Re: Graphics in forked subprocess

[Pavel Hofman]

Hi Mike,
> No, the issue is that your script is using 'fork' without 'exec'. You
> mention 'exec', but it doesn't appear in the above.
>
> It's a well known problem that "fork without exec" does not work with
> large libraries and frameworks linked with a program, including Python,
> Qt, probably Java. In fact, it's considered deprecated on macOS now to
> 'fork' a process and not 'exec' a brand new executable image.
>
> Calling 'fork' by itself for creating worker child processes should only
> be used carefully in small programs that don't link with large
> frameworks, such as Octave.
>
> Can you rework your scripts so that 'fork' is followed immediately by
> 'exec' to start a new separate Octave process for example?
>
>      ctrl_pid = fork ();
>      if (ctrl_pid == 0)
>        exec ('octave', 'testGui.m');
>      else
>        waitpid (ctrl_pid);
>      endif

Thanks a lot for your help. I did use exec. But exec creates two child 
processes:  A) octave which runs B) 
/usr/lib/x86_64-linux-gnu/octave/4.2.2/exec/x86_64-pc-linux-gnu/octave-gui. 
B) runs the actual script while PID of A) is returned by exec(). I need 
to have a way to stop the child process from my control script. I wanted 
to stop the child process by sending kill(pid) - for that I need pid of 
B) - sending kill to A) does not stop B). I have to be able to check B) 
has finished/died to restart if needed

That is why I skipped the exec call and sourced octave code directly 
after forking.

Please what is the best practice for this kind of control? I know I can 
use a side channel, but I want to be able to kill the child reliably 
(kill -15 ?) even if it  gets stuck.

I could write the control script in shell (bash) but I would love to 
write a single code for linux and windows, if actually possible. I see 
Octave for windows uses vbs scripts for startup... perhaps I will have 
to resort to that too, as the last resort...

Thanks a lot,

Pavel.