Qt/FAQ

Why do I need qt3support?
First of all, qt3support is a useflag that enables the module in Qt4, as well as needed code in other Qt4 modules. It does in no way depend on Qt3. It contains classes that make porting Qt3 applications and libraries to Qt4 easier. They are Qt4 classes that emulate Qt3 behavior. This is really only interesting for the developer of that package, not for the user.

Any Qt4-based package that uses these classes from Qt4's qt3support will require the qt3support useflag to be enabled. This means the useflag needs to be enabled for all Qt4 modules that have this useflag. Enabling it for one but not other modules would break things, either at compile time or at runtime, so we force the usage: it must be either enabled or disabled for all Qt ebuilds. And as there is no package other than the Qt libraries themselves that use this useflag, the recommendation is to enable (or disable) it globally in make.conf.

As kdelibs-4 uses these qt3support classes internally, there is a genuine requirement for qt3support to be enabled. There is no way you can have KDE4 without qt3support in Qt4. But this does not at all mean you need to keep Qt3 itself around. We strongly recommend you to remove x11-libs/qt:3.

Users who do not use KDE, or anything that depends on, should be able to have most other Qt4 applications work without qt3support.

Why do I get blockers when trying to emerge Qt?
Gentoo uses split ebuilds of the various components of QT to allow finer-grained control of dependencies from other packages. However, despite there being separate ebuilds all those components must be of the same version, which means the must all be upgraded together.

If some of the updated version packages are keyworded but others are not, you get those blockers.

Another source of QT blocks is incompatible USE flag combinations, the block emerge output will tell you which those are.

Solving the block
After you have checked the keywording is correct, the easiest (not necessarily the best, but the one with the least headaches) way to get the blocks out of the way for a QT upgrade is to uninstall all QT components and then emerge them again:

export INSTALLED_QT_PACKAGES=$(eix -I --only-names x11-libs/qt)   # list all installed qt packages quickpkg ${INSTALLED_QT_PACKAGES}                                 # save tarballs of the old versions should we have to roll back emerge -Ca ${INSTALLED_QT_PACKAGES} && emerge --oneshot -va ${INSTALLED_QT_PACKAGES}

What does the exceptions useflag do?
The useflag description is technical, because the issue is technical. It is enabled by default, because this is the recommended setting for Qt. See for a discussion. When a developer uses exceptions in his program, it will then produce a warning on certain errors, instead of a crash. This is a good thing, which we generally want. The downside is that the application will use some more memory and diskspace. So in cases where those are limited (think for example of embedded environments) it could be useful to turn this off. That is why we have decided to offer a useflag to disable this feature, after some users requested this.