comedi_get_buffer_contents man page
comedi_get_buffer_contents — streaming buffer status
int comedi_get_buffer_contents (comedi_t * device, unsigned int subdevice);
The function comedi_get_buffer_contents() is used on a subdevice that has a Comedi command in progress. The number of bytes that are available in the streaming buffer is returned. If there is an error, -1 is returned.