corbet@STOUT.ATD.UCAR.EDU (Jonathan Corbet) (10/12/90)
I've seen a number of questions in this group about the shared memory
extension, and whether documention exists for it. Well, I, too, have been
digging through the source (even though my name is not Luke) figuring this
thing out, so I thought I would just go ahead and put my findings down on
phosphor for others to use. So, enclosed please find my first attempt at
a shared memory extension document.
Almost certainly some error remains somewhere in all this; please feel free
to politely report any you find back to me, and I'll update things accordingly.
Meanwhile, please realize that while I hope this stuff is useful, neither I
nor NCAR can possibly guarantee it's correctness, or whether or not it causes
cancer in laboratory rats.
Let me just say that for my application (display and animation of high
resolution observational atmospheric data), this extension has proved to
be tremendously valuable. Consider this a plea that shared memory be
retained in future versions of the X window system, preferably with some
sort of X Consortium blessing.
Jonathan Corbet
National Center for Atmospheric Research, Atmospheric Technology Division
corbet@stout.atd.ucar.edu
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How the shared memory extension works
Jonathan Corbet
Atmospheric Technology Division
National Center for Atmospheric Research
corbet@ncar.ucar.edu
This document briefly describes how the X11R4 shared memory extension works
-- at least on my Sun 3 and 4 machines. 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" -- an improvement over what was there
before, but certainly not the definitive word.
REQUIREMENTS
The shared memory extension, as far as I know, is provided only in some
versions of the MIT X11R4 implementation. 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.
WHAT IS PROVIDED
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.
Additionally, some implementations provided shared memory pixmaps. These
are simple pixmaps, 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.
Apparently not all implementations provide shared memory pixmaps -- even
some that provide the shared memory ximage interface. The Sun X server
does provide both.
HOW TO USE THE SHARED MEMORY EXTENSION
Code which uses the shared memory extension must include a number of header
files:
# include <X11/Xlib.h> /* of course */
# include <sys/ipc.h>
# include <sys/shm.h>
# include <X11/extensions/XShm.h>
Of course, if the system you are building on does not support shared
memory, then the file XShm.h may not be present, so you may want to make
liberal use of ifdefs.
Any code which uses the shared memory extension should first check to see
that the server provides said 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:
Status XShmQueryVersion (display, major, minor, pixmaps)
Display *display;
int *major, *minor;
Bool *pixmaps
Where "display" is, of course, the display on which you are running. If
the shared memory extension may be used, the return value will be True;
otherwise your program should operate using conventional Xlib calls. If
the extension is available, "major" and "minor" will contain the version
numbers of the extension implementation, and "pixmaps" will be True iff
shared memory pixmaps are supported.
USE OF SHARED MEMORY XIMAGES
The basic sequence of operations for shared memory XImages is as follows:
(1) create the shared memory XImage structure, (2) create a shared memory
segment to store the image data, (3) inform the server about the shared
memory segment, (4) use the shared memory XImage, much like a normal one.
To create a shared memory XImage, use:
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;
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.
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.
The return value, if all goes well, will be an XImage structure, which you
can use for the subsequent steps.
The first of these steps 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:
shminfo.shmid = shmget (IPC_PRIVATE,
image->bytes_per_line * image->height, IPC_CREAT|0777);
(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 -- they may well be different.
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.
Next, attach this shared memory segment to your process:
shminfo.shmaddr = image->data = shmat (shminfo.shmid, 0, 0);
The address returned by shmat should be stored in *both* the XImage
structure and the shminfo structure.
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:
shminfo.readOnly = False;
If you set it to True, the server will not be able to write to this
segment, and thus XShmGetImage calls will fail.
Finally, tell the server to attach to your shared memory segment with:
Status XShmAttach (display, shminfo);
If all goes well, you will get a true status back, and your XImage is ready
for use.
To write a shared memory XImage into an X drawable, use XShmPutImage:
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;
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.
The completion event has type XShmCompletionEvent, which is defined as the
following:
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;
The event type value that will be used can be determined at run time with a
line of the form:
int CompletionType = XShmGetEventBase (display) + ShmCompletion;
If you modify the shared memory segment before the arrival of the
completion event, the results you see on the screen may be inconsistent.
To read image data into a shared memory XImage, use the following:
Status XShmGetImage (display, d, image, x, y, plane_mask)
Display *display;
Drawable d;
XImage *image;
int x, y;
unsigned long plane_mask;
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.
To destroy a shared memory XImage, you should first instruct the server to
detach from it, then destroy the segment itself, as follows:
XShmDetach (display, shminfo);
XDestroyImage (image);
shmdt (shminfo.shmaddr);
shmctl (shminfo.shmid, IPC_RMID, 0);
USE OF SHARED MEMORY PIXMAPS
To create a shared memory pixmap, you must create a shared memory segment
and "shminfo" structure in exactly the same way as is listed above for
shared memory XImages. While it is, as far as I can tell, 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.
Once you have your shminfo structure filled in, simply call:
Pixmap XShmCreatePixmap (display, d, data, shminfo, width,
height, depth);
Display *display;
Drawable d;
char *data;
XShmSegmentInfo *shminfo;
unsigned int width, height, depth;
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, I believe, be the same as the shminfo.shmaddr
field. I am not sure why this is a separate parameter.
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
probably detach and destroy the shared memory segment itself as shown
above.