author Sam Lantinga <>
Fri, 29 Feb 2008 13:55:44 +0000
changeset 4139 568c9b3c0167
parent 886 05c551e5bc64
child 3083 0bc41e0361d3
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.
     1 README.QNX by Mike Gorchak <>, <>
     2 Last changed at 24 Apr 2004.
     4 ======================================================================
     5 Table of Contents:
     7 1. OpenGL.
     8 2. Wheel and multi-button mouses.
     9 3. CDROM handling issues.
    10 4. Hardware video overlays.
    11 5. Shared library building.
    12 6. Some building issues.
    13 7. Environment variables.
    15 ======================================================================
    16 1. OpenGL:
    18     OpenGL works well and is stable, but fullscreen mode has not  been
    19 heavily tested yet.
    20     If you have QNX RtP version 6.1.0 or above you must  download  the
    21 Photon3D runtime from or install it from the
    22 public repository or from the public CD, available with QNX. OS versi-
    23 ons below 6.1.0 are not supported.
    24     When creating an OpenGL context, software renderer mode is artifi-
    25 cially selected (QSSL made acceleration  only  for  Voodoo  boards  in
    26 fullscreen mode, sorry but I don't have this board  to  test  OpenGL -
    27 maybe it works or maybe not :)). If you  want  acceleration - you  can
    28 remove one line in the source code: find the  file SDL_ph_image.c  and
    29 remove the following
    31     OGLAttrib[OGLargc++]=PHOGL_ATTRIB_FORCE_SW;
    33 line in the ph_SetupOpenGLContext() function or change the argument to
    36 ======================================================================
    37 2. Wheel and multi-button mouses:
    39     Photon emits  keyboard  events (key up and  down)  when the  mouse
    40 wheel is moved. The key_scan field appears valid, and it contains zero.
    41 That is a basic method of detecting  mouse  wheel events under Photon.
    42 It looks like a hack, but it  works for me :) on various PC configura-
    43 tions.
    45 I've tested it on:
    47 1. Genius Optical NetScroll/+ PS/2 (1 wheel)
    48 2. A4Tech Optical GreatEye WheelMouse PS/2,  model: WOP-35.  (2 wheels
    49    + 2 additional buttons). The wheel for vertical scrolling  works as
    50    usual, but the second wheel for horizontal scrolling emits  two se-
    51    quential events up or down, so it can provide faster scrolling than
    52    the first wheel. Additional buttons don't emit  any  events, but it
    53    looks like they're handled by photon in an unusual way - like click
    54    to front, but works not with any window, looks like a fun bug-o-fe-
    55    ature :).
    57 ======================================================================
    58 3. CDROM handling issues:
    60     Access to CDROM can only be  provided  with  'root'  privileges. I
    61 can't do anything about that, /dev/cd0 has brw------- permissions  and
    62 root:root rights.
    64 ======================================================================
    65 4. Hardware video overlays:
    67     Overlays can flicker  during  window  movement,  resizing, etc. It
    68 happens because the photon driver updates the real window contents be-
    69 hind the overlay, then draws the temporary chroma key color  over  the
    70 window contents. It can be done without using the chroma key but  that
    71 causes the overlay to always be on top. So  flickering  during  window
    72 movement is preferred instead.
    73     Double buffering code is temporarily disabled in the photon driver
    74 code, because on my GF2-MX it can accidentally cause a buffer  switch,
    75 which causes the old frame to show. S3 Savage4 has the  same  problem,
    76 but ATI Rage 128 doesn't. I think it can be fixed later. Current  code
    77 works very well, so maybe double buffering is not needed right now.
    78     Something strange happens when you try to move the window with the
    79 overlay beyond the left border  of the screen. The  overlay  tries  to
    80 stay at position x=0, but when attempting to move it  a  bit  more  it
    81 jumps to position x=-60 (on GF2-MX, on ATI Rage128 this  value  a  bit
    82 smaller). It's really strange, looks  like  the  overlay  doesn't like
    83 negative coordinates.
    85 =======================================================================
    86 5. Shared library building:
    88     A shared library can be built, but before running  the
    89 script you must manually delete the libtool.m4 stuff from the acinclu-
    90 de.m4 file (it comes after the ESD detection code up to the end of the
    91 file), because the libtool stuff in the acinclude.m4 file was very old
    92 in SDL distribution before the version 1.2.7 and doesn't knew anything
    93 about QNX. SDL 1.2.7 distribution contains the  new libtool.m4 script,
    94 but anyway  it  is  broken :), Just  remove  it, then  run "libtoolize
    95 --force --copy",  delete the file aclocal.m4 if it is exists and after
    96 that run the script. SDL 1.2.8 contains  fixed libtool.m4,
    97 and config.sub files, so you  can  just  run  the
    98 script.
   100 ======================================================================
   101 6. Some building issues:
   103     Feel free to not use the --disable-shared configure option if you'
   104 ve read the above comment about 'Shared  library  building'. Otherwise
   105 this  option  is  strongly  recommended, as  without it the sdl-config
   106 script will be broken.
   108     Run the configure script without x11 support, e.g.:
   110     a) for OpenGL support:
   111     ./configure --prefix=/usr           \
   112                 --disable-video-x11     \
   113                 --disable-shared
   115     b) without OpenGL support:
   116     ./configure --prefix=/usr           \
   117                 --disable-video-x11     \
   118                 --disable-shared        \
   119                 --disable-video-opengl
   121     And of course dont forget to specify --disable-debug, which  is on
   122 by default, to disable debug and enable the expensive optimizations.
   124     In the test directory also run the ./configure script without
   125 x11 support, e.g.:
   127     ./configure  --with-sdl-prefix=/usr            \
   128                  --with-sdl-exec-prefix=/usr       \
   129                  --prefix=/usr --without-x
   131 ======================================================================
   132 7. Environment variables:
   134     Please note that the photon driver  is  sensible to the  following
   135 environmental variables:
   137  * SDL_PHOTON_FULLSCREEN_REFRESH - this environment variable  controls
   138 the refresh  rate  in  all  fullscreen  modes. Be  carefull !!! Photon
   139 drivers usually do not checking the maximum  refresh rate, which video
   140 adapter or monitor supports.
   142  * SDL_VIDEO_WINDOW_POS - can be  set in the "X,Y" format.  If X and Y
   143 coordinates are bigger than the current desktop resolution, then  win-
   144 dow positioning across  virtual  consoles is activated. If X and Y are
   145 smaller than the desktop resolution  then  window  positioning  in the
   146 current console is activated. The word "center" can be used instead of
   147 coordinates, it  produces  the  same  behavior  as  SDL_VIDEO_CENTERED
   148 environmental variable.
   150  * SDL_VIDEO_CENTERED - if this environmental variable exists then the
   151 window centering is perfomed in the current virtual console.
   153 Notes: The SDL_VIDEO_CENTERED enviromental  variable  has greater pri-
   154 ority than the SDL_VIDEO_WINDOW_POS in case if both variables are sup-
   155 plied to the application.