Two implementations to extend a line.

extend_line.c

// worse for precision
void extend_dividing_line(vec2 pt1, vec2 pt2, float len)
{
    vec2 unit_v;
    vec2_sub(pt1,pt2,unit_v);
    vec2_normalize(unit_v, unit_v);
    vec2_mul(unit_v, len, unit_v);
    vec2_add(pt1, unit_v, pt1);
    vec2_sub(pt2, unit_v, pt2);    
}

// better for precision
void extend_dividing_line_by_units(vec2 pt1, vec2 pt2, float units)
{
    float centroid[2] = {(pt2[0]-pt1[0]) * 0.5f + pt1[0],
                         (pt2[1]-pt1[1]) * 0.5f + pt1[1]};
    
    float lx = pt1[0] - pt2[0];
    float ly = pt1[1] - pt2[1];
    float p_length = sqrtf(lx*lx + ly*ly);

    float s1[2] = {pt1[0] + (pt1[0] - centroid[0]) / p_length * units,
                   pt1[1] + (pt1[1] - centroid[1]) / p_length * units};
    float s2[2] = {pt2[0] + (pt2[0] - centroid[0]) / p_length * units,
                   pt2[1] + (pt2[1] - centroid[1]) / p_length * units};

    memcpy(pt1, s1, sizeof(float)*2);
    memcpy(pt2, s2, sizeof(float)*2);
}