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  4. [Solved]QSqlQuery::prepare segmentation fault.
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[Solved]QSqlQuery::prepare segmentation fault.

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  • L Offline
    L Offline
    luca
    wrote on last edited by
    #3

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

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    • F Offline
      F Offline
      Fahmy
      wrote on last edited by
      #4

      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);

      Stay hungry, stay foolish.

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      • L Offline
        L Offline
        luca
        wrote on last edited by
        #5

        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();
        ...
        ...
        @

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        • L Offline
          L Offline
          lgeyer
          wrote on last edited by
          #6

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

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          • Q Offline
            Q Offline
            qxoz
            wrote on last edited by
            #7

            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!

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            • Q Offline
              Q Offline
              qxoz
              wrote on last edited by
              #8

              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?

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              • L Offline
                L Offline
                luca
                wrote on last edited by
                #9

                [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.

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                • L Offline
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                  luca
                  wrote on last edited by
                  #10

                  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.

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                  • Q Offline
                    Q Offline
                    qxoz
                    wrote on last edited by
                    #11

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

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                    • L Offline
                      L Offline
                      luca
                      wrote on last edited by
                      #12

                      You are welcome...

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