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

[Bruce Rowen <browen@xxxxxxxxxxxx>]

On Aug 4, 2009, at 2:35 PM, Robert P. J. Day wrote:

 i realize that's a potentially dumb question but that doesn't seem
to be explicitly stated in any of the docs i'm reading -- it always
just seems to be assumed.

 i've always assumed that if you just "cat" a /proc file that's based
on seq_file, the offset value that's passed will always be zero, so
you're always starting at the beginning.

 however, if you look at the source for fs/proc/devices.c, you read:

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;
}

note that that routine clearly checks the initial value of the
offset, which i assume you can affect by, say, reading the file with
"dd" and specifying an offset, or something like that.  but unless you
explicitly change the starting location, is it true that the start()
routine always gets an offset of zero?

No, it is up to the programmer to honor the provided offset "loff_t *pos" parameter.

-Bruce

 or are there other ways to affect that value?

rday
--

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Robert P. J. Day                               Waterloo, Ontario, CANADA

       Linux Consulting, Training and Annoying Kernel Pedantry.

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