C implementation of http://stackoverflow.com/a/565282/202451

gistfile1.m

BOOL AGLineIntersection(AGLine l1, AGLine l2, AGPoint *out_pointOfIntersection)
{
    // http://stackoverflow.com/a/565282/202451

    AGPoint p = l1.start;
    AGPoint q = l2.start;
    AGPoint r = AGPointSubtract(l1.end, l1.start);
    AGPoint s = AGPointSubtract(l2.end, l2.start);
    
    double s_r_crossProduct = AGPointCrossProduct(r, s);
    double t = AGPointCrossProduct(AGPointSubtract(q, p), s) / s_r_crossProduct;
    double u = AGPointCrossProduct(AGPointSubtract(q, p), r) / s_r_crossProduct;

    
    if(t < 0 || t > 1.0 || u < 0 || u > 1.0)
    {
        if(out_pointOfIntersection != NULL)
        {
            *out_pointOfIntersection = AGPointZero;
        }
        return NO;
    }
    else
    {
        if(out_pointOfIntersection != NULL)
        {
            AGPoint i = AGPointAdd(p, AGPointMultiply(r, t));
            *out_pointOfIntersection = i;
        }
        return YES;
    }
}