All interfaces (data types and functions) of libassuan are defined in the header file assuan.h. You must include this in all source files using the library, either directly or through some other header file, like this:
#include <assuan.h>
The name space of `assuan' is assuan_* for function
and type names and ASSUAN* for other symbols. In addition the
same name prefixes with one prepended underscore are reserved for
internal use and should never be used by an application.