Segfault taking element from QList - c++

Question

I get a SEGFAULT at the first line in the if-block, takeFirst().
if (!sendQueue.isEmpty()) {
QString nxtcmd = sendQueue.takeFirst();
port->write(nxtcmd.toLatin1());
port->flush();
}
I have code pushing 96 command strings into the queue with
if (completionHandlerQueue.empty()) {
…
} else {
sendQueue.append(cmd_str);
}
The latter happens in my main GUI code. The first bit is called by a signal by QExtSerialPort (when data is there to be read).
sendQueue is a member and declared as such:
class SB::ModuleCommunicator : public QObject
{
…
private:
…
QStringList sendQueue;
Now I'm not using threads and I assumed the Qt Event Loop would make everything work smoothly, but the full backtrace does tell me that there are in fact 3 threads.
According to documentation, Qt Containers are only thread-safe for read only access.
But the Queue is already full with all 96 strings when it crashes. So I don't think anything is appending while the list is being modified.
Is this a thread issue? How can I find out?
Here is the stacktrace:
0 _int_malloc malloc.c 3530 0x7ffff61c7410
1 __GI___libc_malloc malloc.c 2924 0x7ffff61c9f95
2 QListData::detach(int) /usr/lib/x86_64-linux-gnu/libQtCore.so.4 0 0x7ffff6cd397b
3 QList<QString>::detach_helper qlist.h 709 0x412169
4 QList<QString>::detach_helper qlist.h 725 0x411cf2
5 QList<QString>::detach qlist.h 139 0x411e72
6 QList<QString>::begin qlist.h 267 0x423610
7 QList<QString>::first qlist.h 282 0x422dda
8 QList<QString>::takeFirst qlist.h 490 0x422885
9 SB::ModuleCommunicator::handleMsg ModuleCommunicator.cpp 116 0x41f9be
10 SB::ModuleCommunicator::onReadyRead ModuleCommunicator.cpp 393 0x421f4d
11 SB::ModuleCommunicator::qt_static_metacall moc_ModuleCommunicator.cpp 52 0x435e1e
12 QMetaObject::activate(QObject*, QMetaObject const*, int, void**) /usr/lib/x86_64-linux-gnu/libQtCore.so.4 0 0x7ffff6dc9281
13 QextSerialPortPrivate::_q_canRead qextserialport.cpp 313 0x40c505
14 QextSerialPort::qt_static_metacall moc_qextserialport.cpp 97 0x40dbfa
15 QMetaObject::activate(QObject*, QMetaObject const*, int, void**) /usr/lib/x86_64-linux-gnu/libQtCore.so.4 0 0x7ffff6dc9281
16 QSocketNotifier::activated(int) /usr/lib/x86_64-linux-gnu/libQtCore.so.4 0 0x7ffff6e162fe
17 QSocketNotifier::event(QEvent*) /usr/lib/x86_64-linux-gnu/libQtCore.so.4 0 0x7ffff6dd260b
18 QApplicationPrivate::notify_helper(QObject*, QEvent*) /usr/lib/x86_64-linux-gnu/libQtGui.so.4 0 0x7ffff72d7894
19 QApplication::notify(QObject*, QEvent*) /usr/lib/x86_64-linux-gnu/libQtGui.so.4 0 0x7ffff72dc713
20 QCoreApplication::notifyInternal(QObject*, QEvent*) /usr/lib/x86_64-linux-gnu/libQtCore.so.4 0 0x7ffff6db4e9c
... <More>
UDPATE:
So it seems I have a transient error here. Now it crashed at
for (int i = 0; i < msgBuffer->size(); i++) {
with msgBuffer being yet another QStringList member of my Class. This piece of code is in the 'onReadyRead()' Slot that gets called when QExtSerialPort has data for me.
So I think the error is related to that package.
I'm using Qt 4.8 on Ubuntu 12.04 with QExtSerialPort v1.2rc from https://code.google.com/p/qextserialport