If you want to use signalling NaNs, you must indicate this to the compiler by defining the macro _WANT_SNAN in your application. This macro must be defined before you include any standard C headers. If your application is comprised of two or more translation units, either all or none of them must define _WANT_SNAN. That is, the definition must be consistent for any given application.

You must also use the relevant command-line option when you compile your source code. This is associated with the predefined macro, __SUPPORT_SNAN__.