[GSoC] json helper functions

    I am working on providing a new plugin where creates new data type (thrift). What I am currently planning is to translate cstring to byte for thrift_in functions. The cstring should be json format. When I looking at json helper functions I find the header 

./include/server/utils/json.h

where all functions have the signature of Datum function(PG_FUNCTION_ARGS).This signature causes extra overhead when writing the code because I need to convert my intermediate variables to the required format. Wondering are there any json helpers that are used by postgresql and plugins can directly call? 

Thanks Charles