[Qt-interest] bash jobcontrol breaks QT4 and inotify ?

[David Heremans]

On 08/13/2010 07:29 PM, Andreas Pakulat wrote:
> On 13.08.10 15:09:41, David Heremans wrote:
>> Hello,
>>
>> I was trying to find a nice way to have my program react upon new files
>> that could pop-up in a given /var/spool directory. Since it runs on a
>> linux box  I thought about using inotify, and a quick Google got me to
>> this page:
>>
>> http://wahjava.wordpress.com/2007/05/09/my-experiment-with-inotify-and-qt4/
>>
>> I downloaded the small example inotify.tgz and it worked as a charm so I
>> decided to try to integrate the inotify.hh and inotify.cc into my
>> program, but it doesn't seem to work correctly, so I had to look at the
>> example again and (by accident) I found the following strange behavior.
>>
>> If I launch the ./inotify_demo (from an xterm/konsole) and start
>> monitoring a directory it works nicely. But if a press CTRL-Z and bring
>> it back to the foreground (with the fg command) it simply stops reacting
>> to changes? Even pressing the stop/start button doesn't reactivate the
>> program.
>>
>> Can somebody here explain me the impact of the CTRL-Z on the inotify
>> mechanism? Could it be that running my program in the debugger doesn't
>> work because of some unknown-to-me impact from signals (on  the threads?)
>>
>> I'm running kubuntu lucid lynx (64 bit)
>> with qtcreator 1.3.1, based on qt 4.6.2(64 bit)
> 
> Why are you using inotify at all instead of using Qt's existing api to
> watch files/dirs: QFileSystemWatcher? It uses inotify on Linux and
> whatever the OS provides on other platforms. And it still works after
> Ctrl+Z; fg.
> 
> Andreas
> 


Hello,

I'm still puzzled by this program going hay-wire after a 'Ctrl+Z;fg'
I found out the following extra info. I monitored the program with htop
so that I could se the threads and when I launch the program I get
7577 david      20 0 249M 21456 1584 S 0.0 0.4 0:00.10 ./inotify_demo
7578 david      20 0 249M 21456 1584 S 0.0 0.4 0:00.00 ./inotify_demo
7579 david      20 0 249M 21456 1584 S 0.0 0.4 0:00.00 ./inotify_demo

which should be the main process and two threads
If I press CTRL-Z
the stae of all 3 goes from S(leeping) to T(Traced/stopped)

However after giving the fg command
7577 david      20 0 249M 21456 1584 S 0.0 0.4 0:00.18 ./inotify_demo
7578 david      20 0 249M 21456 1584 S 0.0 0.4 0:00.00 ./inotify_demo

It seems that the last thread has vanished!

When I try to performs strace(pressing s in htop) upon those 3 pids
created initialy the first is the one sending output to the console, and
I see read poll events passing by, and everything keeps working. The
second pid doesn't produce any output but still everything works. Howver
when trying to start an strace on the 3th one the output I get this

	close(14)
	close(15)
	madvise(0x7f844d627000, 8368128, MADV_DONTNEED) = 0
	_exit(0)

And afterwards the program doesn't read the changes in the directory anymore

So it seems that trying to trace/debug the program will also cause it to
break...

Can anybody tell me wat is going on here? Or provide some pointers as
where to go from here on so that I understand what is going on..

Thanks in advance

David H