Man page 9 : seq_open

seq_open - initialize sequential file

int seq_open(struct file *  file , const struct seq_operations *  op );

file
    file we initialize

op
    method table describing the sequence

seq_open sets file, associating it with a sequence described by op. op->start sets the iterator up and returns the first element of sequence. op->stop shuts it down. op->next returns the next element of sequence. op->show prints element into the buffer. In case of error ->start and ->next return ERR_PTR(error). In the end of sequence they return NULL. ->show returns 0 in case of success and negative number in case of error. Returning SEQ_SKIP means « discard this element and move on ».