I want to get angles between two line. So I used this code.

int posX = (ScreenWidth) >> 1;

int posY = (ScreenHeight) >> 1;

double radians, degrees;

radians = atan2f( y - posY , x - posX);

degrees = -CC_RADIANS_TO_DEGREES(radians);

NSLog(@"%f %f",degrees,radians);

But it doesn't work . The Log is that: 146.309935 -2.553590

What's the matter? I can't know the reason. Please help me.

If you simply use

radians = atan2f( y - posY , x - posX);

you'll get the angle with the horizontal line y=posY (blue angle).

You'll need to add M_PI_2 to your radians value to get the correct result.