al_get_haptic_gain - Allegro 5 API

       Returns  the  current gain of the device.  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.   Only  works  correctly  if
       al_get_haptic_capabilities(3alleg5)          returns          a          value          that          has
       ALLEGRO_HAPTIC_GAIN(3alleg5)ALLEGRO_HAPTIC_CONSTANTS(3alleg5) set.  If this is  not  set,  this  function
       will simply return 1.0 and all effects will be played without any gain influence.

       al_get_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_get_haptic_gain(3alleg5)

              #include <allegro5/allegro.h>

              double al_get_haptic_gain(ALLEGRO_HAPTIC *hap)