Mail Archives: cygwin/2007/01/05/13:42:36
On Fri, 5 Jan 2007, Corinna Vinschen wrote:
> In the failing case this should still work, since 0x7fff7000 + 0x9000
> (36864 dec) == 0x80000000, so the mapping should fit into the usual 2
> Gig address space. Why Windows fails to do it, I have no idea. The
> error code 487 means invalid address which might mean "already taken"
> address, but that's not visible in the strace. To figure that out would
> require to add a bit of VirtualQuery code to mmap and add appropriate
> debug output.
I'm not quite sure exactly what this means, but I stumbled onto it in gdb:
(gdb) info w32 selector
Selector $fs
0x03b: base=0x7fffe000 limit=0x00000fff 32-bit Data (Read/Write, Exp-up)
Priviledge level = 3. Byte granular.
So, it does indeed look taken.
> Actually this shows a problem in the mmap implementation with respect to
> MEM_TOP_DOWN. I think, what mmap should actually do is to create a
> lightweight MAP_RESERVE anonymous mapping of the whole requested mapping
> size, then close it again and then reopen it with the address it got
> in this first try. This would probably ensure that the subsequent two
> mapping will work.
I don't know what mmap magic Cygwin uses internally to do this, but can't
you just map the big region, then overmap the disk file without the map,
unmap, remap, remap?
--
Brian Ford
Lead Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
the best safety device in any aircraft is a well-trained crew...
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