How to get angle between two points?

FAQ - Look up the atan2() function in your C documentation.

If it's the angle between the two vectors from the origin pointing to p1 and p2, respectively, you can get the angle using the atan() function in the cmath library. atan2 would be even better.

Your question is not well expressed. Two points do not have an "angle from one to another".

If you want the the angle between the line defined by these two points and the horizontal axis:
double angle = atan2(y2 - y1, x2 - x1) * 180 / PI;

If you want the angle bewteen the vectors OP1 and OP2 (O being the origin), you should know that the dot product between two vectors u and v is:
u . v = u.x * v.x + u.y * v.y = |u|*|v|*cos(a)
a being the angle between the vectors.
So the angle is given by:
double n1 = sqrt(x1*x1+y1*y1), n2 = sqrt(x2*x2+y2*y2);
double angle = acos((x1*x2+y1*y2)/(n1*n2)) * 180 / PI;

If you want another "angle", well... I can't think of any other!
jods