Since the synchronous calls are known to be unimplemented in the single threaded version of the client library libzookeeper_st.so, I believe that it would be helpful towards users of the library if that information was also obvious from the header file.
Anecdotally more than one of us here made the mistake of starting by using the synchronous calls with the single-threaded library, and we found ourselves debugging it. An early warning would have been greatly appreciated.
1. Could you please add warnings to the doxygen blocks of all synchronous calls saying that they are not available in the single-threaded API. This cannot be safely done with #ifdef THREADED, obviously, because the same header file is included whichever client library implementation one is compiling for.
2. Could you please bracket the implementation of all synchronous calls in zookeeper.c with #ifdef THREADED and #endif, so that those symbols are not present in libzookeeper_st.so?