Passing enum Parameters to User-defined C-Language Functions

Hi,

I have a (I hope not too dumb) question regarding the use of enum parameters in a user-defined function written in C,
e.g.for the example given in [1]: CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy'); 

- How would a “mood” parameter be encoded when it is passed to the function?  “Four bytes on disk” might imply that its
eitheran int4 or a float4? 
- How is the association between enumlabel (e.g. 'happy') and value?  The same as enumsortorder in pg_enum?
- How are NULL values encoded?

Or is it a bad idea to use an enum in this case – actually, an int2 plus a few #define's would do the job, too, but the
dedicatedtype looks cleaner to me. 

Thanks in advance,
Albrecht.


[1] <https://www.postgresql.org/docs/16/datatype-enum.html>