[Solved]QSqlQuery::prepare segmentation fault.

[qxoz]

Hi everyone!
I know that i'm doing something wrong, but in last 3 hours i can't figure out what exactly.
While run method:
@
int uUserTask::GetId(QString key)
{
int retId = -1;
QSqlQuery query;
query.prepare(QString("select id from table where identifer='%1'").arg(key));
QMutexLocker mxlock(&mutex);
if(query.exec())
{
if(query.next())
{
retId = query.value(0).toInt();
}
}
return retId;
}@

i got segmentation fault in qsql_mysql.cpp at body of prepare method(line 900):
@
r=mysql_stmt_prepare(d->stmt, encQuery.constData(), encQuery.length());
@

Segmentation fault happens not always, and i compared query strings in fault case with other case, they are identical. I think i'm using threads in a wrong way, but how?

tools: QtSDK 1.2.1 Qt4.8.1
Os: Kubuntu 12.04
Db: MySQL 5.5.22

Thank you!

[luca]

I usually use "prepare" in conjunction with "bindValue".

In your code why don't you use directly query.exec("....") ?

[luca]

...and why do you use mutex? Is you application multi-thread?

[Fahmy]

Try this instead:

QSqlQuery query;
query.exec(QString("select id from table where identifer='%1'").arg(key));

If you still want to use prepare(). Try like this:

QSqlQuery query;
query.prepare("select id from table where identifer = ?");
query.addBindValue(key, QSql::Out);

[luca]

In a multi-thread application I call the following code fore every thread:
@
db = new QSqlDatabase();
m_nomeDb = QString::number(qrand());
*db = QSqlDatabase::cloneDatabase(QSqlDatabase::database(), m_nomeDb);
db->open();
@

and for every query in the same thread:
@
QSqlQuery query(*db);
query.exec("select .... bla bla bla ...");
...
query.next();
...
...
@

[lgeyer]

"Threads and the SQL module":http://qt-project.org/doc/qt-4.8/threads-modules.html#threads-and-the-sql-module

[qxoz]

Thank you for replies!!!
As shown by Lukas Geyer "A connection can only be used from within the thread that created it. ..."
So i think solution of Luca prefered :).

But now, i changed code like
@query.exec(QString(“select id from table where identifer=’%1’”).arg(key));@
and not get segmentation fault for now.

Thank you again!

[qxoz]

Luca what about memory leak
@db = new QSqlDatabase();@
when an object will deleted?

Question 2:
In this case(QSqlDatabase for every thread) Mutex are not needed?

[luca]

[quote author="qxoz" date="1353576031"]Luca what about memory leak
@db = new QSqlDatabase();@
when an object will deleted?
[/quote]

I usually put that code in the parent object contructor and this:
@
db->close();
delete db;
QSqlDatabase::removeDatabase(m_nomeDb);
@
in the destruct.

[quote author="qxoz" date="1353576031"]
Question 2:
In this case(QSqlDatabase for every thread) Mutex are not needed?
[/quote]

If you have two threads using two DB connections you don't need mutex to execute queries.

[luca]

If you want to do as I wrote, you also need to make a DB connection in your main :
@
QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
db.setHostName(ip);
db.setDatabaseName(db_name);
db.setUserName(user);
db.setPassword(pwd);

db.open();

@

this way you create a main DB connection and you can use it in the main thread. Next you can "clone" the connections in your threads.

[qxoz]

I changed my code as you said, and it works ok :). Thanks!

[luca]

You are welcome...