C/C++ NaN constant (literal)?

In C, NAN is declared in <math.h>.

In C++, std::numeric_limits<double>::quiet_NaN() is declared in <limits>.

But for checking whether a value is NaN, you can’t compare it with another NaN value. Instead use isnan() from <math.h> in C, or std::isnan() from <cmath> in C++.