> > I don't think mmap() provides exactly the right behaviour.
> It lets you
> > specify that modifications made by the current process
> aren't committed
> > to the file, but what we want is to snapshot the file so
> that subsequent
> > modifications by *other* processes aren't seen by the local process.
>
> The Linux manpage isn't very explicit, so I've tried it. When making a
> private mapping (MAP_PRIVATE), it makes a snapshot which is private to
> the process, and isn't affected by other processes. It's
> exactly what's needed. (see attachment)
It only makes a snapshot of those pages of the file which have been
modified by the current process, because that's when the copy-on-write
happens (a page is 4k on Linux/x86). In unmodified pages of the file
the mapping is still exposed to modifications by other processes, even
if the current process has already read data from that page - try it!
Cheers,
Simon
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