comedi_cancel man page
comedi_cancel — stop streaming input/output in progress
int comedi_cancel (comedi_t * device, unsigned int subdevice);
The function comedi_cancel() can be used to stop a Comedi command previously started by comedi_command() that is still in progress on the subdevice indicated by the parameters device and subdevice. This may not return the subdevice to a ready state, since there may be samples in the buffer that need to be read.
If sucessful, 0 is returned, otherwise -1.