Re: The "black magic" of ioctl

[Simon Wright <simon@pushface.org>]

Francesco Piraneo Giuliano <fpiraneo@gmail.com> writes:

> So, please don't use ioctl -> black magic -> forbidden! :-)
>
> But my application still need to be written and has to run under linux
> so to collect some data about linux' framebuffer I have to use ioctl;
> the only solution is to write all low level interfacing (open the
> device, get informations about, map it into memory) in C then write
> the upper level in Ada?

I've never had to do this. But, for example..

Googling linux ioctl framebuffer led to a piece of code:

   struct fb_var_screeninfo vinfo;
   struct fb_fix_screeninfo finfo;
   ...
   if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
           printf("Error reading fixed information.\n");
           exit(2);
   }

   if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
           printf("Error reading variable information.\n");
           exit(3);
   }

and an Ada way (there are several alternatives) would be to create a
package Framebuffer:

   with Interfaces.C;
   package Framebuffer is

      type Fixed_Screeninfo is record
         --
      end record;
      pragma Convention (C, Fixed_Screeninfo);
      --  probably a good idea to lay out the structure precisely

      type Variable_Screeninfo is record
         --
      end record;
      pragma Convention (C, Variable_Screeninfo);

      Read_Error : exception;

      function Get_Fixed_Screeninfo
        (Fd : Interfaces.C.int) return Fixed_Screeninfo;

      --  etc

   end Framebuffer;

   package body Framebuffer is

      function Get_Fixed_Screeninfo
        (Fd : Interfaces.C.int) return Fixed_Screeninfo
      is
         function ioctl (fildes : Interfaces.C.int;
                         request : Interfaces.C.unsigned_long;
                         finfo : access Fixed_Screeninfo) return Interfaces.C.int;
         pragma Import (C, ioctl, "ioctl");
         FBIOGET_FSCREENINFO : constant Interfaces.C.unsigned_long := ???;
         --  get the appropriate value from the system headers; maybe
         --  write a tiny C program to print the value for you.
         finfo : aliased Fixed_Screeninfo;
         use type Interfaces.C.int;
      begin
         if ioctl (Fd, FBIOGET_FSCREENINFO, finfo'Access) /= 0 then
            raise Read_Error;
         end if;
         return finfo;
      end Get_Fixed_Screeninfo;

      --  etc

   end Framebuffer;