What I don't like is that some functions take as arguments a mix of StringBuffer's and regular C strings. This is confusing. For example why this:

  void StringBuffer_replace(StringBuffer *buf,
    const char *original,
    const char *update,
    size_t from);

instead of this:

  void StringBuffer_replace(StringBuffer *buf,
    const StringBuffer *original,
    const StringBuffer *update,
    size_t from);