C example

I thought an example in C might be slightly helpful for those who solely use C.

Two things though:

1. `err` keeps giving values that are nonzero even when AFAICT there are no errors - so I had to disable the asserts and checks for that.

2. For the CLA, where can I obtain the PDF to print and sign, and where should I send it to?
err is not touched if there's no error. So you need to initialize it to zero before the call.