Have you looked at our Couchbase Mobile platform ? The mobile stack consists of Couchbase Lite embedded DB that can synchronize (one-way or two-way) with the sync gateway. The Sync gateway talks to Couchbase Server.
We have couchbase lite supported on many platforms/languages so you can use the native API on those platforms. For C, You could use the REST API to interact with Couchbase Lite.
Tagging @jens …