QT application crashes when QWebFrame's evaluatingJavascript method called

Question

I am using a QWebFrame to visualize some data and I use evalueateJavascript method to update data on Javascript. Here is my function to do this.

QWebFrame * webFrame;
void setValue(int idx, double value){

   webPage->page()->mainFrame()->evaluateJavaScript(QString("setDataValue(%1,%2)").arg(QString::number(idx)).arg(QString::number(value)));   
}

In QT application I can call this function via a button call back as many time without causing an error.

I want to call this setValue function from a separate thread to visualize incoming data. When I call setValue function from a separate thread, after few or first iteration application crash. I tried with both QThread and boost threads, but results are same.

void dummyTest(){
  for(int i = 0; i < 1000; i++)
    setValue(0,rand() % 150);
}

This dummyTest function is also working without problems when called via a button callback, but crash on running on a separate thread.

Here is the code for thread initialization

 void startSerialProcessing() {
     boost::thread_attributes attr;
     attr.set_stack_size(1024);

     std::cout << "dummy processor started. \n";
     serialThread= new boost::thread(&MavLinkAL::dummyTest, this);
 }

My observation is this crash only happens when setValue is called from a separate thread. Here is important lines from coredump file viewed from gdb.

Program terminated with signal SIGSEGV, Segmentation fault.

#0  0x00007f3f0d6f361f in WTF::StringImpl::~StringImpl() ()
   from /usr/lib/x86_64-linux-gnu/libQtWebKit.so.4
#1  0x00007f3f0d6583f8 in JSC::JSValue::toStringSlowCase(JSC::ExecState*) const
    () from /usr/lib/x86_64-linux-gnu/libQtWebKit.so.4
#2  0x00007f3f0d68a396 in ?? () from /usr/lib/x86_64-linux-gnu/libQtWebKit.so.4
#3  0x00007f3f0d55a9f1 in ?? () from /usr/lib/x86_64-linux-gnu/libQtWebKit.so.4
#4  0x00007f3f0d56313f in ?? () from /usr/lib/x86_64-linux-gnu/libQtWebKit.so.4

Any help to solve this problem is really appreciated. Thanks.

Georgy Pashkov · Accepted Answer · 2016-02-15 08:59:30Z

2

QWebFrame functions are neither reentrant nor thread safe. They can only be invoked from main GUI thread. Qt allows you to deliver a message to another thread quite easily. Make setValue function a slot, and connect it to a signal emitted from the processing thread.

EDIT: As Vladimir Bershov suggested in the comment below, one can also use QMetaObject::invokeMethod() with Qt::QueuedConnection to achieve the same result.

edited Feb 15, 2016 at 8:59

answered Feb 14, 2016 at 19:17

Georgy Pashkov

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2 Comments

Vladimir Bershov Over a year ago

He can also use QMetaObject::invokeMethod() with Qt::QueuedConnection connection type

Georgy Pashkov Over a year ago

@Dinesh, could you mark this as answer in that case? ;)

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