Atomic mmap replacement

[Ken Brown <kbrown@cornell.edu>]

Some code in emacs wants to reserve a chunk of address space with a big 
PROT_NONE anonymous mapping, and then carve it up into separate mappings 
associated to segments of a file.  This fails on Cygwin.  Here's a test 
case that illustrates the problem:

$ truncate -s 64k foo

$ cat mmap_test.c
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/mman.h>

const size_t page_size = 64 * 1024;

int
main ()
{
   void *mem = mmap (NULL, 2 * page_size,
                     PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
   if (mem == MAP_FAILED)
     {
       perror ("mmap");
       exit (1);
     }
   int fd = open ("foo", O_RDONLY);
   void *res = mmap (mem, page_size, PROT_READ | PROT_WRITE,
                     MAP_PRIVATE | MAP_FIXED, fd, 0);
   if (res == MAP_FAILED)
     {
       perror ("mmap");
       exit (2);
     }
}

$ gcc mmap_test.c

$ ./a
mmap: Invalid argument

$ echo $?
2

Is this a bug, or is it simply a limitation of Cygwin's mmap?  If the 
latter, is there a simple workaround?

Ken

P.S. For context, you can start reading here, but it might not make a 
lot of sense:

   https://lists.gnu.org/archive/html/emacs-devel/2018-02/msg00440.html

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