strace is pretty handy on Linux for profiling processes, including attaching to a running one and WTF-ing it.
It turns out that there’s a Mac equivalent, dtrace1. If you skip past the instructions on how to use the terminal and using top, there are some good examples of using dtrace, including a lot more than just profiling system calls.
1 Calling it a Mac app is slightly misleading; it comes from Sun. But to someone used to strace on Linux who is using a Mac, dtrace is the equivalent.