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Kevin Ryde > X11-Protocol-Other > X11::Protocol::Ext::MIT_SHM



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X11::Protocol::Ext::MIT_SHM - images in SysV style shared memory


 use X11::Protocol;
 my $X = X11::Protocol->new;
   or print "MIT-SHM extension not available";

 use IPC::SysV;
 my $shmid = shmget (IPC::SysV::IPC_PRIVATE(),
                     100000,  # bytes
                     IPC::SysV::IPC_CREAT() | 0666);

 my $shmseg = $X->new_rsrc;
 $X->MitShmAttach ($shmseg, $shmid, 0);

 my ($depth, $visual, $size) =  $X->MitShmGetImage
       ($window, 0,0, 16,16, ~0, 'ZPixmap', $shmseg, 0);
 my $image_bytes;
 shmread ($shmid, $image_bytes, 0, 16*16*$bpp) || die "$!";

 # $image_bytes is the top-left 16x16 pixels of the screen


The MIT-SHM extension allows a client and server running on the same machine to transfer image data through System-V shared memory segments.

The client creates a memory segment with shmget() (see "shmget" in perlfunc and "SysV IPC" in perlipc) and asks the server to attach to it and then read or write with equivalents to the core GetImage and PutImage.

The aim is to avoid sending large images through the I/O connection when on the same machine. Memory is faster, and may help avoid request size limits for very big images.

Byte order, padding, etc, required or generated in images is specified by the server $X->{'image_byte_order'}, $X->{'pixmap_formats'}, etc, the same as for the core GetImage() and PutImage(). It's up to the client to adapt to the server's layout, which can be a bit of a chore.

Shm from Perl

A shared memory segment can be created from Perl with shmget(), then read or write its contents with shmread() and shmwrite(). Those functions attach and detach it each time with shmat() and shmdt() system calls, which is fine for grabbing the lot, but will be a bit slow for lots of little accesses.

IPC::SysV (version 2 up) offers a shmat() to keep the block attached and memread() and memwrite() to access it (see IPC::SysV). See IPC::SharedMem for an object-oriented wrapper around this too.

Incidentally, if shmget() is not available on the system then Perl's shmget() croaks. It's always possible for it to return undef too for not enough memory etc. With that, not being on the same machine, not having identifiable perms, etc, there's a quite a few cases where a fallback to plain I/O will be necessary.

Shm Permissions

A SysV shared memory segment has owner/group/other permission bits similar to a file. The server will only attach to segments which the requesting client UID/GID has permission to read or write.

The server can usually determine a client's UID/GID on a local connection such as Unix socket (X11::Protocol::Connection::UNIXSocket, and SO_PEERCRED in socket(7)), and perhaps on a TCP localhost loopback. Failing that the server treats the client as "other" and will only attach to world-readable (or world read-writable) segments.

You can make a shm segment world-readable to ensure the server can read it. If the data for a PutImage etc is already from a world-readable file or is public then it doesn't matter who else reads the segment. Remember to ask for read-only in the MitShmAttach() so the server doesn't want writable too.

There's probably no need to risk relaxing permissions for writing. Chances are that if client UID/GID can't be identified then it's because the connection is not local and the server is on a different machine so shared memory can't be used anyway.

It's usual for the server to run as root, hence it's own permission checks, but it's also possible for the server to be an ordinary user. In that case the shm segments it can access will be limited in the usual way for the user it's running as.


The following requests are available after an init_extension() per "EXTENSIONS" in X11::Protocol.

    my $bool = $X->init_extension('MIT-SHM');

In the following $shmid is the shared memory ID (an integer) as obtained from the kernel with shmget(). $shmseg is an XID (allocated as usual by client $X->new_rsrc()) on the server representing the server attachment to the block.

($server_major, $server_minor, $uid, $gid, $shared_pixmaps, $pixmap_format) = $X->MitShmQueryVersion ()

Return information about the MIT-SHM extension. Unlike other extensions there's no client version / server version negotiation.

$server_major and $server_minor are the extension version number implemented by the server.

$uid and $gid (integers) are the server's effective user ID and group ID (geteuid() and getegid()). Zero means root.

$shared_pixmaps is non-zero if pixmaps in shared memory are supported (see MitShmCreatePixmap() below). $pixmap_format (an ImageFormat) is "XYPixmap" or "ZPixmap" for the layout required in such a pixmap.

$X->MitShmAttach ($shmseg, $shmid, $readonly)

Attach the server to a given shared memory segment. $shmseg is a new XID to represent the attached memory.

    my $shmseg = $X->new_rsrc;
    $X->MitShmAttach ($shmseg, $shmid, 0); # read/write

$shmid is the shared memory ID to attach, as obtained from shmget() (see "shmget" in perlfunc).

