The `s6-fdholder-storec` program

s6-fdholder-storec talks to a fd-holding daemon on its open descriptors 6 and 7, and passes it a copy of its standard input.

Interface

     s6-fdholder-storec [ -T fdtimeout ] [ -t timeout ] id

s6-fdholder-storec expects to talk to a listening s6-fdholderd program on its descriptors 6 and 7.
It attempts to pass a copy of its fd 0 (standard input) to s6-fdholderd, with identifier id.
It then exits 0, or 1 if the server returned an error.

-T fdtimeout : the descriptor is stored with an expiration time of fdtimeout milliseconds, which means the s6-fdholderd daemon will close and get rid of the descriptor after that time. By default, fdtimeout is 0, which means infinite - no expiration time.
-t timeout : if the operation cannot be processed in timeout milliseconds, then fail with an error message. Communications with the server should be near-instant, so this option is only here to protect users against programming errors.

s6-fdholder-storec is the "internal" version of s6-fdholder-store. It simply expects to be run as a UCSPI client, i.e. talking to the server over descriptors 6 and 7, instead of connecting to the server itself.
The error messages for the s6-fdholder suite are explained here.