author Sam Lantinga <>
Fri, 29 Feb 2008 13:55:44 +0000
changeset 4139 568c9b3c0167
parent 1888 488eba319a25
child 3568 8c72321542f6
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 Using SDL under Windows with the OpenWatcom compiler
     3 ====================================================
     5 Prerequisites
     6 -------------
     8 I have done the port under Windows XP Home with SP2 installed. Windows
     9 2000 should also be working. I'm not so sure about ancient Windows NT,
    10 since only DirectX 3 is available there. Building should be possible,
    11 but running the compiled applications will probalbly fail with
    12 SDL_VIDEODRIVER=directx. The windib driver should work, though.
    14 To compile and use the SDL with Open Watcom you will need the following:
    15 - Open Watcom compiler. I used version 1.5. The environment variables
    16   PATH, WATCOM and INCLUDE need to be set appropriately - please consult
    17   the OpenWatcom documentation and instructions given during the
    18   installation of the compiler.
    19   My setup looks like this in owvars.bat:
    20     set WATCOM=C:\watcom
    21     set INCLUDE=%WATCOM%\h;%WATCOM%\h\nt
    22     set PATH=%PATH%;%WATCOM%\binnt;%WATCOM%\binw
    23 - A fairly recent DirectX SDK. The original unmodified DX8 SDK works, as
    24   well as the minimal DirectX 7 SDK from the Allegro download site
    25   (<>).
    26 - The SDL sources from Subversion
    27 - The file (now available in Subversion)
    30 Building the Library
    31 --------------------
    33 1) In the SDL base directory extract the archive This
    34    creates a subdirectory named 'watcom'.
    35 2) The makefile expects the environment variable DXDIR to be set to the
    36    base directory of a DirectX SDK. I have tried a stock DX8 SDK from
    37    Microsoft as well as the minimal DirectX 7 SDK from the Allegro
    38    download site.
    39    You can also edit the makefile directly and hard code your path to
    40    the SDK on your system.
    41    I have this in my setup:
    42      set DXDIR=D:\devel\DX8_SDK
    43 3) Enter the watcom directory and run
    44      wmake sdl
    45 4) All tests from the test directory are working and can be built by
    46    running
    47      wmake tests
    49 Notes:
    51  The makefile offers some options to tweak the way the library is built.
    52  You have at your disposal the option to build a static (default)
    53  library, or a DLL (with tgt=dll). You can also choose whether to build
    54  a Release (default) or a Debug version (with build=debug) of the tests
    55  and library. Please consult the usage comment at the top of the
    56  makefile for usage instructions.
    58  If you specify a test target (i.e. 'wmake tests' for all tests, or
    59  selected targets like 'wmake testgl testvidinfo testoverlay2'), the
    60  tests are always freshly compiled and linked. This is done to
    61  minimise hassle when switching between library versions (static vs.
    62  DLL), because they require subtly different options.
    63  Also, the test executables are put directly into the test directory,
    64  so they can find their data files. The clean target of the makefile
    65  removes the test executables and the SDL.dll file from the test
    66  directory.
    68  To use the library in your own projects with Open Watcom, you can use
    69  the way the tests are built as base of your own build environment.
    71  The library can also be built with the stack calling convention of the
    72  compiler (-6s instead of -6r).
    75 Test applications
    76 -----------------
    78 I've tried to make all tests work. The following table gives an overview
    79 of the current status.
    81  Testname        Status
    82 ~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    83 checkkeys       +
    84 graywin         +
    85 loopwave        +
    86 testalpha       +
    87 testbitmap      +
    88 testdyngl       +
    89 testerror       +
    90 testfile        +
    91 testgamma       +
    92 testgl          +
    93 testhread       +
    94 testiconv       - (all failed)
    95 testkeys        +
    96 testlock        +
    97 testoverlay     + (needs 'set SDL_VIDEODRIVER=directx')
    98 testoverlay2    + (needs 'set SDL_VIDEODRIVER=directx')
    99 testpalette     +
   100 testplatform    +
   101 testsem         +
   102 testsprite      +
   103 testtimer       +
   104 testver         +
   105 testvidinfo     +
   106 testwin         ? (fading doesn't seem right)
   107 testwm          +
   108 torturethread   +
   109 testcdrom       +
   110 testjoystick    not tested
   111 threadwin       +
   112 testcursor      +
   115 TODO
   116 ----
   118 There is room for further improvement:
   119 - Test joystick functionality.
   120 - Investigate fading issue in 'testwin' test.
   121 - Fix the UTF-8 support.
   122 - Adapt the makefile/object file list to support more target systems
   123 - Use "#pragma aux" syntax for the CPU info functions.
   126 Questions and Comments
   127 ----------------------
   129 Please direct any questions or comments to me:  <>
   131    Happy Coding!
   133    Marc Peter