printf hang in retarget code

When I tried to use the sample retarget code on the STM32F4-Butterfly it hangs calling printf. I've created a git repo with all of my code here: https://github.com/ChuckM/retarget-hang you should be able to just clone, make, and run it in arm-gdb to re-create (assuming you have the ST Micro eval board) I'm really curious why it hangs in printf. As I mention in the readme it compiles and links without error so that much is fine at least.

--Chuck