Re: by default, does seq_file start() routine start at zero offset?

[Bruce Rowen <browen@xxxxxxxxxxxx>]

On Aug 5, 2009, at 9:34 AM, Robert P. J. Day wrote:

 i'm still somewhat confused here.  in every example using a seq_file
that i've ever seen, it's *assumed* that the very first call to the
start() routine is going to pass a (*pos) value of zero, which is used
to signify the initial call to that start routine.  that *has* to be
consistent.  if it can be something other than zero for that *initial*
call, how would seq_files ever be guaranteed to work properly?

I pulled this from fs/proc/devices.c, I hope it's version compatible with your query:

static void *devinfo_start(struct seq_file *f, loff_t *pos)

  {
          if (*pos < (BLKDEV_MAJOR_HASH_SIZE + CHRDEV_MAJOR_HASH_SIZE))
                  return pos;
          return NULL;
  }

This example does not assume (*pos) is initialized to zero. Digging further back into the sequence of events that occurs before the start() is called, I can't find anything that explicitly sets the (*pos) to zero, unless it follows philosophy of setting (*pos) to zero when open() is called (this is probably buried in the more generic file I/O layers).

-Bruce