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Date: Mon, 28 Nov 2005 12:55:05 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: mmap() on 64K aligned address fails
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On Nov 28 12:51, Corinna Vinschen wrote:
> On Nov 27 16:47, Ren? Berber wrote:
> > Samuel Thibault wrote:
> > [snip]
> > > Memmapping to malloced memory could very well be forbidden. Why would
> > > one want to do this?
> > 
> > As I said before, the test is one that comes with gcc-4.0.2, it fails only on
> > Windows as far as I know.  To answer your question: is just a test, it should
> > work with some Windows related tweaking but it's just a test.
> > 
> > Besides, what other memory is there available that a process has under its
> > control?  Yes, I know, mmap() should be used w/o MAP_FIXED and let the OS choose
> > where to map, but the interesting part of this test is to use MAP_FIXED.
> 
> The problem is that the application using MAP_FIXED has no idea if the
> address it's using is *allowed*.  There might be already a mapping
> active, maybe just the application's code is mapped at that point, or
> the address is generally invalid for shared memory usage.
> 
> Many mmap tests make such invalid assumptions about the memory.  Just
> because they are part of gcc doesn't make them right.  The only blessed
> usage of MAP_FIXED is if the application knows that the memory address
> is valid.  One way to do this is to establish a non-FIXED mmap first,
> like this:
> 
>   #include <stdio.h>
>   #include <stdlib.h>
>   #include <errno.h>
>   #include <sys/mman.h>
> 
>   int
>   main ()
>   {
>     int pages = 10;
>     void *addr, *addr2;
> 
>     addr = mmap (NULL, 2 * pages * getpagesize(), PROT_READ|PROT_WRITE,
> 		 MAP_SHARED|MAP_ANONYMOUS, -1, 0);
>     if (addr == MAP_FAILED)
>       fprintf (stderr, "Gargl\n");
>     else
>       {
> 	munmap (addr, pages * getpagesize());
> 	addr2 = mmap (addr, pages * getpagesize(), PROT_READ|PROT_WRITE,
> 		      MAP_SHARED|MAP_FIXED|MAP_ANONYMOUS, -1, 0);
> 	if (addr2 == MAP_FAILED)
> 	  fprintf (stderr, "Frtzl\n");
> 	else if (addr2 != addr)
> 	  fprintf (stderr, "Urgl\n");
> 	else
> 	  fprintf (stderr, "Haha\n");
>       }
>     return 0;
>   }

Btw., the above test won't work on Cygwin 1.5.18 due to a incorrect
test in Cygwin's mmap implementation.  This will be fixed in 1.5.19.
For a start, mmap/munmap the whole page area instead just the half of
it.  That should work.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat, Inc.

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