copysign undefined

I want to use the function (double) copysign from math.h but I get the following warning:
warning C4013: 'copysign' undefined; assuming extern returning int
I have used #include <math.h> so I do not understand how this function can not be defined. Furthermore, in Tools-> Options -> Projects and Solutions -> VC++ Directories I have $(VCInstallDir)include in my Include files and $(VCInstallDir)lib in my Library files. What else do I need to do?

There is no function named copysign. There is, however, a function named _copysign (not the underscore).

ok, I clearly didn't pay attention. Thanx.