[SDL] on the dynamic loading of drivers

Sam Hocevar sam at zoy.org
Mon Nov 14 17:09:36 PST 2005

   With the recent addition of the dynamic loading of the X11 driver,
I can see a trend that I think will lead to much code duplication and
uglyfication as well as a more difficult code maintenance.

   As far as I understand, dynamic loading of the drivers serves two
main purposes:
  - avoid linking libSDL with too many extra libraries that may not even
    be installed
  - make it straightforward to activate support for one of these
    libraries once they are installed

   Currently SDL has the following problems:
  - the version of libX11 is hardcoded to 6; on Solaris, the Openwin
    libX11 is libX11.so.4
  - if more than one version of the dynamically opened libraries
    (libasound, libesd, libartsc) is installed on the build system, only
    one version will be used by libSDL and which one will be chosen is
    undetermined (but fixed at build time)

   These are minor details. A more important problem is dependency
tracking. Since the library keeps no link with the library that was on
the build system, there is no easy way to know which version of say,
libartsc is needed for libSDL to work. Linux distributions which use the
linker information in each ELF object to track dependencies will miss
that one.

   Now one may argue that if it causes so many trouble to Linux
distributions then they just need not to activate the feature. But
dynamic loading of drivers is a very useful feature, and I just think
there is a way to do it better.

   The solution I propose is to make the whole driver a separate shared
library that simply exports a unique symbol that is an AudioBootStrap
or VideoBootStrap pointer. The amount of code needed to support that
feature is quite small: it's just a matter of browsing a given directory
(eg. /usr/lib/SDL/1.2/) and dynamically loading what is in there. If the
library required by a given plugin is not here, it will simply fail to


  +----------------------+   dynamically    +-----------+
  | libSDL | ALSA driver |---   loads   --> | libasound |
  +----------------------+    on demand     +-----------+

   My proposal:

  +--------+   dynamically   +-------------+            +-----------+
  | libSDL |---   loads   -->| ALSA driver |-- links -> | libasound |
  +--------+    on demand    +-------------+    with    +-----------+

   The amount of code that needs to be changed in each plugin for this
to work is exactly zero lines. Admittedly, there is some work to do on
the build system, but nothing really hardcore, and it gets rid of the
ugliness that currently sits in configure.in.

   I have already started working on this for the Debian version of
libSDL. I want to provide a way for our users to selectively install
ALSA, esd or aRts support for SDL, and currently we have no other choice
but to provide 4 different packages containing full versions of libSDL
with the given plugin compiled in or not.

   Also, I may have sounded quite Linux centric here, but my proposal
works of course on every Unix platform and BeOS. It will probably not
work on Win32 because of the way DLLs work, but that platform does not
really need it.

   I will send clean patches when this is done, but I just wanted to
know the general opinion about the proposal. FWIW, it is exactly how the
Allegro library works. Let me know what you think.


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