isfinite —
test for finite value
Standard C Library (libc, -lc)
#include <math.h>
int
  
  isfinite(real-floating
    x);
The isfinite() determines whether its argument
  x has a finite value. An argument represented in a
  format wider than its semantic type is converted to its semantic type first.
  The determination is then based on the type of the argument.
It is determined whether the value of x is zero,
  subnormal, or normal, and neither infinite nor NaN.
It is determined whether the value of x is true zero or
  finite, and neither dirty zero nor ROP.
The isfinite() macro returns a non-zero value if the
  value of x is finite. Otherwise 0 is returned.
The isfinite() macro conforms to
  ISO/IEC 9899:1999 (“ISO C99”).