[Interest] Qt/Mac and "Application Extensions Only"

[Jake Petroules]

> On Dec 19, 2017, at 5:39 AM, René J.V. Bertin <rjvbertin at gmail.com> wrote:
> 
> Hi,
> 
> Building Qt 5.9.3 I caught a couple of linker warnings about it being unsafe to use dylib with application extensions. Figuring out the source of that I found a remark that "Qt libraries have to use only APIs that are safe to use for application extensions". This despite the fact that I'm not getting a build that's App Store compatible (which I wouldn't want anyway).
> 
> Is there a post somewhere about why Qt libraries have to be built that way and what it implies in practice (if you just want to be able to build and run cross-platform FOSS software including KDE/KF5)?
> 
> I see the `CONFIG += app_extension_api_only` line appearing in a number of mkspecs/*.prf files which suggests that the setting *might* be applied to user projects too (like when building a Qt plugin).
> 
> Is there any risk to commenting those lines out before building Qt, following the principle of "don't use compiler options that you do not understand fully and/or do not really need"?

What about the principle of "don't remove compiler options that you do not understand fully and/or may need"? ;)

Leave these alone; do not change them. All this does it makes it into a compiler error to use an API *within Qt libraries themselves* that would make such libraries incompatible with an App Extension environment. Since the developers of Qt are the ones who decide which APIs Qt uses, only the developers of Qt should be adding or removing this flag; users have no need to ever touch it unless they're forking Qt. There's actually only a very small amount of APIs incompatible with app extensions anyways (and primarily on iOS) so for the most part the warnings are harmless.

The philosophy is this: you should always build LIBRARIES (or App Extensions themselves, which is a logical requirement) with CONFIG+=app_extension_api_only so that anyone - regular apps OR extensions - can use them, unless you understand why you don't want it. For APPLICATIONS you do not want CONFIG+=app_extension_api_only.

It's perfectly OK for an app/library WITHOUT app_extension_api_only to depend on a library WITH app_extension_api_only, but obviously not the reverse.

That said, I'd like to know exactly WHERE you're seeing these errors. If they're coming from third party dependencies like libpng, etc., then those dependencies should be built with -fapplication-extension in order to get rid of the warnings. Otherwise they can be safely ignored because there's no way anything coming from a KDE/KF5 environment is using any App Extension unsafe APIs anyways.

It's slightly unfortunate the way this flag works, because it requires the whole world to declare its status. I wish they'd done it in a slightly more backwards incompatible way and required modules to explicitly state that they are NOT app extension safe instead of the other way around.

> Thanks,
> R.
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-- 
Jake Petroules - jake.petroules at qt.io
The Qt Company - Silicon Valley
Qbs build tool evangelist - qbs.io