USRP N210 Tuning

Hi,

I have a basic question from rx_samples_to_file.cpp example. When
exactly
USRP N210 gets tuned or locked?

If I issue two commands
of uhd::stream_cmd_t::STREAM_MODE_NUM_SAMPS_AND_DONE, will the usrp will
be
re-tuned between them.

Thanks,

Khalid.