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SubjectRe: procfs feature or bug?
At 3:53 PM +0530 2001-09-19, Sandip Bhattacharya wrote:
>I was trying out the multipage procfs entries, when i found out that i
>was having a problem.
>
>I am trying to use the "hack" by Paul Russell to allow mangling of
>filepos using ``*start'' entries as my personal page offset. Now, for
>multipage entries, whatever i set as *start should be coming back to
>me as offset when the next page is called.
>
>But in fs/proc/generic.c in proc_file_read() at the end we have (
>after the first page has been "copied_to_user")
>
> *ppos += start < page ? (long) start : n;
>
>But this _adds_ the contents of start to the offset, in the case where
>I am supplying the offset in ``start''. Shouldn't this just be
>_replacing_ ? 'Cause in this case the offsets get cumulatively added,
>causing an oops at the end for me :(
>
>Or am I missing something big???

Per the comment:

> /* This is a hack to allow mangling of file pos independent
> * of actual bytes read. Simply place the data at page,
> * return the bytes, and set `start' to the desired offset
> * as an unsigned int. - Paul.Russell@rustcorp.com.au
> */

*ppos represents the offset in the *file*, not the buffer, so
cumulative is correct. So when you do your read, you need to
interpret it thus. I've been using it in one case as a record number,
where the records are variable length. I set *start to 1, and treat
is as an index into a (virtual) array of records.
--
/Jonathan Lundell.
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