Re: mmap(), MAP_SHARED, MAP_ANONYMOUS and sharing memory

Bahadir <Bilgehan.Balban@xxxxxxxxx> writes:
I have checked various unix specs and I am puzzled about how to share
memory between two processes using mmap().

AFAIK MAP_ANONYMOUS + MAP_PRIVATE would map /dev/zero (in linux man
pages don't mention /dev/zero) so the process can get some private
zero-initialised free memory, correct?

Before MAP_ANONYMOUS existed, the same effect could be accomplished on
'some operating system' (notably, Solaris) by mmap'ing /dev/zero.

Then what would MAP_ANONYMOUS + MAP_SHARED do?

Create a memory region shared among all processes created by the
process who mmap'ed it and all processes created by these processes
and ...

If Linux specifies no particular file for MAP_ANONYMOUS where would
the shared mapping be found by another process?

Not at all. To share memory among unrelated processing using mmap, one
would either use shm_open or mmap real files.
.

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