static/life.o
ar rcs bin/static/liblife.a bin/static/life.o

# Create a shared library
mkdir -p bin/shared
gcc -c -fPIC src/life/life.c -o bin/shared/life.o
gcc -shared bin/shared/life.o -o bin/shared/liblife.so

# Create a binary with a static library
gcc src/main.c -Lbin/static -llife -o bin/static-bin

# Create a binary with a shared library
gcc src/main.c -Lbin/shared -llife -o bin/static-bin

Prior to execution of the binary which uses the shared library, you will need to move liblife.so to /usr/lib to make it globally available. You can mitigate this by creating a temporary environment variable which references the location of the shared library:

LD_LIBRARY_PATH=${PWD}/bin/shared bin/use-shared-library