CODEWITHSUNDEEP
c++mysqlqtdatabase-connection
metdos
metdos

Reputation: 13929

How to Notice Close of MySql Server in Qt

When I closed MySql server, how can I understand that mysql server is gone away from my Qt program?

Edit:

Here my trial:

When I close MySql, I get these results, and I can't catch that MySql is closed.

My Code Snippet is

QSqlQuery query(db);
query.exec("SELECT * From RequestIds");
qDebug()<<query.lastError();
qDebug()<<db.lastError()<<QTime::currentTime();
qDebug()<<db.isOpen();
qDebug()<<db.isValid();

and output is:

QSqlError(2006, "QMYSQL: Unable to execute query", "MySQL server has gone away") 
QSqlError(-1, "", "") QTime("14:22:58") 
true 
true

I don't understand why db.isOpen() returns true.

Upvotes: 5

Views: 3651

Answers (4)

metdos
metdos

Reputation: 13929

There is a bug related with QSqlDatabase::isOpen() in Qt.

https://bugreports.qt.io/browse/QTBUG-223

Upvotes: 3

Ton van den Heuvel
Ton van den Heuvel

Reputation: 10528

You can use isOpenError to determine whether opening the initial database connection was successfull. I agree that isOpen returning true is confusing.

To monitor the database connection I repeatedly try to open and close a lightweight MySQL connection (e.g. every 3 seconds):

    #include <mysql/mysql.h>

    mysql_init(&connection);
    MYSQL *result = mysql_real_connect(&connection,
            host.isNull() ? static_cast<const char *>(0) : host.toLocal8Bit().constData(),
            user.isNull() ? static_cast<const char *>(0) : user.toLocal8Bit().constData(),
            pass.isNull() ? static_cast<const char *>(0) : pass.toLocal8Bit().constData(),
            dbName.isNull() ? static_cast<const char *>(0) : dbName.toLocal8Bit().constData(),
            0,
            0,
            0);

    bool currentlyConnected = (result != 0);

In the above example, host, user, pass, and dbName are QString instances containing the connection information. Note that you need the MySQL development headers.

Upvotes: 1

Harald Scheirich
Harald Scheirich

Reputation: 9764

QSqlQuery::lastError() should give you an error if your query via QSqlQuery::exec() has failed. Also QSqlDatabase::isOpen() should report the state of your connection, QSqlDatabase::lastError() is also available

Upvotes: 1

Thomas Matthews
Thomas Matthews

Reputation: 57678

Your program has no idea of its surroundings. If something changes, you may be able to have the OS notify your program, or you'll have to test yourself.

If the database connection closes before your program, the status from the connection should return some kind of error code. You are checking status from the connection functions?

Write a simple program that opens a window and upon the click of a button, writes to the database. After writing to the database, the program should display the status in the window. Run your program. Press button to get the "controlled" response. Close the database then click on the button again.

You may be able to do this with a debugger, depending on the ability of the debugger & OS to queue up messages.

Upvotes: 1

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