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To: "Dirk Bull" <dirkbull102@hotmail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: shmat problem
References: <F10zrLCCtgx8VUkRacU000005ae@hotmail.com>
From: Doug McNaught <doug@mcnaught.org>
Date: 06 Jan 2003 10:32:50 -0500
In-Reply-To: "Dirk Bull"'s message of "Mon, 06 Jan 2003 14:53:41 +0000"
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"Dirk Bull" <dirkbull102@hotmail.com> writes:

> Doug, thanks for the reply. I've set SHM_RND in the call and used
> "__attribute__ ((aligned(4096)))" during the the declaration of
> variable global01_
> (as shown below) such that it is aligned on a page boundary. I'm
> porting code that was
> written for a Unix system to Linux and the example shown below is how
> the code is
> implemented on Unix.

Hmmm, I may be ignorant, but I don't see how the below could work.
You're asking for the kernel to map your shared memory in the middle
of an already-mapped area (the bss segment).  I'm actually surprised
it works on any Unix.

If you can point out a clear violation of the POSIX spec, someone may
be willing to change it, but it's not clear to me that you're
guaranteed to be able to map a shared memory area inside your existing
data segment.

> union {
> 	long IN[2048];
> } global01_ __attribute__ ((aligned(4096)));

[...]

> 	if ( (shmptr = shmat(shmid, &global01_, SHM_RND)) == (void *) -1)
> 		printf("shmat error: %d %s\n",errno, strerror(errno));
> 	else
> 		printf("shared memory attached from %x to %x\n",
> 				shmptr, shmptr+sizeof(global01_));

-Doug
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