MAP_PRIVATE that stays private even on external write

From: Miquel van Smoorenburg
Date: Tue Dec 02 2008 - 05:14:48 EST

I have an application that mmaps blocks from a file and then
sends it out over a TCP socket.

The contents of the file may be overwritten while the blocks
are sent out, but there is a very low chance of that.

To prevent this, I can ofcourse just use read() all blocks into
memory in advance, check consistency, and go ahead .. but since
there may be a lot of network connections active this might cost
a lot of memory.

What I am looking for is a MAP_PRIVATE type flag that, when another
process modifies pages of the file (through mmap() or write())
makes sure that my mapping never sees that.

I found out that mmap(MAP_PRIVATE|MAP_POPULATE) does exactly what
I want, but ofcourse it reads in the entire mapping all at once,
which is equivalent to plain read().

There has been talk of a MAP_SNAPSHOT flag before, e.g.

Has anyone ever looked at implementing something like this ?

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at