author Sam Lantinga <>
Fri, 29 Feb 2008 13:55:44 +0000
changeset 4139 568c9b3c0167
parent 386 2c5d4c22a2ac
permissions -rw-r--r--
* Added configure option --enable-screensaver, to allow enabling the screensaver by default.
* Use XResetScreenSaver() instead of disabling screensaver entirely.

Full discussion summary from Erik on the SDL mailing list:

Current behaviour

SDL changes the user's display power management settings without
permission from the user and without telling the user.

The interface that it uses to do so is DPMSDisable/DPMSEnable, which
should only ever be used by configuration utilities like KControl, never
by normal application programs, let alone by the libraries that they
use. Using an interface that is not at all intended for what SDL tries
to achieve means that it will not work as it should. Firstly, the power
management is completely disabled during the whole lifetime of the SDL
program, not only when it should be. Secondly, it makes SDL
non-reentrant, meaning that things will break when multiple SDL programs
are clients of the same X server simultaneously. Thirdly, no cleanup
mechanism ensures that the setting is restored if the client does not do
that (for example if it crashes).

In addition to that, this interface is broken on xorg,
[], so what SDL tries
to do does not work at all on that implementation of the X Window
System. (The reason that the DPMSEnable works in KControl is that it
calls DPMSSetTimeout immediately after,

The problems that the current behaviour causes
1. Information leak. When the user is away, someone might see what the
user has on the display when the user counts on the screensaver
preventing this. This does not even require physical access to the
workstation, it is enough to see it from a distance.
2. Draining battery. An SDL program that runs on a laptop will quickly
drain the battery while the user is away. The system will soon shut down
and require recharging before being usable again, while it should in
fact have consumed very little energy if the user's settings would have
been obeyed.
3. Wasting energy. Even if battery issues are not considered, energy as
such is wasted.
4. Display wear. The display may be worn out.

The problems that the current behaviour tries to solve

1. Preventing screensaver while playing movies.
Many SDL applications are media players. They have reasons to prevent
screensavers from being activated while a movie is being played. When a
user clicks on the play button it can be interpreted as saying "play
this movie, but do not turn off the display while playing it, because I
will watch it even though I do not interact with the system".

2. Preventing screensaver when some input bypasses X.
Sometimes SDL uses input from another source than the X server, so
that the X server is bypassed. This obviously breaks the screensaver
handling. SDL tries to work around that.

3. Preventing screensaver when all input bypasses X.
There is something called Direct Graphics Access mode, where a
program takes control of both the display and the input devices from the
X server. This obviously means that the X server can not handle the
screensaver alone, since screensaver handling depends on input handling.
SDL does not do what it should to help the X server to handle the
screensaver. Nor does SDL take care of screeensaver handling itself. SDL
simply disables the screensaver completely.

How the problems should be solved

The correct way for an application program to prevent the screensaver
under X is to call XResetScreenSaver. This was recently discovered and
implemented by the mplayer developers,
[]. SDL needs to
wrap this in an API call (SDL_ResetScreenSaver) and implement it for the
other video targets (if they do not have a corresponding call, SDL
should do what it takes on that particular target, for example sending
fake key events).

1. When a movie is played, the player should reset the screensaver when
the animation is advanced to a new frame. The same applies to anything
similar, like slideshows.

2. When the X server is handling input, it must handle all input
(keyboards, mice, gamepads, ...). This is necessary, not only to be able
to handle the screensaver, but also so that it can send the events to
the correct (the currently active) client. If there is an input device
that the X server can not handle for some reason (such as lack of Plug
and Play capability), the program that handles the device as a
workaround must simulate what would happen if the X server would have
handled the device, by calling XResetScreenSaver when input is received
from the device.

3. When the X server is not handling the input, it depends on the
program that does to call XResetScreenSaver whenever an input event
occurs. Alternatively the program must handle the screensaver countdown
internally and call XActivateScreenSaver.
     2 ==============================================================================
     3 Using the Simple DirectMedia Layer with Qtopia/OPIE
     4 ==============================================================================
     6 ==============================================================================
     7 I.  Setting up the Qtopia development environment.
     9   This document will not explain how to setup the Qtopia development
    10   environment. That is outside the scope of the document. You can read
    11   more on this subject in this excellent howto:
    15 ==============================================================================
    16 II.  Building the Simple DirectMedia Layer libraries using the arm
    17      cross-compiler
    19   This is somewhat tricky since the name of the compiler binaries
    20   differ from the standard. Also you should disable features not
    21   needed. The command below works for me. Note that it's all one
    22   line. You can also set the NM, LD etc environment variables
    23   separately.
    25 	NM=arm-linux-nm LD=arm-linux-ld CC=arm-linux-gcc CXX=arm-linux-g++ RANLIB=arm-linux-ranlib AR=arm-linux-ar ./configure --enable-video-qtopia --disable-video-dummy --disable-video-fbcon  --disable-video-dga --disable-arts --disable-esd --disable-alsa --disable-cdrom --disable-video-x11 --disable-nasm --prefix=/opt/Qtopia/sharp/ arm-unknown-linux-gnu
    27   One thing to note is that the above configure will include joystick
    28   support, even though you can't have joysticks on the Zaurus. The
    29   reason for this is to avoid link / compile / runtime errors with
    30   applications that have joystick support.
    32 ==============================================================================
    33 III.  Building the Simple DirectMedia Layer test programs:
    35   After installing, making sure the correct sdl-config is in your
    36   path, run configure like this:
    38 	NM=arm-linux-nm LD=arm-linux-ld CC=arm-linux-gcc CXX=arm-linux-g++ AR=arm-linux-ar ./configure arm-unknown-linux-gnu
    40 ==============================================================================
    41 IV.  Application porting notes
    43   One thing I have noticed is that applications sometimes don't exit
    44   correctly. Their icon remains in the taskbar and they tend to
    45   relaunch themselves automatically. I believe this problem doesn't
    46   occur if you exit your application using the exit() method. However,
    47   if you end main() with 'return 0;' or so, this seems to happen.
    49   Also note that when running in landscape mode - i.e requesting a
    50   window that is HEIGHT pixels wide and WIDTH pixels high, where WIDTH
    51   and HEIGHT normally is 240 and 320 - the image is blitted so that
    52   the hardware buttons are on the left side of the display. This might
    53   not always be desirable but such is the code today.
    56 ==============================================================================
    57 V.  Enjoy! :)
    59   If you have a project you'd like me to know about, or want to ask questions,
    60   go ahead and join the SDL developer's mailing list by sending e-mail to:
    64   and put "subscribe" into the subject of the message. Or alternatively you
    65   can use the web interface:
    69 ==============================================================================
    70 VI.  What is supported:
    72 Keyboard (Sharp Zaurus)
    73 Hardware buttons
    74 Stylus input (mouse)
    75 Video. Allows fullscreen both in portrait mode (up to WIDTHxHEIGHT
    76 size window) and in landscape mode (up to HEIGHTxWIDTH). 
    78 All other SDL functionality works like a normal Linux system (threads,
    79 audio etc).
    81 -- 
    82 David Hedbor <>