$readonly is 1 to have the server attach read-only, or 0 for read-write. Read-only suffices for MitShmPutImage(), but read-write is needed for MitShmGetImage() and MitShmCreatePixmap().

$X->MitShmDetach ($shmseg)

Detach the server from shared memory $shmseg (an XID) and release that XID.

    $X->MitShmDetach ($shmseg);
$X->MitShmPutImage ($drawable, $gc, $depth, $total_width, $total_height, $src_x, $src_y, $src_width, $src_height, $dst_x, $dst_y, $format, $send_event, $shmseg, $offset)

Put image data from $shmseg (an XID) to $drawable. The parameters are similar to the core PutImage().

$depth is the depth of the image. For $format "Bitmap" this must be 1 and the foreground and background colours of $gc are then drawn. For $format "XYPixmap" and "ZPixmap" $depth must be the depth of $drawable.

$total_width,$total_height is the full size of the image in the shared memory. $src_x,$src_y and $src_width,$src_height are the portion of it to draw. $dst_x,$dst_y is where in $drawable to put it.

$format is "Bitmap", "XYPixmap" or "ZPixmap" (an ImageFormat).

$send_event is 1 to have an MitShmCompletionEvent sent to the client when drawing is finished (see "EVENTS" below), or 0 if that's not wanted.

$offset is a byte offset into the shared memory where the image starts.

($depth, $visual, $size) = $X->MitShmGetImage ($drawable, $x, $y, $width, $height, $planemask, $format, $shmseg, $offset)

Copy an image from $drawable to shared memory $shmseg (an XID). The parameters are similar to the core GetImage().

$x,$y, $width,$height are the part of $drawable to get. $planemask is a bit mask for which bit planes of the pixels are wanted.

$format is "XYPixmap" or "ZPixmap" for the layout to be written to the shared memory, and $offset is a byte offset into the memory where the image should start.

The returned $depth (an integer) is the depth of $drawable. $visual (integer ID) is its visual for a window, or "None" for a pixmap. $size is how many bytes were written.

$shmseg must be attached read-write in the MitShmAttach() or an Access error results.

$X->MitShmCreatePixmap ($pixmap, $drawable, $depth, $width, $height, $shmseg, $offset)

Create $pixmap (a new XID) as a pixmap with contents in shared memory $shmseg (an XID). When the client reads or writes that memory it changes the pixmap contents. The parameters are similar to the core CreatePixmap().

    my $pixmap = $X->new_rsrc;
    $X->MitShmCreatePixmap ($pixmap,         # new XID
                            $X->root,        # for the screen
                            $X->root_depth,  # depth
                            10,10,           # width,height
                            0);      # byte offset into shm

The MitShmQueryVersion() request above reports whether shared memory pixmaps are supported, and if so whether they're "XYPixmap" or "ZPixmap" layout.

$drawable is used to determine the screen for the new $pixmap. $offset is a byte offset into the shared memory where the pixmap data will begin.

If damage objects from the DAMAGE extension (see X11::Protocol::Ext::DAMAGE) are monitoring a shared $pixmap then client writes to the shared memory generally don't produce DamageNotify events. The client can use DamageAdd() requests (in Damage version 1.1) to tell the server about changes made, which it will broadcast to interested damage objects. It's probably unusual to have damage objects listening to a shared pixmap though.


MitShmCompletionEvent is sent to the client when requested in an MitShmPutImage(). It says the server has finished reading the memory. The event has the usual fields

    name             "MitShmCompletionEvent"
    synthetic        true if from a SendEvent
    code             integer opcode
    sequence_number  integer

and event-specific fields

    drawable       XID, target as from request
    shmseg         XID, source as from request
    offset         integer, byte offset as from request
    major_opcode   integer, MIT-SHM extension code
    minor_opcode   integer, 3 for MitShmPutImage

major_opcode and minor_opcode are the codes of the originating MitShmPutImage(). These fields are similar to the core GraphicsExposure and NoExposure events, but here there's only one request (MitShmPutImage()) which gives a completion event.


Error type "ShmSeg" is a bad $shmseg resource XID in a request.


X11::Protocol, "shmget" in perlfunc, "SysV IPC" in perlipc, IPC::SysV, IPC::SharedMem


/usr/share/doc/x11proto-xext-dev/shm.txt.gz, /usr/share/X11/doc/hardcopy/Xext/mit-shm.PS.gz



Copyright 2011 Kevin Ryde

X11-Protocol-Other is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.

X11-Protocol-Other is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with X11-Protocol-Other. If not, see <>.

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