359 lines
13 KiB
Plaintext
359 lines
13 KiB
Plaintext
.\" Use -ms
|
|
.de Cs
|
|
.IP
|
|
.nf
|
|
.ft C
|
|
..
|
|
.de Ce
|
|
.ft P
|
|
.fi
|
|
..
|
|
.de Bu
|
|
.br
|
|
.ti +.5i
|
|
.ie \\n(.$ \\$1
|
|
.el \\(bu
|
|
..
|
|
.EH ''''
|
|
.OH ''''
|
|
.EF ''''
|
|
.OF ''''
|
|
.ps 10
|
|
.nr PS 10
|
|
\&
|
|
.TL
|
|
\s+2\fBMIT-SHM\(emThe MIT Shared Memory Extension\fP\s-2
|
|
.sp
|
|
How the shared memory extension works
|
|
.AU
|
|
Jonathan Corbet
|
|
.AI
|
|
Atmospheric Technology Division
|
|
National Center for Atmospheric Research
|
|
corbet@ncar.ucar.edu
|
|
.sp
|
|
Formatted and edited for release 5 by
|
|
.AU
|
|
Keith Packard
|
|
.AI
|
|
MIT X Consortium
|
|
.ps 9
|
|
.nr PS 9
|
|
.sp 8
|
|
.AB
|
|
This document briefly describes how to use the MIT-SHM shared memory
|
|
extension. I have tried to make it accurate, but it would not surprise me
|
|
if some errors remained. If you find anything wrong, do let me know and I
|
|
will incorporate the corrections. Meanwhile, please take this document ``as
|
|
is''\(eman improvement over what was there before, but certainly not the
|
|
definitive word.
|
|
.AE
|
|
.LP
|
|
.DS C
|
|
Copyright \(co 1991 X Consortium
|
|
.DE
|
|
.sp 2
|
|
.LP
|
|
Permission is hereby granted, free of charge, to any person obtaining a copy
|
|
of this software and associated documentation files (the ``Software''), to deal
|
|
in the Software without restriction, including without limitation the rights
|
|
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
|
|
copies of the Software, and to permit persons to whom the Software is
|
|
furnished to do so, subject to the following conditions:
|
|
.LP
|
|
The above copyright notice and this permission notice shall be included in
|
|
all copies or substantial portions of the Software.
|
|
.LP
|
|
THE SOFTWARE IS PROVIDED ``AS IS'', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
|
|
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
|
|
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
|
|
X CONSORTIUM BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
|
|
AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
|
|
CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
|
|
.LP
|
|
Except as contained in this notice, the name of the X Consortium shall not be
|
|
used in advertising or otherwise to promote the sale, use or other dealings
|
|
in this Software without prior written authorization from the X Consortium.
|
|
.ps 10
|
|
.nr PS 10
|
|
.bp 1
|
|
.de PT
|
|
.tl ''MIT Shared Memory Extension''
|
|
..
|
|
.de BT
|
|
.tl ''\fB % \fP''
|
|
..
|
|
.NH 1
|
|
REQUIREMENTS
|
|
.LP
|
|
The shared memory extension is provided only by some X servers. To find out
|
|
if your server supports the extension, use xdpyinfo(1). In particular, to
|
|
be able to use this extension, your system must provide the SYSV shared
|
|
memory primitives. There is not an mmap-based version of this extension.
|
|
To use shared memory on Sun systems, you must have built your kernel with
|
|
SYSV shared memory enabled -- which is not the default configuration.
|
|
Additionally, the shared memeory maximum size will need to be increased on
|
|
both Sun and Digital systems; the defaults are far too small for any useful
|
|
work.
|
|
.NH 1
|
|
WHAT IS PROVIDED
|
|
.LP
|
|
The basic capability provided is that of shared memory XImages. This is
|
|
essentially a version of the ximage interface where the actual image data
|
|
is stored in a shared memory segment, and thus need not be moved through
|
|
the Xlib interprocess communication channel. For large images, use of this
|
|
facility can result in some real performance increases.
