I can't find any API documentation, using those APIs I googled a bit:
A google(rrdc_connect site:oss.oetiker.ch) is empty.
A google(rrdc_connect site:oetiker.ch) gives only the patches
on the mailinglist.
The Debian librrd-dev does not have anything either and I can't
seem to find it in the source tarballs (see below) unless I am
looking in the wrong location.
Is there a small minimal example application that shows the ropes?
Also, if rrdcached opens the files, how are permissions handled?
Especially when the calling process runs as user X and the daemon
as user Y. Or is it expected that rrdcached runs as X or at least
in the right group?