[Dovecot] plugin development: retrieve username -d -f inside plugins

[Alano Conraz]

>
> struct mail_user->username has it. How you get to struct mail_user depends
> on where you want to access it from. Note that each process is capable of
> handling multiple different users (although it's not commonly done).
>

i'm not sure to understand the last part.  mail_user->username is a char *,
right ? What is the content of the string if there is multiple users ?
Thanks for your answer btw.