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Message-ID:	<5177CA05.3010500@cs.utoronto.ca>
Date:	Wed, 24 Apr 2013 08:03:17 -0400
From:	Ryan Johnson <ryan DOT johnson AT cs DOT utoronto DOT ca>
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To:	"cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
Subject:	mmap(MAP_FIXED) vs mprotect

Hi all,

I'm trying to port a linux program that uses mmap to implement a 
growable array; the ideas is to mmap(PROT_NONE, MAP_NORESERVE) a chunk 
of address space (corresponding to the maximum array size) and then call 
mmap(PROT_READ|PROT_WRITE, MAP_FIXED) to allocate actual memory in the 
"blank" region. This works well in Linux but fails with EINVAL in cygwin.

My code aligns all sizes up to 2MB boundaries, so it's not a 64kB 
boundary problem. My code reports the failing call as:
> 22 Invalid argument addr=0xffdb0000, sz=2097152

A peek in /proc/self/maps confirms that the address is correct:
> FFDB0000-FFFB0000 ===p 00000000 0000:0000 0

Oddly, trying to map in blank pages in with mprotect succeeds on cygwin 
but fails with ENOMEM on linux...

Am I missing something here, or is this just a place where different 
behavior between the two platforms is a fact of life? Which version is 
the posixly "correct" way to reserve a chunk of address space and later 
back it with actual memory?

Thanks,
Ryan


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