//My static library (compiled to .a in OSX) is just the following:

#include<stdio.h>

void HelloWorld()
{
    printf("Hello World!\n");
}

//My program (which I link to the above library) is this:

#include <stdio.h>
#include <stdlib.h>

extern void HelloWorld();

int main()
{
    HelloWorld();
    return 0;
}