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Date: Fri, 25 Feb 2005 13:00:45 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: mmap and MAP_FIXED
Message-ID: <20050225120045.GO18314@cygbert.vinschen.de>
Reply-To: cygwin AT cygwin DOT com
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References: <421E60A0 DOT 8000102 AT stambulchik DOT net>
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On Feb 25 01:17, Evgeny Stambulchik wrote:
> Hello,
> 
> There is something strange about mmap(addr, ..., MAP_FIXED) under 
> Cygwin. For a reason, it always fails if addr is not on a 16xPAGE_SIZE 
> boundary. Here is a short demo:
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <errno.h>
> #include <string.h>
> #include <sys/mman.h>
> 
> #define NRUNS                 20
> #define CHUNK_SIZE          4096
> #define START_ADDR      0x640000
> 
> int main(void)
> {
>     unsigned int i, pagesize, prot, flags;
>     void *addr, *maddr;
> 
>     pagesize = getpagesize();
>     prot = PROT_READ | PROT_WRITE | PROT_EXEC;
>     flags = MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED;
> 
>     for (i = 0; i < NRUNS; i++) {
>         addr = (void *) START_ADDR + pagesize*i;
>         maddr = mmap(addr, CHUNK_SIZE, prot, flags, 0, 0);
>         if (maddr != MAP_FAILED) {
>             fprintf(stderr, "OK:   %p -> %p\n", addr, maddr);
>         } else {
>             fprintf(stderr, "FAIL: %p (%s)\n", addr, strerror(errno));
>         }
>     }
>     exit(0);
> }
> 
> And the output is:
> 
> OK:   0x640000 -> 0x640000

Pure coincidence.  You shouldn't just use some arbitrary address and
use MAP_FIXED with it, otherwise you're making invaild assumptions
about the memory layout of your machine/os/application.

The reason that MAP_FAILED only works on 64K boundaries so far is,
that I didn't handle this case.  Usually there are not many good
reasons to use MAP_FIXED.  However, I've checked in a patch which
tries to handle MAP_FIXED on 4K boundaries, but only in the anonymous
case.


Corinna

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

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