tanh, tanhf —
hyperbolic tangent function
#include <math.h>
double
  
  tanh(double
    x);
float
  
  tanhf(float
    x);
The tanh() and tanhf() functions
  compute the hyperbolic tangent of x. For a discussion of
  error due to roundoff, see
  math(3).
Upon successful completion, these functions return the hyperbolic tangent value.
  The following may also occur:
  - If x is ± 0, x is
      returned.
- If x is NaN, a NaN is returned.
- If x is positive infinity, a value 1 is returned; if
      x is negative infinity, -1 is returned.
- If x is subnormal, a range error can occur and
      x is returned.
The described functions conform to ISO/IEC 9899:1999
  (“ISO C99”).