The command capabilities of the subdevice indicated by the parameters device and subdevice are probed, and the results placed in the command structure pointed to by the parameter command. The command structure pointed to by the parameter command is modified to be a valid command that can be used as a parameter to comedi_command(). The command measures samples at a rate that corresponds to the period period_ns. The rate is adjusted to a rate that the device can handle. If sucessful, 0 is returned, otherwise -1.