al_lock_bitmap_blocked - Allegro 5 API

       Like  al_lock_bitmap(3alleg5),  but allows locking bitmaps with a blocked pixel format (i.e. a format for
       which al_get_pixel_block_width(3alleg5) or al_get_pixel_block_height(3alleg5) do not return  1)  in  that
       format.   To  that  end,  this function also does not allow format conversion.  For bitmap formats with a
       block size of 1, this function is identical  to  calling  al_lock_bitmap(bmp,  al_get_bitmap_format(bmp),
       flags).

              Note:  Currently  there  are  no  drawing  functions  that  work  when the bitmap is locked with a
              compressed format.  al_get_pixel(3alleg5) will also not work.

       al_lock_bitmap_blocked - Allegro 5 API

al_lock_bitmap(3alleg5), al_lock_bitmap_region_blocked(3alleg5)

Allegro reference manual                                                         al_lock_bitmap_blocked(3alleg5)

       5.1.9

              #include <allegro5/allegro.h>

              ALLEGRO_LOCKED_REGION *al_lock_bitmap_blocked(ALLEGRO_BITMAP *bitmap,
                 int flags)