postgresql jsonb processing with c api

I am working with C API for postgres-9.4 installed from ubuntu trusty main repo.

I would like to use a function that converts a cstring to Jsonb* structure defined in

http://doxygen.postgresql.org/jsonb_8h.html

There are functions doing exactly this already defined in

http://doxygen.postgresql.org/jsonb_8c.html

Namely, the function Datum jsonb_in ( PG_FUNCTION_ARGS ), however I am not sure if I can call this function from C API in a portable and safe manner. As it seems it is intended for being called by postgres from first glance.

I could also use the function jsonb_from_cstring

http://doxygen.postgresql.org/jsonb_8c.html#ab23eca28d5880f86a0943d71c90d6654

but it is declared and defined in jsonb.c and not declared in json.h, and hence linking with this function is not a very clean solution. I tried finding the symbols for jsonb_from_cstring in libpq.so, however there are none. I am guessing I need a non-standard build of postgres?

So the question is, what is the best way to convert a cstring to a Jsonb* structure from within C API?

Thanks,

Igor