qt application crash.

[Anas_Deshmukh]

hi everyone,
i am getting below error, dont know how to figure out what exactly goin wrong.

[xcb] Unknown sequence number while processing queue
[xcb] Most likely this is a multi-threaded client and XInitThreads has not been called
[xcb] Aborting, sorry about that.
sps_v1: ../../src/xcb_io.c:274: poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.
The program has unexpectedly finished.

thanx in advance.

P.S. edit : also getting
QPixmap: It is not safe to use pixmaps outside the GUI thread

[mrjj]

Hi
you should provide more details about OS version/platform
and Qt version,
when error happens.

Also try it on other computer.

[kshegunov]

I suspect you're using the GUI classes from different threads. Please provide code to confirm or deny this.

[Anas_Deshmukh]

@mrjj @kshegunov Thanks for reply.

result of command : lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 14.04.2 LTS
Release: 14.04
Codename: trusty

Qt version : 4.8.6

@kshegunov : i cant provide code, its big application.

I am having scanner class which continuously running in thread and detecting scanner available or not and emit the signal when scanning event happen. further i used this signal to various slot as per need.

I create one more class to monitor all USB devices including scanner which are connected.
error occurred when specifically scanner not connected.

I am very thankful if you suggest me correct approach to monitor all usb connection status. so that any thread wont get affected.

[Anas_Deshmukh]

meanwhile tried with flag Qt::AA_X11InitThreads. but nothing works
for now i add below lines in .pro file

QMAKE_CXXFLAGS += -std=c++0x -pthread
LIBS += -pthread

and add libraires in main.cpp and simply called XInitThreads();
but after calling function XInitThreads().
my application crash when i tried to call qmessagebox from some function.

#include <thread>
#include <X11/Xlib.h>

Q_DECL_EXPORT int main(int argc, char *argv[])
{
    QScopedPointer<QApplication> app(createApplication(argc, argv));

    XInitThreads();  // if coment  -  `!xcb_xlib_threads_sequence_lost' failed. appeared.


//    app->setAttribute(Qt::AA_X11InitThreads, true);  // setting flag has no effect so far

    QmlApplicationViewer viewer;
...
...

}

// test.cpp

someFunction() {
...
    QMessageBox *msgBox = new QMessageBox();
    int retValue=msgBox->exec(); // crash here.
...
}

[Anas_Deshmukh]

Hi everyone,

I read about Qt::AA_X11InitThreads and it clearly mention as "Calls XInitThreads() as part of the QApplication construction in order to make Xlib calls thread-safe. This attribute must be set before QApplication is constructed."

now I removed these flags from .pro file

QMAKE_CXXFLAGS += -std=c++0x -pthread
LIBS += -pthread

and updated main.cpp file

Q_DECL_EXPORT int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::AA_X11InitThreads);

    QScopedPointer<QApplication> app(createApplication(argc, argv));

//    XInitThreads();
//    app->setAttribute(Qt::AA_X11InitThreads, true);

    QmlApplicationViewer viewer;

...
}

Problem is fixed so far. thanks.