Messages in this thread | | | Date | Thu, 07 Aug 2014 15:09:13 +0100 | From | Rob Jones <> | Subject | Re: [PATCH] seq_file: Allow private data to be supplied on seq_open |
| |
Hi Steve,
On 07/08/14 14:32, Steven Whitehouse wrote: > Hi, > > On 07/08/14 13:58, Rob Jones wrote: > [snip] >> >> On a related subject, Having looked at a few uses of seq_file, I must >> say that some users seem to make assumptions about the internal >> workings of the module. Dangerous behaviour as only some behaviours are >> documented. >> >> e.g. The behaviour that "struct seq_file" pointer is stored in >> file->private_data is documented and can therefore be relied upon but >> the fact that the output buffer and its size are only defined at the >> first output (and can therefore be pre-defined and pre-allocated by >> user code) is not documented and could therefore change without warning. >> >> This second behaviour is assumed in, for example, module fs/gfs2/glock.c >> which could, therefore, stop working properly without warning if the >> internal behaviour was changed. >> > While it is undocumented, it is I understand, how this feature was > intended to be used, so I think that it is safe to do this in the GFS2 > case. Here is a ref to the thread which explains how it landed up like > that: > https://www.redhat.com/archives/cluster-devel/2012-June/msg00000.html
No criticism was intended of that particular piece of code, It has been there for a couple of years and is, presumably, still working :-)
It was just a general point about things needing to be written down. A behaviour such as you were relying on can be a very positive thing but it would be of much greater use if it was written down in the file docs.
I completely missed seq_file_private() because I was looking at the docs more than the code. If it had been written down in the docs it would have saved me quite a bit of time, similarly, if the buffer allocation behaviour was documented, changes to seq_file.c would not be made that could break your code.
God knows, I'm not a fan of unnecessary documentation but where it's useful I'm all for it.
> > Steve.
-- Rob Jones Codethink Ltd mailto:rob.jones@codethink.co.uk tel:+44 161 236 5575
| |