Selecting data from mysql using qt?

Dii · wrote on 12 Oct 2010, 12:28

doforumda: the problem was ~small... the Qt mysql driver looks for the socket file (the connection to mysql) in the default location (var/run/mysqld/mysqld.sock), though you had it somewhere else, because your lampp package changed the path.

The "ln -s" made a symbolic link to that, so the /var/run/mysqld/mysqld.sock point to the /opt/lampp/var/mysql/mysql.sock. This way Qt can find it. If you do a new installation, with lampp, you should do it again, if you use the regular mysql package, probably, you won't face this problem.

lyuts: great work, we deserve these points :)

lyuts · wrote on 12 Oct 2010, 12:33

I think our solution is rough enough, but at least it works. I think the best solution is to set the socket parameter in my.cnf to /var/run/mysqld/mysqld.sock. But it is up to you doforumda.

bq. millions of thanks to both of you TYUT and Dii.

Hey, I'm lyuts, not TYUT :)

doforumda and ¤ Dii ¤, my congratulations, we did it.

Dii · wrote on 12 Oct 2010, 12:45

lyuts: I think that's the best solution, if he is changing the socket parameter of mysql probably he will screw up some other components in his LAMPP configuration. I don't recommend that.

lyuts · wrote on 12 Oct 2010, 13:34

I don't know what's going on within LAMPP and you might be right. It's just creating this symlink looks more like a workaround.

doforumda · wrote on 12 Oct 2010, 14:48

[quote author="lyuts" date="1286886793"]I think our solution is rough enough, but at least it works. I think the best solution is to set the socket parameter in my.cnf to /var/run/mysqld/mysqld.sock. But it is up to you doforumda.

bq. millions of thanks to both of you TYUT and Dii.

Hey, I'm lyuts, not TYUT :)

doforumda and ¤ Dii ¤, my congratulations, we did it.[/quote]

sorry I wrote your name wrong

? This user is from outside of this forum · wrote on 12 Oct 2010, 15:17

with 131 replies, I think this must be the longest threaded discussion in devnet forum so far !!!
congrats to you guys for sticking till the end and solving it :)

doforumda · wrote on 13 Oct 2010, 15:52

hi
i have got the same problem again after restarting my system.
when i execute code it again fails to connect. it returns this problem.
@
Starting /home/zafar/c++/insertquery-build-desktop/insertquery...
DB could be opened QSqlError(2002, "QMYSQL: Unable to connect", "Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)")
/home/zafar/c++/insertquery-build-desktop/insertquery exited with code 0
@

i ran this command
@
sudo ln -s /opt/lampp/var/mysql/mysql.sock /var/run/mysqld/mysqld.sock
@
but this displays this error
@
ln: creating symbolic link `/var/run/mysqld/mysqld.sock': No such file or directory
@
i have already created this directory while lyut and Dii were solving this problem. at that time when i ran this command it worked really perfect but now it is not working.
Please help

Dii · wrote on 13 Oct 2010, 17:02

Welcome back :)

Let's check to existence of those sock files...

sudo ls -l /opt/lampp/var/mysql/mysql.sock

sudo ls -l /var/run/mysqld/mysqld.sock

doforumda · wrote on 13 Oct 2010, 17:13

well mysql.sock is there
but there is no such thing in mysqld. i mean there is no mysqld.sock file in mysqld folder

doforumda · wrote on 13 Oct 2010, 17:15

thank god your are here

doforumda · wrote on 13 Oct 2010, 17:40

i am waiting for help Dii

Dii · wrote on 13 Oct 2010, 19:50

hmmm pretty strange, does /var/run/mysqld exits? if you try

sudo mkdir /var/run/mysqld

and then the

sudo ln -s /opt/lampp/var/mysql/mysql.sock /var/run/mysqld/mysqld.sock

Though restart should not delete the directory, but I can't imagine anything else at the moment.

doforumda · wrote on 13 Oct 2010, 20:12

it works now thanks again. any clue what happened to this file after restarting.

Dii · wrote on 13 Oct 2010, 20:20

Hmmm... /var/run will be clear on each start, so you have to do a directory and the symlink again. You can do it with a script.

Or get rid of the whole LAMPP setup and use the regular mysql package. Maybe it's possible to run two mysql parallel, but I don't recommend that, too confusing.

lyuts · wrote on 14 Oct 2010, 06:01

If the problems after restart persists, then I think it will be time to change socket parameter in my.cnf. configs are created for changing settings and I don't think it will harm any component in lampp, because if it would then 1) a special note must have been written somewhere and 2) it makes no sense to allow user to change this setting if it breaks lampp.

Dii · wrote on 14 Oct 2010, 07:44

I don't know LAMPP, but I have seen similar packages, where the packager wanted to keep everything in /opt and for that reason all settings and installation directories were changed. It might be, that the apache or php is configured, to look for the socket in the /opt/lampp.

However, I looked around in the documentation, and I found setConnectOptions(..) which could be a good workaround. I think it should work like this:

db.setConnectOptions("UNIX_SOCKET=/opt/lampp/var/mysql/mysql.sock");

The only problem is, if you move the program to a different system, it won't work again, so you should make some lookups in the code, if the above is correct or not in the actual system (usually it won't be).

If this problem heads up more often (the distribution packagers change the socket location for fun), maybe it's good to implement a socket search function somewhere in QSqlDatabase::open().

doforumda · wrote on 14 Oct 2010, 08:05

thanks for that exaplanation

lyuts · wrote on 14 Oct 2010, 10:11

I was reading last posts and wondered, what if doforumda changes the way of connecting to mysql server. I just noticed that you can connect via tcp or socket, and here socket, as I understand is unix domain socket. Connecting via tcp might eliminate the need for looking sock files.

UPD: just a little bit more words. Unix domain sockets limit communicating process to be within the same OS, which is not true in general. At least I think the original application is not for local usage only.

Dii · wrote on 14 Oct 2010, 10:45

Yes, it must be possible to connect to 127.0.0.1:3306, though it will not work, as all mysql default configuration comes with "--skip-networking" option, for good reason. You don't want your sql server to be reachable directly from the outside. Ofcourse you can turn this off, but probably that's not the way.

I still find the above with the connect options better, like this:

@db.setConnectOptions("UNIX_SOCKET=/opt/lampp/var/mysql/mysql.sock");
if (!db.open()) {
db.setConnectOptions(); // clears the connect option string
}
if (!db.open()) ... some error handling@

lyuts · wrote on 14 Oct 2010, 11:11

Isn't this limiting an application to have mysql server on the same machine? Moving an application will make doforumda put mysql server on that machine too.