|
|
.LP
|
|
Additionally, some implementations provided shared memory pixmaps. These
|
|
are 2 dimensional arrays of pixels in a format specified by the X server,
|
|
where the image data is stored in the shared memory segment. Through use of
|
|
shared memory pixmaps, it is possible to change the contents of these
|
|
pixmaps without using any Xlib routines at all. Shared memory pixmaps can
|
|
only be supported when the X server can use regular virtual memory for
|
|
pixmap data; if the pixmaps are stored in some magic graphics hardware, your
|
|
application will not be able to share them with the server. Xdpyinfo(1)
|
|
doesn't print this particular nugget of information.
|
|
.NH 1
|
|
HOW TO USE THE SHARED MEMORY EXTENSION
|
|
.LP
|
|
Code which uses the shared memory extension must include a number of header
|
|
files:
|
|
.Cs
|
|
# include <X11/Xlib.h> /* of course */
|
|
# include <sys/ipc.h>
|
|
# include <sys/shm.h>
|
|
# include <X11/extensions/XShm.h>
|
|
.Ce
|
|
.LP
|
|
Of course, if the system you are building on does not support shared
|
|
memory, the file XShm.h may not be present. You may want to make
|
|
liberal use of #ifdefs.
|
|
.LP
|
|
Any code which uses the shared memory extension should first check to see
|
|
that the server provides the extension. You could always be running over
|
|
the net, or in some other environment where the extension will not work.
|
|
To perform this check, call either
|
|
.Cs
|
|
Status XShmQueryExtension (display)
|
|
Display *display
|
|
.Ce
|
|
.LP
|
|
or
|
|
.Cs
|
|
Status XShmQueryVersion (display, major, minor, pixmaps)
|
|
Display *display;
|
|
int *major, *minor;
|
|
Bool *pixmaps
|
|
.Ce
|
|
.LP
|
|
Where ``display'' is, of course, the display on which you are running. If
|
|
the shared memory extension may be used, the return value from either
|
|
function will be True; otherwise your program should operate using
|
|
conventional Xlib calls. When the extension is available,
|
|
\fCXShmQueryVersion\fP also returns ``major'' and ``minor'' which are the
|
|
version numbers of the extension implementation, and ``pixmaps'' which is
|
|
True iff shared memory pixmaps are supported.
|
|
.NH 1
|
|
USE OF SHARED MEMORY XIMAGES
|
|
.LP
|
|
The basic sequence of operations for shared memory XImages is as follows:
|
|
.LP
|
|
.Bu "1 \-"
|
|
Create the shared memory XImage structure
|
|
.Bu "2 \-"
|
|
Create a shared memory segment to store the image data
|
|
.Bu "3 \-"
|
|
Inform the server about the shared memory segment
|
|
.Bu "4 \-"
|
|
Use the shared memory XImage, much like a normal one.
|
|
.LP
|
|
To create a shared memory XImage, use:
|
|
.Cs
|
|
XImage *XShmCreateImage (display, visual, depth, format, data,
|
|
shminfo, width, height)
|
|
Display *display;
|
|
Visual *visual;
|
|
unsigned int depth, width, height;
|
|
int format;
|
|
char *data;
|
|
XShmSegmentInfo *shminfo;
|
|
.Ce
|
|
.LP
|
|
Most of the arguments are the same as for XCreateImage; I will not go
|
|
through them here. Note, however, that there are no ``offset'', ``bitmap_pad'',
|
|
or ``bytes_per_line'' arguments. These quantities will be defined by the
|
|
server itself, and your code needs to abide by them. Unless you have already
|
|
allocated the shared memory segment (see below), you should pass in NULL for
|
|
the ``data'' pointer.
