al_set_haptic_gain - Allegro 5 API

       Sets  the  gain of the haptic device if supported.  Gain is much like volume for sound, it is as if every
       effect’s intensity is multiplied by it.  Gain is a value between  0.0  and  1.0.   Returns  true  if  set
       successfully,  false  if not.  Only works if al_get_haptic_capabilities(3alleg5) returns a value that has
       ALLEGRO_HAPTIC_GAIN(3alleg5)ALLEGRO_HAPTIC_CONSTANTS(3alleg5) set.  If not, this function returns  false,
       and all effects will be played without any gain influence.

       al_set_haptic_gain - Allegro 5 API

       5.1.8

              [UnstableAPI]:  Perhaps  could be simplified due to limited support for all the exposed features
              across all of the platforms.  Awaiting feedback from users.

Allegro reference manual                                                             al_set_haptic_gain(3alleg5)

              #include <allegro5/allegro.h>

              bool al_set_haptic_gain(ALLEGRO_HAPTIC *hap, double gain)