author Sam Lantinga <>
Fri, 29 Feb 2008 13:55:44 +0000
changeset 4139 568c9b3c0167
parent 3926 9785a5be718e
child 4159 a1b03ba2fcd0
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 /*
     2     SDL - Simple DirectMedia Layer
     3     Copyright (C) 1997-2006 Sam Lantinga
     5     This library is free software; you can redistribute it and/or
     6     modify it under the terms of the GNU Lesser General Public
     7     License as published by the Free Software Foundation; either
     8     version 2.1 of the License, or (at your option) any later version.
    10     This library is distributed in the hope that it will be useful,
    11     but WITHOUT ANY WARRANTY; without even the implied warranty of
    13     Lesser General Public License for more details.
    15     You should have received a copy of the GNU Lesser General Public
    16     License along with this library; if not, write to the Free Software
    17     Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
    19     Sam Lantinga
    21 */
    23 #ifndef _SDL_config_macosx_h
    24 #define _SDL_config_macosx_h
    26 #include "SDL_platform.h"
    28 /* This gets us MAC_OS_X_VERSION_MIN_REQUIRED... */
    29 #include <AvailabilityMacros.h>
    31 /* This is a set of defines to configure the SDL features */
    33 #define SDL_HAS_64BIT_TYPE	1
    35 /* Useful headers */
    36 /* If we specified an SDK or have a post-PowerPC chip, then alloca.h exists. */
    37 #if ( (MAC_OS_X_VERSION_MIN_REQUIRED >= 1030) || (!defined(__POWERPC__)) )
    38 #define HAVE_ALLOCA_H		1
    39 #endif
    40 #define HAVE_SYS_TYPES_H	1
    41 #define HAVE_STDIO_H	1
    42 #define STDC_HEADERS	1
    43 #define HAVE_STRING_H	1
    44 #define HAVE_INTTYPES_H	1
    45 #define HAVE_STDINT_H	1
    46 #define HAVE_CTYPE_H	1
    47 #define HAVE_MATH_H	1
    48 #define HAVE_SIGNAL_H	1
    50 /* C library functions */
    51 #define HAVE_MALLOC	1
    52 #define HAVE_CALLOC	1
    53 #define HAVE_REALLOC	1
    54 #define HAVE_FREE	1
    55 #define HAVE_ALLOCA	1
    56 #define HAVE_GETENV	1
    57 #define HAVE_PUTENV	1
    58 #define HAVE_UNSETENV	1
    59 #define HAVE_QSORT	1
    60 #define HAVE_ABS	1
    61 #define HAVE_BCOPY	1
    62 #define HAVE_MEMSET	1
    63 #define HAVE_MEMCPY	1
    64 #define HAVE_MEMMOVE	1
    65 #define HAVE_MEMCMP	1
    66 #define HAVE_STRLEN	1
    67 #define HAVE_STRLCPY	1
    68 #define HAVE_STRLCAT	1
    69 #define HAVE_STRDUP	1
    70 #define HAVE_STRCHR	1
    71 #define HAVE_STRRCHR	1
    72 #define HAVE_STRSTR	1
    73 #define HAVE_STRTOL	1
    74 #define HAVE_STRTOUL	1
    75 #define HAVE_STRTOLL	1
    76 #define HAVE_STRTOULL	1
    77 #define HAVE_STRTOD	1
    78 #define HAVE_ATOI	1
    79 #define HAVE_ATOF	1
    80 #define HAVE_STRCMP	1
    81 #define HAVE_STRNCMP	1
    82 #define HAVE_STRCASECMP	1
    83 #define HAVE_STRNCASECMP 1
    84 #define HAVE_SSCANF	1
    85 #define HAVE_SNPRINTF	1
    86 #define HAVE_VSNPRINTF	1
    87 #define HAVE_SIGACTION	1
    88 #define HAVE_SETJMP	1
    89 #define HAVE_NANOSLEEP	1
    91 /* Enable various audio drivers */
    93 #define SDL_AUDIO_DRIVER_SNDMGR	1
    94 #define SDL_AUDIO_DRIVER_DISK	1
    95 #define SDL_AUDIO_DRIVER_DUMMY	1
    97 /* Enable various cdrom drivers */
    98 #define SDL_CDROM_MACOSX	1
   100 /* Enable various input drivers */
   101 #define SDL_JOYSTICK_IOKIT	1
   103 /* Enable various shared object loading systems */
   104 #ifdef __ppc__
   105 /* For Mac OS X 10.2 compatibility */
   106 #define SDL_LOADSO_DLCOMPAT	1
   107 #else
   108 #define SDL_LOADSO_DLOPEN	1
   109 #endif
   111 /* Enable various threading systems */
   112 #define SDL_THREAD_PTHREAD	1
   115 /* Enable various timer systems */
   116 #define SDL_TIMER_UNIX	1
   118 /* Enable various video drivers */
   119 #define SDL_VIDEO_DRIVER_DUMMY	1
   122 #else
   123 #define SDL_VIDEO_DRIVER_QUARTZ	1
   124 #endif
   126 /* Enable OpenGL support */
   127 #define SDL_VIDEO_OPENGL	1
   129 /* Disable screensaver */
   132 /* Enable assembly routines */
   133 #define SDL_ASSEMBLY_ROUTINES	1
   134 #ifdef __ppc__
   135 #define SDL_ALTIVEC_BLITTERS	1
   136 #endif
   138 #endif /* _SDL_config_macosx_h */