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handling GUI resource locks between event handler and boost::thread

I have this wxWidgets OnButtonClick event handler, that apparently holds
a lock on all widgets in my form, but this event handler is supposed to
end a thread in the background - while that thread is supposed to update
widget content. So if I want to wait for the thread to end in the event
handler, I have 2 threads waiting on each other - with the one in the
background waiting to access the widget content.

My current workaround is to introduce a boost::mutex, have the
background thread acquire a lock on it upon start, release upon end, and
the OnButtonClick event handler will check (or try to acquire and
instantly release) the lock on that mutex to make sure the previous
background thread has ended before starting a new one.

The according code looks like this:

boost::mutex mutexChecka;
typedef boost::mutex::scoped_lock lockaChecka;

void demonstrateFrameLock()
{
cout << "child thread: acquiring lock" << endl;
lockaChecka lc (mutexChecka);
cout << "child thread: got lock" << endl;

sleep (1);
cout << "child thread: taking a nap (2 seconds)" << endl;
sleep (2);
cout << "child thread: woke up!" << endl;

cout << "child thread: mainWindow->txt1->GetValue = " <<
mainWindow->txt1->GetValue() << endl;

cout << "child thread: unlocking" << endl;
lc.unlock();
cout << "child thread: unlocked" << endl;

cout << "child thread: thread finished" << endl;
}

void debuggingGUImainFrame::OnToggle( wxCommandEvent& event )
{
cout << "main thread: acquiring lock" << endl;
lockaChecka lc (mutexChecka);
cout << "main thread: got lock, unlocking" << endl;
lc.unlock();
cout << "main thread: unlocked" << endl;
if (myThread) {
cout << "main thread: deleting old thread" << endl;
delete myThread;
myThread = 0;
}

myThread = new boost::thread(&demonstrateFrameLock);
cout << "main thread: sleeping 5 seconds" << endl;
sleep (5);
cout << "main thread: done sleeping" << endl;
// myThread.join(); // old code, never got this to work due to the lock
on form controls
cout << "main thread: thread finished" << endl;
}

This gets the job done, but it really doesn't seem very pretty to me -
so being a newbie with thread programming, I would appreciate any
comments on how to improve this code (which is, of course, incomplete
and just the sample that remained after long hours of debugging).

Thank you in advance!

Lars
Mar 11 '08 #1
3 2174
On 11 Mar, 16:01, Lars Uffmann <a...@nurfuerspam.dewrote:
I have this wxWidgets OnButtonClick event handler, that apparently holds
a lock on all widgets in my form, but this event handler is supposed to
<snip>
boost::mutex mutexChecka;
typedef boost::mutex::scoped_lock lockaChecka;
wxWidgets have a support forum at http://wxforum.shadonet.com/ and
boost have user support mailing lists at http://www.boost.org/more/mailing_lists.htm#users

HTH.
Mar 11 '08 #2
bytebro wrote:
wxWidgets have a support forum at http://wxforum.shadonet.com/ and
boost have user support mailing lists at http://www.boost.org/more/mailing_lists.htm#users
HTH.
No, not at all. You told me two things that I do already know, and btw.
there is no newsgroup where boost is more on-topic than this one,
because the boost mailing list - while having a newsgroup portal -
doesn't allow posting via the latter.

I guess I'll add a disclaimer in my signature saying which resource
sites I know and which I do not :) Otherwise yes, your information would
have been partially helpful - but a direct answer was what I was looking
for.

Thanks anyways,

Lars
Mar 12 '08 #3
Hey Paavo,

I'll have to chew a bit on your example, doesn't seem so obvious to me -
I haven't yet learned even nearly all aspects of C++.
Put it in my "re-read" folder :)

Meanwhile, I managed to solve my locking problem with the help of input
from this thread, in the way stated below.

Best Regards & thanks to everyone involved,

Lars

solution relevant code excerpt (missing includes and wxWidgets
initialization):
/* *** */

debuggingGUImainFrame *mainWindow; // wxWidgets main window,
// controls cmd1 (wxButton) and txt1 (wxTextCtrl)

int glbSERVICING = 0; // "keepalive flag" for the background thread

// mutex to ensure alternate access to window controls and glbSERVICING
boost::mutex mutexChecka;

typedef boost::mutex::scoped_lock lockaChecka;

void demonstrateFrameLock()
{
sleep (1);
cout << "child thread: acquiring lock" << endl;
lockaChecka lc (mutexChecka);
cout << "child thread: got lock, child thread: ready for servicing"
<< endl;
mainWindow->cmd1->Enable();

int i = 0;
while (glbSERVICING) {
lc.unlock(); // unlock r/w access to keepalive flag
cout << "child thread: unlocked" << endl;
++i;
sleep (2);
cout << "child thread: servicing... (i = " << i << ")" << endl;
lc.lock(); // prepare read access to keepalive flag
cout << "child thread: got lock" << endl;
}

cout << "child thread: mainWindow->txt1->GetValue = " <<
mainWindow->txt1->GetValue() << endl;
mainWindow->cmd1->Enable();

lc.unlock();
cout << "child thread: unlocked, thread finished" << endl;
}

void debuggingGUImainFrame::OnToggle( wxCommandEvent& event )
{
cmd1->Disable();

cout << "main thread: acquiring lock" << endl;
lockaChecka lc (mutexChecka);
cout << "main thread: got lock" << endl;

if (glbSERVICING) {
glbSERVICING = 0;
mainWindow->cmd1->SetLabel (wxT ("Enable Service"));
}
else {
glbSERVICING = 1;
mainWindow->cmd1->SetLabel (wxT ("Disable Service"));
boost::thread *helperThread;
helperThread = new boost::thread(&demonstrateFrameLock);
delete helperThread;
helperThread = 0;
}

cout << "main thread: unlocking" << endl;
lc.unlock();

cout << "main thread: thread finished" << endl;
}
/* *** */
Mar 13 '08 #4

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