#define silc_buffer_datalen ...

    Macro that can be used in function argument list to give the data
    pointer and the data length, instead of calling both silc_buffer_data
    and silc_buffer_len separately.

    // Following are the same thing
    silc_foo_function(foo, silc_buffer_datalen(buf));
    silc_foo_function(foo, silc_buffer_data(buf), silc_buffer_len(buf));

    #define silc_buffer_datalen(x) (x) ? silc_buffer_data((x)) : NULL, \
      (x) ? silc_buffer_len((x)) : 0