Re: 1.3.2-1: mmap and MAP_PRIVATE|MAP_FIXED|MAP_ANON

Re: 1.3.2-1: mmap and MAP_PRIVATE|MAP_FIXED|MAP_ANON

[Kandziora Jan]

>
> > You are right. But it does *not* return MAP_FAILED, it returns
0x60001000,
> > which is a valid address.
>
> Ok, I figured out what's going wrong.  A fix will be in the next
> developers snapshot.
>
For all you who are interested: I was able to walk around this problem by
using a disk file for each area and mmap that file with
MAP_PRIVATE|MAP_FIXED, avoiding MAP_ANON. The disk file must be at least as
long as requested in the following mmap call, though; But that can be done
easily by creating, lseeking, writing and immediately unlinking it from
within the program.

	Jan




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Re: 1.3.2-1: mmap and MAP_PRIVATE|MAP_FIXED|MAP_ANON

[Corinna Vinschen]

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On Tue, Sep 18, 2001 at 11:02:22AM +0200, Kandziora Jan wrote:
> 	if ((buf2=mmap((void*)0x70000000,1,
> 	
> PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_FIXED|MAP_ANON,0,0))==NULL)
                                                             ^^^^
							     Hmm!

$ man mmap
[...]
RETURN VALUE
       On success, mmap returns a pointer to the mapped area.  On
       error,  MAP_FAILED  (-1)  is  returned,  and  errno is set
       appropriately...

> Is it impossible with windows to mmap a couple of anonymous areas anywhere I
> want and no memory is already mapped?

More or less.  Windows is somewhat bulky in giving you the area
you want.  However:

$ man mmap
[...]
       MAP_FIXED  Do  not select a different address than the one
                  specified.  If the specified address cannot  be
                  used,  mmap  will fail.  If MAP_FIXED is speci­
                  fied, start must be a multiple of the pagesize.
                  Use of this option is discouraged.
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                  Note that.  A normal application should have no
                  need ever to desire a specific address.

Corinna

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Corinna Vinschen                  Please, send mails regarding Cygwin to
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Red Hat, Inc.

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1.3.2-1: mmap and MAP_PRIVATE|MAP_FIXED|MAP_ANON

[Kandziora Jan]

Hello,

I got a problem using mmap with MAP_PRIVATE|MAP_FIXED|MAP_ANON several times
in a program. It does not reserve the desired address range the second time
I call it, neither it returns an error. The test case is:

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>

int main(int argc, char* argv[])
{
	int   fd1;
	void *buf1;
	void *buf2;

/*
	if ((fd1=open("file1",O_RDWR))<0)
	{
		perror("f1");
		exit(EXIT_FAILURE);
	}
*/

	if ((buf1=mmap((void*)0x60000000,1,
	
PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_FIXED|MAP_ANON,0,0))==NULL)
//		PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_FIXED,fd1,0))==NULL)
	{
		perror("m1");
		exit(EXIT_FAILURE);
	}

	if ((buf2=mmap((void*)0x70000000,1,
	
PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_FIXED|MAP_ANON,0,0))==NULL)
	{
		perror("m2");
		exit(EXIT_FAILURE);
	}


	printf("buf1: %p\n",buf1);
	printf("buf2: %p\n",buf2);

	return 0;
}

The program prints
	buf1: 0x60000000
	buf2: 0x60001000
and not as desired 
	buf1: 0x60000000
	buf2: 0x70000000

Moreover, I get the desired output and can use the mapped memory if I do not
use MAP_ANON with the first mmap.

Is it impossible with windows to mmap a couple of anonymous areas anywhere I
want and no memory is already mapped?
At least cygwin should return an error if the desired address and the
address returned by the windows system call do not match and MAP_FIXED is
returned. That problem was not easy to find.

My system:

WinNT Ver 4.0 build 1381 Service Pack 6
   Cygwin DLL version info:
        dll major: 1003
        dll minor: 2
        dll epoch: 19
        dll bad signal mask: 19005
        dll old termios: 5
        dll malloc env: 28
        api major: 0
        api minor: 39
        shared data: 3
        dll identifier: cygwin1
       build date: Sun May 20 23:28:17 EDT 2001

Thanks in advance

	Jan Kandziora


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