Re: kevent monitoring of file system changes

[Dave Tyson <dtyson%anduin.org.uk@localhost>]

On Sunday 14 Mar 2021 18:29:58 Mouse wrote:
> > 1) If I am monitoring a file, for example, called 'a' and I copy
> > another file over it then I get a write event and an extend event.
> > If I copy the same file over it again I would have thought I should
> > just get a write event as the file size would be the same (no need to
> > extend), but I find that I still get both events reported.  The same
> > is true if the target is overwritten with a smaller file!
> 
> Most file copy programs open the target with O_TRUNC, then write.  THis
> means that the target is always being extended, since it is truncated
> to zero size as of the open().
> 
> Does it still work the same way if you copy using dd conv=notrunc?
> (That's one of the few ways in the stock system to copy a file's
> contents without truncating the target first.)
> 
> > 2) If I remove the NOTE_EXTEND attribute from the list [...]
> 
> No idea here.  I don't really know kevent.
> 

Thank you - I hadn't though about copy using O_TRUNC!

Testing with dd conv=notrunc showed just writes if the file being copied over 
was equal or less than the size of the existing file. Problem 1 explained, 
I'll have a look in kevent.c and fathom out what's going.

Thanks again,
Dave