[unwind]

Can you elaborate on how it detects and signals if it runs out of output buffer space? I couldn't see how the amount of available space was even communicated to `fc_enc()`.

Also there some "C icks" (to me, I'm very picky and used to know the standard awfully well from answering many SO questions) that you might want to look into. The two I remember now are the casting of `void` pointers from allocation functions, and (worse) the assumption that "all bits zero" is how a NULL pointer is represented.