Archive for April, 2010

CoreFoundation plus QuickTime on Windows

Monday, April 26th, 2010

We are writing portable cross-platform software, primarily running on Macs, Windows PCs, iPhones and iPads. Recently we had a problem to solve, that I’ll describe here.

Our code uses Apple’s CoreFoundation. On Mac OS X or iPhone OS it’s “just there” as part of the System Frameworks, and on Windows we use CoreFoundation in form of the great OpenCFLite port. Now we had a project where we’re also using Apple’s QuickTime. Again, on the Mac it’s “just there”, but on Windows we have to use the QuickTime SDK for Windows. Now when trying to build this project that uses both CoreFoundation and QuickTime on Windows, we got a lot of Linker errors complaining about duplicate symbols. The reason is that the QuickTime SDK for Windows itself contains its own subset of CoreFoundation. So we searched for a solution.

I already had the idea to write a wrapper around all necessary QuickTime calls and move this wrapper into a DLL to separate the QuickTime stuff from the main binary. So QuickTime called from the wrapper DLL uses its own embedded CoreFoundation implementation, and the main binary can still use OpenCFLite. Most probably there are some things to keep in mind, e.g. the API to the QuickTime wrapper must not pass any CoreFoundation objects across the boundary. CFStringRefs had to be passed e.g. as plain C strings UTF-8 encoded and the CFStringRef has then to be reconstructed from that inside the wrapper. But all this shouldn’t be much of a problem.

But we were not 100% sure if our theory really solves the problem, or wether there is an easier solution. Maybe some secret linking tricks or something like that.

We thought somebody else must already had the same problem!? And, yes, it came to my mind that the developers of WebKit – the engine of Apple’s web browser Safari – must have solved that. I know that WebKit uses CoreFoundation. And I also was pretty sure that WebKit uses QuickTime for displaying Video. Luckily WebKit is open source, so I downloaded the WebKit sources and digged into them to see if the WebKit engineers had the same problem and how they solved it.

Short time later I had the answer. Really, they obviously are in the same boat. And it was solved pretty much the same way as the idea that I already had in my mind. WebKit contains classes like “QTMovieWin” that are build into a separate DLL. So all QuickTime calls are done in those special wrapper classes from the DLL and WebKit itself does not directly call any QuickTime routines.

It’s always nice to see being not the only one having strange problems to solve. 😉

I published this article, because I googled a lot for CoreFoundation and QuickTime linking conflicts on Windows, but nobody seems to either have this constellation or did not write or ask about it. Maybe this article will help somebody out there …