The stdio table is conceptually similar to table-socketmap in
OpenSMTPD-extras: it allows to write programs that implements the
tables api speaking a simple text-based protocol.
Unlike table-socketmap, it provides a better protocol that's similar
to the one used by the filters.
The end goal is to have OpenSMTPD speak natively this protocol and thus to deprecate this repository. In the meantime, it allows to write and test new table implementations in any programming language without changing smtpd itself.
It works by "wrapping" the new tables and speaking with smtpd the legacy protocol. Here's an example usage with table-passwd:
table virtuals "stdio:/usr/local/libexec/smtpd/table-passwd /etc/mail/users"
Please note that the legacy tables in OpenSMTPD-extras implement the old protocol and so they don't need table-stdio. The new implementations are now in separate repositories and share the same name, usage and configuration with the old ones to ease the transition.