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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?


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