QSqlQuery issue

fahmouch

Hi All,

I am developing a qt application using a SQLite database. In fact, I collect data periodically and I store them in the database.

The code compiles fine but after a few minutes, the application craches. I tried to debug the application using the gdb tool. Here is what the debugger appears:

__memmove_ssse3_back () at ../sysdeps/x86_64/multiarch/memcpy-ssse3-back.S:1547
1547 ../sysdeps/x86_64/multiarch/memcpy-ssse3-back.S: Aucun fichier ou dossier de ce type.
(gdb) where
#0 __memmove_ssse3_back () at ../sysdeps/x86_64/multiarch/memcpy-ssse3-back.S:1547
#1 0x0000000000dbd399 in QListData::remove(int) ()
#2 0x00000000004b1f11 in QList<QSQLiteResult*>::removeOne(QSQLiteResult* const&) ()
#3 0x00000000004b0116 in QSQLiteResult::~QSQLiteResult() ()
#4 0x00000000004b0189 in QSQLiteResult::~QSQLiteResult() ()
#5 0x000000000049f4ae in QSqlQueryPrivate::~QSqlQueryPrivate() ()
#6 0x000000000049f7df in QSqlQuery::~QSqlQuery() ()
#7 0x0000000000545a20 in QSqlQueryModelPrivate::~QSqlQueryModelPrivate() ()
#8 0x0000000000541ff8 in QSqlTableModelPrivate::~QSqlTableModelPrivate() ()
#9 0x0000000000f6e7a3 in QObject::~QObject() ()
#10 0x0000000000439516 in WritingData::updateNodeStatus(QString, int) ()
#11 0x0000000000414a53 in CollectScheduler::updateNodeStatus(QString, int) ()
#12 0x000000000041635b in CollectScheduler::setWorkerPing(QString, bool) ()
#13 0x0000000000453dd4 in CollectScheduler::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) ()
#14 0x0000000000f67be6 in QObject::event(QEvent*) ()
#15 0x000000000056442c in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
#16 0x00000000005692e6 in QApplication::notify(QObject*, QEvent*) ()
#17 0x0000000000f451e0 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) ()
#18 0x0000000000f8525c in QEventDispatcherUNIX::processEvents(QFlagsQEventLoop::ProcessEventsFlag) ()
#19 0x0000000000f403d1 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag >) ()
#20 0x0000000000d54568 in QThread::run() ()
#21 0x0000000000d598ef in QThreadPrivate::start(void*) ()
#22 0x00007ffff6174182 in start_thread (arg=0x7ffff3140700) at pthread_create.c:312
#23 0x00007ffff568147d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111**

How can i fix this bug?
Any help would be appreciated.

mcosta

Hi and welcome to devnet,

very hard to help without the code.
BTW, the debug info says that the error is inside WritingData::updateNodeStatus

fahmouch

@mcosta

Thanks for your reply, here the funtion WritingData::updateNodeStatus

bool WritingData::updateNodeStatus(QString node_Id , int node_status)
{
bool submit=false;
if(writingDb.isOpen())
{
QSqlTableModel model(NULL, writingDb);
model.setTable("nodes");
model.setFilter(QString("node_id = '%1'").arg(node_Id));
model.select();
if (model.rowCount() == 1) {
QSqlRecord record = model.record(0);
record.setValue("node_id", node_Id);
record.setValue("node_status_id", QString::number(node_status));
model.setRecord(0, record);
submit=model.submitAll();
}
}
return submit;
}

This function is called by another thread for updating the last pool column of a specified node (filtered by its id). Moreover, this function uses a writing connexion (called here "writingDb").

mcosta

Hi,

the error happens in the QSqlModel destructor. (you should run in debug mode to try to understand why it crashes)

QUESTION: are you sure your code does what you want? each time you overwrite the first record without storing all data model.setRecord(0, record)

fahmouch

@mcosta
Hi,

I have tried to use another method to update the database. Here the implementation of the new function: (this function updates the status of one node (network node), that's why i overwrite each time the first record)

bool WritingData::updateNodeStatus(QString node_Id , int node_status)
{
    bool submit=false;
    if(writingDb.isOpen())
    {
        QSqlQuery query(writingDb);
        submit=query.exec(QString("UPDATE nodes set node_status_id='%1' where   node_id='%2'").arg(QString::number(node_status)).arg(node_Id));
    }
    return submit;
}

There still the same problem. I think the problem is caused by the function
QListData::remove(int)

Note : I have this problem only on Linux Os (ubuntu 14.04). It doesn't crash on windows OS.

Have you any idea?

Edited: Please use ``` (3 back-ticks) for code blocks - p3c0

mcosta

@fahmouch said:

writingDb

I think is a QSqlDatabase instance. Is it used across threads??
According to this you should use it only in the thread that created it

fahmouch

@mcosta

Hi,

I read in the offical documentation this phrase : The driver is locked for updates while a select is executed. This may cause problems when using QSqlTableModel because Qt's item views fetch data as needed (with QSqlQuery::fetchMore() in the case of QSqlTableModel).

In fact, i tested my application by disabling the reading component (thread) and i haven't any problem. That's why, i think that i have this problem.

There is any way to solve this problem?

mcosta

Hi,

the solution is to use the connection only inside the GUI thread. Or, if you need to work with multiple threads, you have to use more connections.

fahmouch

@mcosta

I'm already using two connections (readingDB and writingDB) and each connection has a specific name.

mcosta

mmmm,

the problem is that SQLite (the DB not the Qt driver) cannot handle multiple connections in the right way (is a file access problem).
DO you need to open the database in ReadWrite mode in both threads (do you need to modify through the view)??
If the view is read-only you can try to specify QSQLITE_OPEN_READONLY for the UI thread connection

fahmouch

@mcosta

Thanks for your reply,

I have already tried this solution, but unfortantly, it doesn't resolve the problem (after a few minutes, the application crashes on Linux).