Allegro Linux-specific information

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                     Linux-specific information.

             See readme.txt for a more general overview.

       Also see docs/build/unix.txt for general Unix information.

# General notes

On Linux you have two different system drivers -- one for running using X, and one for running without X. For information on the X version, and instructions telling how to install Allegro and compile your programs on a Unix system, see `docs/build/unix.txt`. This file only describes the Linux console routines.

Normally the X version is used in preference to the console version, if an X server is contactable (this means that your programs will use X if you launch them from inside X, and otherwise use the console).

Unless you have specific needs, we recommend using the X version. The Linux console port is not enabled by default. It never worked that well and there is no real interest in it any more. We may remove it in a future release.

# Installing Allegro

See `docs/build/unix.txt`: this process is identical for the console and X versions.

You must set the CMake option WANT_LINUX_CONSOLE to enable the console version, as it is not built by default.

# Using Allegro

Some parts of Linux console Allegro need root privileges. Others don't. Specifically, the VGA and mode-X graphics drivers do. The fbcon driver doesn't. Also, if you intend to run a program from something which isn't a VT (e.g. an xterm (but not using the X version), a remote login, a screen session) then you need to either own a (free) console numbered from 1 to 15 or to have root privileges.

If you want a program to run with privileges even though the user running it is not root, make root own the binary and set the suid bit:

>           # chown root.allegro exhello
>           # chmod 4770 exhello

That makes the executable be owned by root and group ``allegro'`. It's readable, writable and executable by root and people in that group. Also, when it runs, it has root privileges. For that reason, it's not executable by the general public. Put trusted users into group ``allegro'`.

You might prefer to use ``4750'` instead of ``4770'` -- then people in the group cannot write to the binary.

Allegro will drop the privileges almost immediately after initialisation, using ``seteuid'`. This means that you can still get the permissions back later on. Allegro needs to be able to regain the privileges if you intend to call ```allegro_init`'` again in the future. If this isn't the case, you can completely give up the privileges using ``setuid(getuid());`' -- then they are lost for good.

Security note: If you don't do this, any buffer overflows are a severe security problem. In any case, you should take great care when allowing random people to execute third-party programs suid root.

If, as root, you run ``make suid`', it will do this to all of the programs generated -- the demo game, the examples, the tests, etc.

# Notes on drivers

This section contains Linux-specific notes on the drivers available.

> System:
>   On initialisation, Allegro will try to get a certain amount of control over the console. This doesn't generally require root privileges, but if the program isn't being run from a plain VC (e.g. it's running from within X or screen), Allegro must try to allocate a spare VC and move the program to it. This requires a free console we can read from and write to; unless the user has allocated some, we need to be root here.

>   Display switching is possible; all of Allegro's modes are supported. The default is `SWITCH_PAUSE`. `SWITCH_BACKGROUND` will only work if the application takes special precautions.

> Graphics:
>   Linux Allegro supports standard VGA by direct writes to the video card. You need root privileges for this to work.

>   It also supports fbcon, for which you do not need privileges, but you do need a recent kernel, correctly configured.

>   The SVGAlib driver should be fairly stable and fast now, and can use most of the video modes that SVGAlib provides. It requires root privileges if your version of SVGAlib requires them.

> Note: Allegro used to have a mode-X driver, but it does not work as of 4.4.0. Someone may restore it in the future, but it's likely.

> Keyboard:
>   The keyboard driver uses the kernel mapping tables to map scancodes to character codes and key functions, so your keyboard mapping should be fine without special configuration in the setup program.

> Joystick:
>   To compile Allegro with joystick support you must have a 2.2.* or newer kernel, i.e. with kernel-based joystick support. Look for `/usr/src/linux/include/linux/joystick.h`. You shouldn't have to actually configure the kernel with joystick support though, you just need that header file.

>   Similarly, to run Allegro programs with joystick support you must have configured joystick support into the kernel. You also need to create the device file `/dev/input/js0`, if it's not already there.

>   See `/usr/src/linux/Documentation/joystick.txt` for details.

> Mouse:
>   Currently Linux Allegro supports PS/2 and Microsoft mice directly, and other mouse types via GPM. To use GPM you must turn on its ``repeater'` feature, where it copies all mouse activity to the FIFO `/dev/gpmdata`. Edit your init scripts and add the ``-R`' option. The GPM driver can also be used with native Mouse Systems mice by changing the ```mouse_device`'` variable in the config file.

>   When using the PS/2 Intellimouse protocol, it is necessary to explicitly put the mouse into wheel mode, which requires Allegro to have write permissions to the device.

>   There is an input events-based driver which reads events from `/dev/input/eventN`.