Hey Howard:
I don’t have any problems with fprintf(stderr,…) on XCode with VST3 clients, however Pro Tools will prevent stderr and stdout from appearing (which sounds like what you may be experiencing). I got a great tip from a friend at SoundToys who uses Terminal for all logging. You start the client from the command prompt, then fprintf() will go there. For Cubase Elements 8 that would be:
arch -x86_64 /Applications/Cubase\ LE\ AI\ Elements\ 8.app/Contents/MacOS/Cubase\ LE\ AI\ Elements\ 8
Next attach to the process in XCode.
Then you should be able to see your data-logs in Terminal, bypassing XCode altogether. Hope that helps (and sorry if you already tried/knew that…)!
- Will