1  #include <math.h>
2  
tanh(double x)3  double tanh(double x) {
4      int sign = 0;
5      if (x < 0) {
6          sign = 1;
7          x = -x;
8      }
9      x = expm1(-2 * x);
10      x = x / (x + 2);
11      return sign ? x : -x;
12  }
13