fpclassify —
classify real floating type
Standard C Library (libc, -lc)
#include <math.h>
int
  
  fpclassify(real-floating
    x);
The fpclassify() macro performs classification of its
  argument x. An argument represented in a format wider
  than its semantic type is converted to its semantic type first. The
  classification is then based on the type of the argument.
The fpclassify() macro returns the value of the number
  classification macro appropriate to its argument x as
  described above.
The fpclassify() macro conforms to
  ISO/IEC 9899:1999 (“ISO C99”).