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Date: Fri, 1 Jul 2022 12:45:31 -0400
From: gs-cygwin.com@gluelogic.com
To: David Allsopp <David.Allsopp@cl.cam.ac.uk>
Subject: Re: mmap failing with MAP_FIXED
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On Fri, Jul 01, 2022 at 05:33:31PM +0100, David Allsopp wrote:
> This program fails at the second mmap call with EINVAL:
> 
>   #include <stdio.h>
>   #include <sys/mman.h>
>   #include <error.h>
> 
>   int main (void) {
>     void * mem;
>     /* Reserve 256MB address space for the minor heaps */
>     mem = mmap(0, 268439552, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>     if (mem == MAP_FAILED)
>       error(1, 0, "Reservation failed");
>     /* Commit the first 2MB heap */
>     if (mmap(mem, 2097152, PROT_READ | PROT_WRITE, MAP_PRIVATE |
> MAP_ANONYMOUS | MAP_FIXED, -1, 0) == MAP_FAILED)
>       error(1, 0, "Commit failed");
>   }
> 
> Is this something that's expected to fail for Cygwin, or a bug? The example
> is extracted from OCaml 5.0's runtime, which reserves an area of address
> space and then commits chunks of it as required. The above snippet comes
> from the Linux side, on Windows we're using VirtualAlloc with PAGE_NOACCESS
> to reserve the address space and then VirtualAlloc with MEM_COMMIT and
> PAGE_READWRITE to commit smaller portions of it.
> 
> Is there a way to do that with Cygwin's mmap?

Have you tried 'msync()' with MS_ASYNC flag as an alternative approach?

Cheers, Glenn

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