|
|
.LP
|
|
There is one additional argument: ``shminfo'', which is a pointer to a
|
|
structure of type XShmSegmentInfo. You must allocate one of these
|
|
structures such that it will have a lifetime at least as long as that of
|
|
the shared memory XImage. There is no need to initialize this structure
|
|
before the call to XShmCreateImage.
|
|
.LP
|
|
The return value, if all goes well, will be an XImage structure, which you
|
|
can use for the subsequent steps.
|
|
.LP
|
|
The next step is to create the shared memory segment. This is
|
|
best done after the creation of the XImage, since you need to make use of
|
|
the information in that XImage to know how much memory to allocate. To
|
|
create the segment, you need a call like:
|
|
.Cs
|
|
shminfo.shmid = shmget (IPC_PRIVATE,
|
|
image->bytes_per_line * image->height, IPC_CREAT|0777);
|
|
.Ce
|
|
.LP
|
|
(assuming that you have called your shared memory XImage ``image''). You
|
|
should, of course, follow the Rules and do error checking on all of these
|
|
system calls. Also, be sure to use the bytes_per_line field, not the width
|
|
you used to create the XImage as they may well be different.
|
|
.LP
|
|
Note that the shared memory ID returned by the system is stored in the
|
|
shminfo structure. The server will need that ID to attach itself to the
|
|
segment.
|
|
.LP
|
|
Also note that, on many systems for security reasons, the X server
|
|
will only accept to attach to the shared memory segment if it's
|
|
readable and writeable by ``other''. On systems where the X server is
|
|
able to determine the uid of the X client over a local transport, the
|
|
shared memory segment can be readable and writeable only by the uid of
|
|
the client.
|
|
.LP
|
|
Next, attach this shared memory segment to your process:
|
|
.Cs
|
|
shminfo.shmaddr = image->data = shmat (shminfo.shmid, 0, 0);
|
|
.Ce
|
|
.LP
|
|
The address returned by shmat should be stored in *both* the XImage
|
|
structure and the shminfo structure.
|
|
.LP
|
|
To finish filling in the shminfo structure, you need to decide how you want
|
|
the server to attach to the shared memory segment, and set the ``readOnly''
|
|
field as follows. Normally, you would code:
|
|
.Cs
|
|
shminfo.readOnly = False;
|
|
.Ce
|
|
.LP
|
|
If you set it to True, the server will not be able to write to this
|
|
segment, and thus XShmGetImage calls will fail.
|
|
.LP
|
|
Finally, tell the server to attach to your shared memory segment with:
|
|
.Cs
|
|
Status XShmAttach (display, shminfo);
|
|
.Ce
|
|
.LP
|
|
If all goes well, you will get a non-zero status back, and your XImage is
|
|
ready for use.
|
|
.LP
|
|
To write a shared memory XImage into an X drawable, use XShmPutImage:
|
|
.Cs
|
|
Status XShmPutImage (display, d, gc, image, src_x, src_y,
|
|
dest_x, dest_y, width, height, send_event)
|
|
Display *display;
|
|
Drawable d;
|
|
GC gc;
|
|
XImage *image;
|
|
int src_x, src_y, dest_x, dest_y;
|
|
unsigned int width, height;
|
|
bool send_event;
|
|
.Ce
|
|
.LP
|
|
The interface is identical to that of XPutImage, so I will spare my fingers
|
|
and not repeat that documentation here. There is one additional parameter,
|
|
however, called ``send_event''. If this parameter is passed as True, the
|
|
server will generate a ``completion'' event when the image write is complete;
|
|
thus your program can know when it is safe to begin manipulating the shared
|
|
memory segment again.
