headers/foo.h:

#ifndef foo_h__
#define foo_h__
 
extern void foo(void);
 
#endif  // foo_h__

utils/foo.c:

#include <stdio.h>

void foo(void)
{
    puts("Hello, I am a shared library.");
}

main.c:

#include <stdio.h>
#include "headers/foo.h"
 
int main(void)
{
    puts("This is a shared library test...");
    foo();
    return 0;
}

Step 1: Compiling with Position Independent Code

Inside the utils directory we need to compile our library source code into position-independent code (PIC):

gcc -c -Wall -Werror -fpic foo.c

Step 2: Creating a shared library from an object file

Now we need to actually turn this object file into a shared library. We will call it libfoo.so:

gcc -shared -o libfoo.so foo.o

Step 3: Making the library available at runtime

sudo mv libfoo.so /usr/lib/

Now the file is in a standard location, we need to tell the loader it is available for use, so let us update the cache:

sudo ldconfig

That should create a link to our shared library and update the cache so it is available for immediate use. Let us double check:

ldconfig -p | grep foo

Link our executable. Notice we do not need the -l option since our library is stored in a default location:

gcc -Wall -o test main.c -lfoo

Let us make sure we are using the /usr/lib instance of our library using ldd:

ldd test | grep foo
libfoo.so => /usr/lib/libfoo.so (0x00a42000)

Good, now let us run it:

./test
This is a shared library test...
Hello, I am a shared library.

That about wraps it up. We have covered how to build a shared library.