How to make Qt aware of the QMYSQL driver

Question

I'm trying to access a MySql database from a Qt application but I get the following error:

QSqlDatabase: QMYSQL driver not loaded
QSqlDatabase: available drivers: QSQLITE QSQLITE2

I find this very strange cause I have libqsqlmysql.so on my Qt folder. I have even tried to compile the MySql driver as a static plugin and add it to my .pro file as:

QTPLUGIN += qsqlmysql

But this also generates the same runtime error (it must've found the plugin cause there's no error compiling the application)

What am I missing? I would like to avoid having to compile Qt from source cause this will have to work seamlessly on the deploy machines as well.

BTW: Even though I'm developing and testing on Linux I will need to support Windows. Will I experience this same issue on Windows? How can I compile and link the MySql driver in both Linux and Windows?

The solution:

After following @Sergey's recommendations I did an strace of the application redirecting the output to grep so I could search for 'mysql' and for my surprise the application wasn't looking for the plugin at QTDIR/plugins/sqldrivers where I had libqsqlmysql.so, it was looking at QTDIR/lib. After copying the plugin to the lib folder the MySql connection worked.

Answer 1

Try opening the shared library with dlopen() and see if it loads and if not, what dlerror() tells you. I always run into similar problems on Windows. LoadLibrary()/GetLastError() saved me numerous times (last time it was because of a wrong version of some libiconv/libintl DLL). Running ldd on the plugin may also help.

If dlopen() works fine, try to load the plugin with QPluginLoader. If it doesn't load, then check the buildkey of the plugin. I usually do it the dirty way by running strings on the plugin and then looking for strings like "buildkey" or "QT_PLUGIN_VERIFICATION_DATA". Just looking at the build key and around it may give you an idea. For example, you may realize that you have compiled your plugin in the release mode while your application is compiled in the debug mode. In such case the build key won't match and the plugin won't load. Everything in the build key must match your configuration. Note that the version and the build key are checked differently: the build key must match exactly (or match some black magic called QT_BUILD_KEY_COMPAT), but in the version only the major version must match exactly, the minor version must be the version of Qt the plugin was compiled with or later and the patch level is ignored. So if your plugin was compiled with Qt 4.x.y then it will work with Qt versions 4.z.* where z>=x. This actually makes sense.

If the build key looks okay (which is unlikely if you got to this point), you may wish to look at QLibraryPrivate::isPlugin() source code to figure out what's wrong, but that doesn't look like an easy task to me (although running this in a debugger may help).

If QPluginLoader does load the plugin, check if it is in the right directory and has correct permissions. If you still didn't solve the problem by this point, it's time to look at the SQL module source code that actually loads these plugins. But it is extremely unlikely. I ran into this problem many, many times and it was always either the library not loading or the build key not matching.

Another way to go after QPluginLoader loads the plugin successfully is to use strace to figure out whether the program at least tries to open the plugin file. Searching for something like "sqldrivers" or "plugins" in the strace output should also give away the directory where Qt is searching for its plugins and specifically SQL drivers.

Update

Is it possible to compile the driver as a static plugin and don't worry about anything? Let's try:

d:\Qt4\src\plugins\sqldrivers\psql>qmake CONFIG+=static LIBS+=-Ld:/programs/Post
greSQL/lib INCLUDEPATH+=d:/programs/PostgreSQL/include
d:\Qt4\src\plugins\sqldrivers\psql>make

It compiles fine and now I got libqsqlpsql.a (release) and libqsqlpsqld.a (debug) in QTDIR/plugins/sqldrivers (it is the right place on Windows). I am using PostgreSQL driver here, but I don't think it will be any different for MySQL which I just don't have installed. Ok, let's compile some real program with it:

d:\alqualos\pr\archserv>qmake QTPLUGIN+=qsqlpsql PREFIX=d:/alqualos LIBS+=-Ld:/g
nu/lib INCLUDEPATH+=d:/gnu/include LIBS+=-Ld:/programs/PostgreSQL/lib LIBS+=-lpq

Note that I had to manually link to libpq, otherwise the linker would complain about undefined references. The funny thing is, qmake knows that qsqlpsql is located in QTDIR/plugins/sqldrivers and sets compiler and linker options accordingly. So it still needs to be in the right place to work, only you don't have to worry about your users running into the same problem as it is only used during compilation. An alternative would be to just use LIBS+=-Lpath/to/plugin LIBS+=-lqsqlpsql instead of QTPLUGIN+=qsqlpsql, at least the docs say that it should work, but I haven't tested it.

In order for the application to actually use the plugin I had to put the following in my main unit (CPP file):

#include <QtPlugin>
Q_IMPORT_PLUGIN(qsqlpsql)

It works! Also, from what I've been able to figure out from the sources, the build key and the version are checked only when a plugin is dynamically loaded (all the relevant stuff is in the QLibrary's private class, not even QPluginLoader's). So the resulting executable may (or may not, depending on the binary compatibility) work even with different versions and builds of Qt, although using it with older versions may trigger some bugs that were fixed later.

It is also worth noting that the order for loading SQL drivers is this: use the driver statically linked into Qt if available, then look for a driver registered manually with QSqlDatabase::registerSqlDriver(), then look for a driver statically imported into the application (the way described above), and finally try to load a shared plugin. So when you link statically, your users won't be able to use dynamically linked drivers they may already have, but will be able to use drivers linked statically into Qt (like in Ubuntu).