[: More must be said here about the complex functions' ranges.]

#include <math.h>
float tanhf(float a);
double tanh(double a);
long double tanhl(long double a);

#include <complex.h>
float complex ctanhf(float complex a);
double complex ctanh(double complex a);
long double complex ctanhl(long double complex a);

#include <tgmath.h>
floating-type tanh(floating-type a);

The tanh functions take a hyperbolic angle a and return the hyperbolic tangent of that angle, tanh a, in the range [−1, +1].

#include <math.h>
float atanhf(float r);
double atanh(double r);
long double atanhl(long double r);

#include <complex.h>
float complex catanhf(float complex r);
double complex catanh(double complex r);
long double complex catanhl(long double complex r);

#include <tgmath.h>
floating-type atanh(floating-type r);

The atanh functions take a ratio r and return the area hyperbolic tangent of that ratio, tanh⁻¹ r.