16 Dec
2024
16 Dec
'24
9:40 p.m.
> Does anyone know whether there are implementations
of mmap that
> do transparent file sharing? It seems to me that should be possible by
> making the buffer cache share pages with mmapping processes.
These days they all do. The POSIX rationale says:
... When multiple processes map the same memory
object, they can
share access to the underlying data.
Notice the weasel word "can". It is not guaranteed that they will do so
automatically without delay. Apparently each process may have a physically
distinct copy of the data, not shared access to a single location.
The Linux man page mmap(2), for example, makes it very clear that mmap
has a cache-coherence problem, at least in that system. The existence
of msync(2) is a visible symptom of the problem.
[Weasel words of my own: I have not read the POSIX definition of mmap.]
Doug