Angle between 3 points?

Given points ABC, how could I find angle ABC? I'm making a feehand tool for a vector drawing application and to minimize the number of points it generates, I wont add a points unless the angle of the mouse position and the last 2 points is greater than a certain threshold. Thanks

what I had:

int CGlEngineFunctions::GetAngleABC( POINTFLOAT a, POINTFLOAT b, POINTFLOAT c )
{
    POINTFLOAT ab;
    POINTFLOAT ac;

    ab.x = b.x - a.x;
    ab.y = b.y - a.y;

    ac.x = b.x - c.x;
    ac.y = b.y - c.y;

    float dotabac = (ab.x * ab.y + ac.x * ac.y);
    float lenab = sqrt(ab.x * ab.x + ab.y * ab.y);
    float lenac = sqrt(ac.x * ac.x + ac.y * ac.y);

    float dacos = dotabac / lenab / lenac;

    float rslt = acos(dacos);
    float rs = (rslt * 180) / 3.141592;
     RoundNumber(rs);
     return (int)rs;


}