Some weeks after Neal Walfield [interview] implemented POSIX semaphores for libpthread [story], Marcus Brinkmann has now implemented SysV shared memory for the GNU Hurd in glibc, based on some earlier work done by Neal. This was probably the last commonly used POSIX feature missing in the Hurd, and having shared memory available will make several programs work much better. Success has already been reported with mplayer; it was very cludgy before and works smoothly with shared memory enabled.

From: Marcus Brinkmann
Subject: [PATCH] system V shared memory support for the Hurd in glibc
Date: Mon, 11 Jul 2005 23:52:35 +0200 

Hi,

this patch is for glibc, but it needs to get more testing before it
can go in.  Please give it a good beating.

This patch adds System V Shared Memory support to the GNU Hurd.

There is a small extension to the filesystem interface that allows the
filesystem to report to the application if the file may currently be
mapped by other applications.  A separate patch to report this
information from ext2fs and other filesystems will follow.

These features are implemented:
Creating shared memory segments.
Attaching to them.
Detaching from them.
Removing them.
Get the uid/gid/mode.

These features are not, and can't be within the standard filesystem
interface:
Getting the exact number of nattch (not even if it is just 1).
Getting pid/lpid (always zero).
Getting time stamps (last attach, last detach, last modification).
Getting the cuid/cgid (always just uid/gid).
Verifying the cuid/cgid when accessing.

Compatibility note:
Most programs should work.
POSIX says RMID removes all mappings, but no system does it that way
(and neither do we).

From: Marcus Brinkmann
Subject: Re: [PATCH] system V shared memory support for the Hurd in glibc
Date: Tue, 12 Jul 2005 00:20:05 +0200

At Mon, 11 Jul 2005 23:52:35 +0200, Marcus wrote:
>
> Hi,
>
> this patch is for glibc, but it needs to get more testing before it
> can go in.  Please give it a good beating.

I should add some information on how to test this.

The shm node is /dev/shm.  Normally, tmpfs is installed on this node
by MAKEDEV.  But this won't work yet, because tmpfs is broken (d'oh).

So, you put on the node an ext2fs filesystem and it will work.  For
example, you can use a ramdisk.  Or just don't set any translator on
it and use the root filesystem.  In either case, you need to use the
--no-inherit-dir-group (--sysvgroups) option so the group permissions
will be right.  The permissions on the node should be like for the
/tmp directory: world rwx, sticky bit set.

Below is a temporary patch for ext2fs to support the bit used for
nattch.  Only few programs will require this, though.

Thanks,
Marcus