invokeMethod in other thread

VRonin

invokemethod with direct connection is basically the same as a direct call to the method so I'd investigate why it generates a crash

McLion

The invokeMethod now calls it the same as the direct call - To be honest, I have no clue why the call works today - maybe the full rebuild did the trick ... and overnight ... ;-)
However, it still crashes.
Isn't calling this in a different thread the issue? Or I have some other issue - some code segments follow. I'm creating everything in the constructor of the mainwindow:

// start a new thread and put the LoLaWorker to it
  bLoLaIsInitiated = false;
  QThread* lolathread = new QThread;
  LoLaWorker* lolaworker = new LoLaWorker();
  lolaworker->moveToThread(lolathread);
  connect(lolathread, SIGNAL(started()), lolaworker, SLOT(InitLoLaThread()));
  lolathread->start();
... some more connects here

I have #include "LoLaWorkerThread.h" and LoLaWorker *lolaworker; public in header of mainwindow and I am calling

QMetaObject::invokeMethod(lolaworker, "InitLoLaUDPsocket", Qt::DirectConnection, 
                          Q_ARG(QString, ip4addr),
                          Q_ARG(quint16, port),
                          Q_ARG(QString, location),
                          Q_ARG(QString, name),
                          Q_ARG(QString, MAC),
                          Q_ARG(QString, loglevel));

When emitting the signal everything works - just not immediate, which is what I need.

I suspect I'm doing something wrong. Any ideas?
Thanks
McL

kshegunov

@McLion said in invokeMethod in other thread:

QMetaObject::invokeMethod(lolaworker, "InitLoLaUDPsocket", Qt::DirectConnection, ...

This is a race condition if you're calling this from a thread different than lolathread .

If you want something to be called first in a thread post the event to the object immediately after moving it to the thread (i.e. it becomes the first event) and don't use direct connection across threads if you don't provide manual access serialization.

    // ...
    lolaworker->moveToThread(lolathread);
    // What's wrong with this?
    QMetaObject::invokeMethod(lolaworker, "InitLoLaUDPsocket", Qt::QueuedConnection, ... );
    // And ... start the event processing
    lolathread->start();

McLion

@kshegunov
Sorry for the delay ... was called off to work on some other software.

OK, I moved the moveToThread and the start from the constructor to the function where the initialization takes place (when, at runtime, I got the parameters needed), as suggested to try.

This gives me a crash at runtime (signal 11 - sigsegv - invalid memory reference).

VRonin

that's segmentation fault... did you try debugging your code?

McLion

@VRonin
Working on Windows, compiling on/for Linux as well, running on eLinux device with no debugger.

VRonin

Can you compile for windows just for debug? or run a linux virtual machine to debug it?

I'm sorry to be this useless but that error is quite generic. Main things to investigate:

Are you accessing an object through a pointer before creating it (calling new)?
Are you accessing an object through a pointer after it was deleted?
Are you trying to access a null pointer?

McLion

@VRonin
Thanks for trying to help!

As far as I can tell everything is ok.

in constructor of qtgui mainwindow:

QThread* lolathread = new QThread;
LoLaWorker* lolaworker = new LoLaWorker();
connect(lolathread, SIGNAL(started()), lolaworker, SLOT(InitLoLaThread()));
connect(this, SIGNAL(TerminateLoLa()), lolaworker, SLOT(TerminateLoLaUDPsocket()));
.. and some more conects

later, in function LoLaInit:

lolaworker->moveToThread(lolathread);
QMetaObject::invokeMethod(lolaworker, "InitLoLaUDPsocket", Qt::QueuedConnection, Q_ARG(QString, ip4addr), Q_ARG(quint16, port), Q_ARG(QString, location), Q_ARG(QString, name), Q_ARG(QString, MAC), Q_ARG(QString, loglevel));
lolathread->start();

Anyhow, if moveToThread and start the thread are in the constructor and later emit a signal only, everything works, despite the fact that it is not immediate.

I start to believe that what I want is not possible.
I just want to have it to execute this function immediately (switching threads or whatever is needed) and following it up where it has left later (there is a queue of commands that now are worked off first, unfortunately).

VRonin

I just want to have it to execute this function immediately

Then just call the function from the main thread. invokeMethod with direct connection will execute it in the calling thread anyway so I still stuggle to understand why you are doing what you are doing

McLion

@VRonin said in invokeMethod in other thread:

Then just call the function from the main thread. invokeMethod with direct connection will execute it in the calling thread anyway so I still stuggle to understand why you are doing what you are doing

I tried to do this - to no avail. It may well be that it's me that is doing something wrong.

The debug console actually shows that it seems to really directly call the functions in the other thread. However, it also ends up in a strange behavior on the debug console - dbg strings already written out are written out again - and then I again get a seg fault crash.

There are 3 threads: gui-main, serial and lola. I want the following to happen

serial gets parameters for lola-init and emit to gui-main (master cmd-interpreter)
gui-main initializes lola (works with emit, but not immediately)
lola tells gui-main success or not (works with emit, but not immediately)

When I replace the 2 emit with direct calls, I get the crash.
Maybe I'm doing something wrong .. or if what I want is not possible I'll think about a different approach/solution.
Thanks anyway.

kshegunov

There's some confusion here. What does "immediately" mean in this context and why is this required? You can't have queued signal-slot invocation (the default) between threads executing immediately, it's not how the event loop is designed to work; and even with manual sync there's waiting for the synchronization primitive. I suspect you want to provide a blocking queued call (i.e. Qt::BlockingQueuedConnection) to wait for the target slot's end of execution, or perhaps you want a manual thread synchronization, but in any case there's no "fast and easy way", you have to make sure you don't get race conditions whatever you choose as an approach.

As for the segafult, as @VRonin said, track it through the debugger. This is the most common error one can get and while it usually has easily identifiable causes (i.e. buffer overflows, dangling pointers and nullptr dereferencing) there are the cases that the error might be delayed due to stack overflows in release mode with protection dummies off. In any case the cause for it can be anything, debug it!

Also please provide the relevant parts of the code - the signal-slot connections and a clear statement of which object is (living) in which thread when the signal emissions are happening.

McLion

@kshegunov said in invokeMethod in other thread:

There's some confusion here. What does "immediately" mean in this context and why is this required?

I'd like to answer to this first:
Serial is receiving many commands in short sequence. The first is the one to initialize lola, which should be initialized before any of the other commands are executed. When everything is set up with signal-slots, the thread switches needed (gui-main to lola to make the initialization and then back to gui-main with the result of the initialization) take too long and in the meantime many of the other commands received have been executed by gui-main.

Of course, I can put everything in a buffer and keep it there until the initialization has taken place. In that case I'd use the signal-slot connection as it is and as it works perfectly.

However, I was looking for a solution to influence this sequence without the need for an additional buffer only used once for the initialization.
If there's no other way- OK.

btw. The idea of the blocking connection is what I think could do it and I'm somehow trying to do.
It should block gui-main from when I emit to lola to start the initialization until the answer/result is back from lola in gui-main, and then pick up on the next commands.