Re: mmap and MAP_FIXED

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Feb 25 06:35, Eric Blake wrote:
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> According to Corinna Vinschen on 2/25/2005 5:00 AM:
> > The reason that MAP_FAILED only works on 64K boundaries so far is,
> > that I didn't handle this case.  Usually there are not many good
> > reasons to use MAP_FIXED.  However, I've checked in a patch which
> > tries to handle MAP_FIXED on 4K boundaries, but only in the anonymous
> > case.
> 
> While you are at it, POSIX requires an update to underlying file's ctime
> sometime between mmap(... MAP_SHARED, PROT_WRITE ...) and the
> corresponding munmap() or msync() if the mapped region was written to.  If
> Windows correctly updates mtime on a mapped file, then one possible way to
> do this might be: when munmap()ing, if mtime is newer than ctime (but less
> than or equal to now), adjust ctime to match mtime.

Well... no, not now.  That's more tricky than you think.  If you have
an open mmap to a file, there's not necessarily also an open file
descriptor available.  This is perfectly fine:

  fd = open ("foo", O_RDWR);
  addr = mmap (NULL, getpagesize (), PROT_WRITE, MAP_SHARED, fd, 0);
  close (fd);
  *addr = 'x';
  munmap (addr, getpagesize ());

The problem is that right now the mmap records don't keep track of the
file name, nor do they keep a file handle open since that was never
necessary for the mmap/munmap functionality.  Adding this just for the
sake of setting st_ctime makes the code rather more complicated than
it already is.  I have to think about this a while.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          mailto:cygwin@cygwin.com
Red Hat, Inc.

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