How to debug a QString causing a bus error? - c++

Question

I have a Raspberry Pi 3B+ with my Qt code on it (Qt 5.12.5). When I run my code, it randomly crashes with a Bus Error after a few hours. I am not sure how to work out the exact cause. I cross compile on Ubuntu for the RPi using the latest Raspberry Pi OS (2020-05-27).
The core dump (I have replaced some irrelevant parts with ...)
pi#raspberrypi: $ gdb TEST core
GNU gdb (Raspbian 8.2.1-2) 8.2.1
...
Type "apropos word" to search for commands related to "word"...
Reading symbols from TEST...done.
[New LWP 1233]
...
[New LWP 1226]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/arm-linux-gnueabihf/libthread_db.so.1".
Core was generated by `./TEST'.
Program terminated with signal SIGBUS, Bus error.
#0 0x755b5b7c in QString::arg(long long, int, int, QChar) const () from /usr/local/qt5pi/lib/libQt5Core.so.5
[Current thread is 1 (Thread 0x6feff440 (LWP 1233))]
(gdb) bt full
#0 0x755b5b7c in QString::arg(long long, int, int, QChar) const () at /usr/local/qt5pi/lib/libQt5Core.so.5
#1 0x0001c0bc in QString::arg(int, int, int, QChar) const (this=0x6fefdc78, a=12, fieldWidth=0, base=10, fillChar=...) at ../raspi/qt5pi/include/QtCore/qstring.h:976
#2 0x00039ff8 in StageState::getStateString() (this=0x6fefdf00) at ../TEST/stage.h:35
...
#7 0x00142124 in TEST::timerExpired() (this=0x7ecd4708) at ../TEST/TEST.cpp:51
__PRETTY_FUNCTION__ = "void TEST::timerExpired()"
locker = {val = 2127384345}
#8 0x0015c188 in TEST::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (_o=0x7ecd4708, _c=QMetaObject::InvokeMetaMethod, _id=2, _a=0x6fefe9d8) at moc_TEST.cpp:122
_t = 0x7ecd4708
#9 0x75722b08 in QMetaObject::activate(QObject*, int, int, void**) () at /usr/local/qt5pi/lib/libQt5Core.so.5
#10 0x75730c1c in QTimer::timeout(QTimer::QPrivateSignal) () at /usr/local/qt5pi/lib/libQt5Core.so.5
#11 0x75730fc8 in QTimer::timerEvent(QTimerEvent*) () at /usr/local/qt5pi/lib/libQt5Core.so.5
#12 0x75724194 in QObject::event(QEvent*) () at /usr/local/qt5pi/lib/libQt5Core.so.5
#13 0x768c6b88 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /usr/local/qt5pi/lib/libQt5Widgets.so.5
#14 0x768ce29c in QApplication::notify(QObject*, QEvent*) () at /usr/local/qt5pi/lib/libQt5Widgets.so.5
#15 0x759b62ec in QCoreApplication::self () at /usr/local/qt5pi/lib/libQt5Core.so.5
(gdb)
../TEST/stage.h:35 refers to this code:
return QString("text %1").arg(aInt);
Possible causes and fixes I am considering:
To me, from the stack trace, it looks like the crashes are occurring inside QString, possibly due to a Qt bug? However I am not sure. I can try a newer version of Qt.
I am also using a QTimer in a thread, maybe this could cause a issue, accessing a object from other threads? I am already using QMutex. I could instead only use the gui thread short term and test if crashes still occur.
Searching online, I found a comment to look a the dmesg output (below), maybe my MicroSD card is dying? I am waiting for a new one to arrive.
Anything else?
pi#raspberrypi:~ $ dmesg
[ 0.000000] Booting Linux on physical CPU 0x0
[ 0.000000] Linux version 5.4.44-v7+ (dom#buildbot) (gcc version 4.9.3 (crosstool-NG crosstool-ng-1.22.0-88-g8460611)) #1320 SMP Wed Jun 3 16:07:06 BST 2020
[ 0.000000] CPU: ARMv7 Processor [410fd034] revision 4 (ARMv7), cr=10c5383d
...
[ 17.898294] Bluetooth: BNEP filters: protocol multicast
[ 17.898313] Bluetooth: BNEP socket layer initialized
[16092.809350] Alignment trap: not handling instruction e1903f9f at [<755b5b78>]
[16092.809367] 8<--- cut here ---
[16092.815191] Unhandled fault: alignment exception (0x001) at 0x6f577277
[16092.820865] pgd = 6aa8fcbe
[16092.826468] [6f577277] *pgd=3278c835, *pte=2812175f, *ppte=28121c7f
Here is another bus error that is similar but not quite the same as previously:
pi#raspberrypi:~/TEST/bin $ gdb TEST core
GNU gdb (Raspbian 8.2.1-2) 8.2.1
...
Type "apropos word" to search for commands related to "word"...
Reading symbols from TEST...done.
[New LWP 767]
...
[New LWP 763]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/arm-linux-gnueabihf/libthread_db.so.1".
Core was generated by `./TEST'.
Program terminated with signal SIGBUS, Bus error.
#0 0x7568d470 in QString::fromAscii_helper(char const*, int) () from /usr/local/qt5pi/lib/libQt5Core.so.5
[Current thread is 1 (Thread 0x709bf440 (LWP 767))]
(gdb) bt
#0 0x7568d470 in QString::fromAscii_helper(char const*, int) () at /usr/local/qt5pi/lib/libQt5Core.so.5
#1 0x75830da4 in () at /usr/local/qt5pi/lib/libQt5Core.so.5
#2 0x0001c030 in QString::QString(char const*) (this=0x709be71c, ch=0x15fa88 "[0-9| ]{3}") at ../raspi/qt5pi/include/QtCore/qstring.h:700
#3 0x00029da8 in Measurement::doesRececivedDataFormatMatchRegex(QString) (this=0x709be7f4, receivedData=...) at ../TEST/Measurement.h:103
...
#12 0x7580c58c in QSocketNotifier::activated(int, QSocketNotifier::QPrivateSignal) () at /usr/local/qt5pi/lib/libQt5Core.so.5
#13 0x7580c90c in QSocketNotifier::event(QEvent*) () at /usr/local/qt5pi/lib/libQt5Core.so.5
#14 0x769a2b88 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /usr/local/qt5pi/lib/libQt5Widgets.so.5
#15 0x769aa29c in QApplication::notify(QObject*, QEvent*) () at /usr/local/qt5pi/lib/libQt5Widgets.so.5
#16 0x75a922ec in QCoreApplication::self () at /usr/local/qt5pi/lib/libQt5Core.so.5
Edit: added getStateString() and doesRececivedDataFormatMatchRegex() functions
QString getStateString() {
switch (state) {
case StageState::AEnum:
return QString("text %1").arg(aInt);
...
default:
return QString("Unknown");
}
}
bool Measurement::doesRececivedDataFormatMatchRegex(QString receivedData)
{
QRegExp regExp("[0-9| ]{3}");
return receivedData.indexOf(regExp) != -1;
}