printf is not thread-safe on Redox
It could panic when it's simultaneously called from multiple threads.
An example:
thread '<unnamed>' panicked at 'assertion failed: end <= len', /home/user/git/redox/rust/src/liballoc/vec.rs:1119:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
fatal runtime error: failed to initiate panic, error 5