Retrieving the result of a 'cat' command into a Lazarus app
lazarus / 32-Linux

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MartynA · 2017-12-09 13:34

Hi,

I'm using Rasbian on an RPi 3.

If I execute this

cat /sys/class/gpio/gpio22/value

in Terminal, it correctly reports the state of a push-to-make switch
across port#22 and Gnd (with pull-up resistor) as 1 or 0.  The 1 or
zero is displayed in Terminal on the next line, of course.

If I do this in my app:

  S := 'cat /sys/class/gpio/gpio22/value >';
  Res := fpsystem(S);

, Res always returns zero, which I guess is telling me that the
command executed without error, rather than what the value on port#22 
was.  So, My question is, how do I read the 1 or 0 from my Lazarus
app? 

Btw, in my app, I want to be able to poll the input from the port many
times per second, so I doubt that sending the output of cat to a disk
file would be fast enough.

TIA, Martyn

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Thaddy · 2017-12-09 15:32

/sys/class/gpio/gpio22/value  does not exist on my Raspberry Pi 3's
all of them run stretch.

But something along this line should work:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i program
catweasel;{$mode objfpc}{$H+}uses sysutils, classes;var
s:TStringlist;begin  s:= TStringlist.Create;  try    if
FileExists('/sys/class/gpio/gpio22/value') then    begin     
s.LoadFromFile('/sys/class/gpio/gpio22/value');      writeln(s.text); 
  end else      writeln('File does not exist');  finally    s.free; 
end;end.

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MartynA · 2017-12-09 15:44

Not quite sure what you mean by "does not exist".  Physically, port 22
is pin 15 on my RPi3.

And, if I redirect the output of the cat command I quoted to a file,
it works fine.  Did you do the necessary set-up:  exporting the port
and setting its direction to "In"?

Cheers, Martyn

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Thaddy · 2017-12-09 15:46

Post crossed. Did you check my example code?

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MartynA · 2017-12-09 16:22

Yes, it works thanks, which would save me the trouble of redirecting
it elsewhere.

But the purpose of my q was to see  if there is a way of doing it
without accessing the file system.

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Thaddy · 2017-12-09 16:28

In linux "everything is a file". It will be fast enough.
Then again, look at several of the GPIO options in the wiki.
http://wiki.freepascal.org/Lazarus_on_Raspberry_Pi

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MartynA · 2017-12-09 16:38

Thanks.

>Then again, look at several of the GPIO options in the wiki.

I have been, that's what my other 2 qs here have been about.

>In linux "everything is a file". It will be fast enough.

I'm not sure that the file system will be fast enough.  It wasn't for
the "encapsulated shell calls" example - see my thread
here:https://forum.lazarus.freepascal.org/index.php/topic,39177.0.html

It turned out that the reason I couldn't get the encapsulated shell
calls to work initially was because the system didn't react fast
enough until I introduced some Sleep()s between calls.

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Thaddy · 2017-12-09 16:44

Quote from: MartynA on December 09, 2017, 05:38:44 pmI'm not sure that
the file system will be fast enough.
 It is a *virtual* file system: Linux doesn't access the *disk* but
the kernel space for such device directories. A common misconception
for people coming from windows.

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MartynA · 2017-12-09 16:55

Yes, I already got that thanks.