#include <math.h>
#include <string.h>

typedef struct
{
   double x;
   double y;
} point;


typedef struct
{
   point vertices[3];
   double perimeter;
   float extra;
} triangle;


double distance(point a, point b)
{
   return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));
}


int dylib_demo(int n1, int n2, float copy, triangle* t, 
   double* avgx, double* avgy, char* str)
{
   t->perimeter = 0.0;
   t->perimeter += distance(t->vertices[0], t->vertices[1]);
   t->perimeter += distance(t->vertices[1], t->vertices[2]);
   t->perimeter += distance(t->vertices[2], t->vertices[0]);
   t->extra = copy;
   
   *avgx = (t->vertices[0].x + t->vertices[1].x + t->vertices[2].x) / 3.0;
   *avgy = (t->vertices[0].y + t->vertices[1].y + t->vertices[2].y) / 3.0;

   strcpy(str, "done");

   return n1 + n2;
}

gcc -dynamic -c main.c
libtool -dynamic -o exlib.dylib -lc -macosx_version_min 10.11.1 main.o