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Newsgroups: comp.lang.ada
Date: Tue, 28 Jan 2020 10:53:53 -0800 (PST)
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Subject: Re: GNATCOLL-Mmap example
From: Bob Goddard <1963bib@googlemail.com>
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Date: 2020-01-28T10:53:53-08:00
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On Tuesday, 28 January 2020 08:25:25 UTC, Dmitry A. Kazakov  wrote:
> On 2020-01-25 22:52, Bob Goddard wrote:
> 
> > I need to map the RPi's GPIO memory addresses so that I can do what I need to do.
> 
> See files under:
> 
>     /sys/class/gpio
> 
> You can use standard Ada I/O to read and set GPIO pins. E.g.
> 
>     /sys/class/gpio/gpio/XXX/value

This was the way I started, but could not get it to read anything. It just blocks trying to read a single character. My c code works fine using the wiringPi libs.

Stephen Leake:
Mmap is used to read more than just disk files. Opening /dev/mem, or even /dev/gpiomem, is just another file.

Regardless, I got it to work. The moral of the story, is never assume example code in the library source works, or even can compile. I got it running a couple of days ago, and have since spent time banging my head against a wall.

This works, both with System.Mmap & GNATCOLL.Mmap...

with Ada.Unchecked_Conversion;
with Ada.Text_IO;
with GNATCOLL.Mmap; use GNATCOLL.Mmap;
with Ada.Strings.Unbounded;
with Ada.Strings.Unbounded.Text_IO;

procedure Mmaptest is
   package Mmap renames GNATCOLL.Mmap;

   File : Mmap.Mapped_File;
   Reg  : Mmap.Mapped_Region;
   Str  : Mmap.Str_Access;
   Offs : Mmap.File_Size := 0;
   Page : constant Mmap.File_Size := Mmap.File_Size (Mmap.Get_Page_Size);
begin
   File := Mmap.Open_Write ("/tmp/file_on_disk");
   
   while Offs < Mmap.Length (File) loop
      Mmap.Read (File, Reg, Offs, Length => Page, Mutable => False);
      Str := Mmap.Data (Reg);
      Ada.Strings.Unbounded.Text_IO.Put (Ada.Strings.Unbounded.To_Unbounded_String (
                                              Str (Integer (Offs - Mmap.Offset (Reg)) + 1 .. Mmap.Last (Reg)))
                                             );
      Offs := Offs + Page; --  Mmap.File_Size (Mmap.Last (Reg));
   end loop;
   
   Mmap.Free (Reg);
   
   if Reg /= Mmap.Invalid_Mapped_Region then
      Ada.Text_IO.Put_Line ("Mapped region still valid");
   end if;
   
   Mmap.Close (File);
   
   if File /= Mmap.Invalid_Mapped_File then
      Ada.Text_IO.Put_Line ("Mapped file still valid");
   end if;
end Mmaptest;