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Subject: Memory mapping: =?UTF-8?Q?MAP=5FPRIVATE=20and=20msync=28=29?=
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Date: Sun, 07 Apr 24 13:34:43 +0000
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From: pehache <pehache.7@gmail.com>
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Hello,

When memory mapping a file with the MAP_PRIVATE flag, the modifications 
(writes) only exist in memory and are not written back to the file.

According to the man pages, calling msync (3) on a such a mapping does NOT 
writes the changes back:

"When the msync() function is called on MAP_PRIVATE mappings, any modified 
data shall not be written to the underlying object and shall not cause 
such data to be made visible to other processes"
https://linux.die.net/man/3/msync

So: is there a way to write the changes back to the file? 

An obvious application is:
- mapping the file with MAP_PRIVATE
- make some modifications in memory only (fast) while keeping the original 
version on disk (safe)
- at some point (when the user decides, and once the consistency of the 
changes have been verified) writing the modifications to the disk

I'm pretty sure it exists some way or another, but I don't know how.