comedi_get_buffer_offset man page
comedi_get_buffer_offset — streaming buffer status
int comedi_get_buffer_offset (comedi_t * device, unsigned int subdevice);
The function comedi_get_buffer_offset() is used on a subdevice that has a Comedi command in progress. This function returns the offset in bytes of the read pointer in the streaming buffer. This offset is only useful for memory mapped buffers. If there is an error, -1 is returned.