Unexpected behaviour in slot with Qt::DirectConnection

loladiro

Are you yourself moving the plugin to a different thread or is automatically done by Qt?

goetz

This sounds really weird. It should work the way you designed it. Actually it's pretty just the same as the Qt Designer plugins work. As you speak of DLLs - there's one thing: check that your plugin and application uses the very same compiler settings as the Qt libs (mainly threaded/nonthreaded stdlib, debug/release settings). Windows is known for being very picky about this.

Gourmand

bq. Are you yourself moving the plugin to a different thread or is automatically done by Qt?

It was made automatically by Qt. I did nothing for that. Just downloaded plugin using

@QPluginLoader PluginLoader;
PluginLoader.setFileName( dir.absoluteFilePath( abspath ) );
QObject* plugin = qobject_cast<QObject*>( PluginLoader.instance() );@

and later called it's start method:

@ .....
interface* plugin_interface = qobject_cast<interface*>( plugin );
if( plugin_interface )
{
SEGINTERFACES ifr = plugin_interface->start( this );
.....@

bq. check that your plugin and application uses the very same compiler setting

I'm pretty sure they use all same settings. There is nothing about settings. In application console I see ASSERT error message about different threads.

Exactly Qt::BlockingQueuedConnection works well.

goetz

If the sender of your signal really is in another plugin, then you must use queued connections (automatic detections does the trick too). Direct connections will crash the application (but you know that already). You must not call GUI functions (including creating and/or showing a dialog) outside the main thread of a Qt application.

Gourmand

Now I see that. But this must be noticed in plugin docs. Exactly following: "Loaded plugin runs in different to main application thread, That means GUI functions from main application cannot be used from plugin not directly, nor from signal/slot pair with Qt::DirectConnection."

Now there is nothing about plugin threading in it's doc.

goetz

No it must not.

A plugin does not necessarily live in a different thread. Actually most of them do not. I don't know why your's is, but that usually is a deliberate decision made by the plugin developer or the developer using the plugin.

Gourmand

I AM of those plugins developer and main application developer... I made them all "by default" as recommended in Qt 4.7 docs and Max's Shlee book. By default plugin starts in different thread. If it should not - then there is Qt bug. If it should start in different thread by default - then I must be warned in docs about it. Now there is a hole about threading in plugins doc. Nothing.

Uwe Kindler

A Qt plugin is simply a shared library with a root component object like stated here:

"http://doc.qt.nokia.com/4.7/qpluginloader.html":http://doc.qt.nokia.com/4.7/qpluginloader.html

So by default a Qt plugin does not live in another thread and also the QObjects that are created in the plugin do not live in another thread. That means connectinng signals and slots from Qt plugins with signals and slots from your application via direct connection is no problem and should work fine.

Gourmand

May be should. But does not. They run by default in separate threads. All plugins (now I created 3). Probably this is a bug in Qt.

I must tell - one of my plugins is exactly multi threaded. It has QThread object which runs inside this plugin to perform specific tasks. But it starts not always. All other code is single threaded. All settings are default.

goetz

Sorry, you are the only one reporting this. And with plugins working with a charm so very often, I seriously doubt that this is a bug in Qt.

And as you are introducing threads in at least one of the plugins, it is very likely that this causes the problems. Creating a well designed multithreaded application can be very tricky, just as debugging it.

I suspect some flow of control problem to cause the troubles, eg. by some object/method of a plugin which is meant to work single threaded being called from the second thread of another plugin, probably without you knowing or being intended by you.

giesbert

Hi Gourmand,

If you really think, the problem is Qt located, you could post your code via some share hoster or gitorious (the best in simplified form). Then some people could have a look, whether it is really a Qt bug. But I don't think so. I already created plug-ins and it never was a problem.

so the interesting point would be, where do you use QPluginLoader ?

Gourmand

Exactly second thread in one of plugins calls another but not directly, via signal/slot, default connection. I expected this will not force other plugins start in separate threads. Docs say nothing about this. But I've got multithreaded plugins. Neverthless... I can work with multithreading too. I only want all these aspects be documented.

Gourmand

bq. where do you use QPluginLoader ?

In MainWindow constructor. I will not provide all the code, it is proprietary, LGPLed.

giesbert

So, if you can't provide a (simplified, reduced) example, where it happened, it's up to you to find the reason of the problem, sorry. I didn't want to read you full source code, the best is to reduce it to a minimum.

Gourmand

Sorry... But I have no much free time now to create branch with all significant code. Just imagine how it would be:

main application loads plugin which exports slots in special structure
main application loads another plugin which exports signals in special structure
main application connects signals from 2nd plugin to slots in 1st, but one of senders is QThread object in 2nd plugin
main application shows it's window and enters to it's main loop, it has a button which starts some method from 2nd plugin
this method in 2nd plugin starts 2nd thread which sends signals - they are accepted by first plugin

This all requires some time to be created from scratch clearly and accurately. But I must do main job.

I think problem appeared by the following reason: one of loaded plugins IS multithreaded. PluginLoader does not know how it will later communicate between threads. It just turns on multithreading. I can check this - first plugin loaded is NOT multithreading for all plugins. I can look at how it communicates with main application. I expect it will work in single thread model. But after 2nd plugin loaded - then multithreading will be turned on. I'll try do that later, at free time.

If I'm right - this is not exactly a bug. This is just undocumented behavior.

tobias.hunger

We use lots of plugins in Qt Creator (actually everything but the plugin loader is a a plugin there) and so far never had any issues with connections between plugins. All plugins are in the same thread, too.

Gourmand

Probably you ever did not following:

one of plugins is multithreaded
in second thread it communicates to another plugin in some manner

tobias.hunger

We do use threading in Qt Creator quite heavily in places. We do also use Direct Connections - even in threads.

Maybe we are just doing it right;-)

Seriously: The plugin loader does not "turn on multithreading". It is just a nice wrapper around dlopen which loads a piece of code into the address space of your application. It does not care what that code does at all.

Gourmand

bq. Maybe we are just doing it right

May be. Then just write to documentation how it must be done right. Or Qt still sometimes can work unexpectedly ;-) to it's creators...

I already shown pieces of code which do plugin loading. What is wrong there?
The thread is just like this:

@DThread::DThread(MainWindow *parent) : QThread(parent)@

It was developed long time ago and works fine.

@We do also use Direct Connections – even in threads.@

Direct Connections in threads work fine for me except with QtGui - as signed in docs.

tobias.hunger

I am not a lawyer, but the LGPL is not considered to be a proprietary license. You need to pass the your source code on to your customers according to the LGPL. Of course we here are not your customers, so there is no obligation to show it to us:-)

Actually the LGPL does not really make much sense for applications at all I think. You might want to contact a lawyer before sending out your product to your customers.