|
|
.LP
|
|
The completion event has type XShmCompletionEvent, which is defined as the
|
|
following:
|
|
.Cs
|
|
typedef struct {
|
|
int type; /* of event */
|
|
unsigned long serial; /* # of last request processed */
|
|
Bool send_event; /* true if came from a SendEvent request */
|
|
Display *display; /* Display the event was read from */
|
|
Drawable drawable; /* drawable of request */
|
|
int major_code; /* ShmReqCode */
|
|
int minor_code; /* X_ShmPutImage */
|
|
ShmSeg shmseg; /* the ShmSeg used in the request */
|
|
unsigned long offset; /* the offset into ShmSeg used */
|
|
} XShmCompletionEvent;
|
|
.Ce
|
|
.LP
|
|
The event type value that will be used can be determined at run time with a
|
|
line of the form:
|
|
.Cs
|
|
int CompletionType = XShmGetEventBase (display) + ShmCompletion;
|
|
.Ce
|
|
.LP
|
|
If you modify the shared memory segment before the arrival of the
|
|
completion event, the results you see on the screen may be inconsistent.
|
|
.LP
|
|
To read image data into a shared memory XImage, use the following:
|
|
.Cs
|
|
Status XShmGetImage (display, d, image, x, y, plane_mask)
|
|
Display *display;
|
|
Drawable d;
|
|
XImage *image;
|
|
int x, y;
|
|
unsigned long plane_mask;
|
|
.Ce
|
|
.LP
|
|
Where ``display'' is the display of interest, ``d'' is the source drawable,
|
|
``image'' is the destination XImage, ``x'' and ``y'' are the offsets within
|
|
``d'', and ``plane_mask'' defines which planes are to be read.
|
|
.LP
|
|
To destroy a shared memory XImage, you should first instruct the server to
|
|
detach from it, then destroy the segment itself, as follows:
|
|
.Cs
|
|
XShmDetach (display, shminfo);
|
|
XDestroyImage (image);
|
|
shmdt (shminfo.shmaddr);
|
|
shmctl (shminfo.shmid, IPC_RMID, 0);
|
|
.Ce
|
|
.NH 1
|
|
USE OF SHARED MEMORY PIXMAPS
|
|
.LP
|
|
Unlike X images, for which any image format is usable, the shared memory
|
|
extension supports only a single format (i.e. XYPixmap or ZPixmap) for the
|
|
data stored in a shared memory pixmap. This format is independent of the
|
|
depth of the image (for 1-bit pixmaps it doesn't really matter what this
|
|
format is) and independent of the screen. Use XShmPixmapFormat to get the
|
|
format for the server:
|
|
.Cs
|
|
int XShmPixmapFormat (display)
|
|
Display *display;
|
|
.Ce
|
|
.LP
|
|
If your application can deal with the server pixmap data format (including
|
|
bits-per-pixel et al.), create a shared memory segment and ``shminfo''
|
|
structure in exactly the same way as is listed above for shared memory
|
|
XImages. While it is, not strictly necessary to create an XImage first,
|
|
doing so incurs little overhead and will give you an appropriate
|
|
bytes_per_line value to use.
|
|
.LP
|
|
Once you have your shminfo structure filled in, simply call:
|
|
.Cs
|
|
Pixmap XShmCreatePixmap (display, d, data, shminfo, width,
|
|
height, depth);
|
|
Display *display;
|
|
Drawable d;
|
|
char *data;
|
|
XShmSegmentInfo *shminfo;
|
|
unsigned int width, height, depth;
|
|
.Ce
|
|
.LP
|
|
The arguments are all the same as for XCreatePixmap, with two additions:
|
|
``data'' and ``shminfo''. The second of the two is the same old shminfo
|
|
structure that has been used before. The first is the pointer to the shared
|
|
memory segment, and should be the same as the shminfo.shmaddr field. I am
|
|
not sure why this is a separate parameter.
|
|
.LP
|
|
If everything works, you will get back a pixmap, which you can manipulate in
|
|
all of the usual ways, with the added bonus of being able to tweak its
|
|
contents directly through the shared memory segment. Shared memory pixmaps
|
|
are destroyed in the usual manner with XFreePixmap, though you should detach
|
|
and destroy the shared memory segment itself as shown above.
|