
			The xlock Hacker's Guide
			      Ron Hitchens


INTRODUCTION

   This document is meant to serve as a guide for those who wish to hack
xlock; to make changes, add new modes, fix bugs, etc.  The intent is to
explain non-obvious things about how the various pieces of xlock
are organized, to help the casual hacker avoid common coding mistakes
and to stay away from "off limits" parts of xlock.


What Is Xlock?

   Xlock is an application for the X Window system which will cover up
one or more X screens to prevent access.  It may be run manually by a
user to lock the display or, more commonly, xlock may be run automatically
by a daemon utility after a period of inactivity.

   Xlock creates a "blanket" window to cover the entire screen, and also
grabs the X server to prevent access by external clients.  When the
user presses a key or clicks a mouse button, xlock will prompt for a
password.  When the proper password is provided, xlock releases the
server and removes its blanket window.

   While xlock has the display locked, it runs one or more "modes" which
are code modules that draw various things on the xlock window.  These
modes act as screen savers.  They attempt to provide amusing and/or
entertaining displays, while avoiding static imagery which could lead
to screen phosphor "burn-in".

   The xlock application began life at Sun Microsystems many years
ago.  It was written by Patrick J. Naughton and was much simpler then.
That original xlock is almost ubiquitous in the X Windows world.  This
distribution, known as xlockmore, is maintained by David Bagley and
is not officially connected with the original xlock, (although it
received Patrick's blessing).  Major enhancements have been made to
xlock - many, many new modes have been added and significant structural
changes have been made.  This document will attempt to inform you of
how xlock is structured and how the pieces fit together.


ORGANIZATION

   Xlock is organized into two basic parts: the "mainline" code, which
handles startup, window creation, passwords, etc, and the modes,
which are self-contained modules that draw things on the window(s)
provided by the mainline code.

   The code which makes up an xlock mode is accessed through a few
well-defined callback functions, or "hooks".  The mode should not
concern itself with anything not provided to it via these hooks.
As of the xlockmore-3.8 release, these hooks have been restructured
to provide all the environmental information a mode needs to do its
task.  Prior to this, it was necessary for the modes to access global
variables.  This is no longer condoned, it is now strongly suggested
that modes only trust the passed-in information.  These globals will
probably go away once all the naive modes have been updated.


MAINLINE CODE

   The mainline xlock code is concerned with preventing unauthorized
access to the X server and creating the environment in which the modes
run.  It also calls the hooks for the current mode in response to
external events and timing intervals.  The mainline code keeps track
of the clock and determines when to make calls to a mode.

        < unfinished >

MODES

   The primary focus of this document is writing and maintaining
modes.  The remainder will be concerned with how to write a mode,
how a mode accesses the display, what a mode should not do, etc.


HOOKS

   Xlock modes are driven entirely through their externally visible
hook functions.  There are currently five hooks defined, with a sixth
reserved for future expansion.  The first two, init and callback, are
the same as in older versions of xlock.  The release, refresh and
change hooks are new to xlockmore-3.8:

	o init   This hook will be called to prepare a mode for running.
		The mode should allocate resources, establish its
		initial state, etc.  This hook may be called
		multiple times, and the window and/or screen
		values may be different each time.

	o callback  This is the main driver hook for a mode.  It
		is called repeatedly, at a time interval determined
		by defaults or command line arguments.  A mode sees
		each call as one "tick", it may chose to do something
		on every tick, or count the calls and only update
		the screen periodically.  A mode should not spend a
		lot of time executing the callback function.  If it
		has a lot of screen updating to do, it should spread
		the work across multiple calls.  A mode can depend
		on the init hook being called at least once before
		the callback hook is called.  But it should not depend
		on the callback hook ever being called following an init.

	o release  This hook will be called when some other mode is
		about to be initialized, or when xlock is shutting
		down.  This hook should free up any long-lived, 
		dynamically allocated resources the mode has acquired.
		This would include memory and X resources, such as
		Pixmaps and GCs.

	o refresh  This hook is called when the drawing window may have
		been damaged.   It should take steps to repaint the
		window.  No information about which part(s) of the
		window have been damaged is provided.  The mode should
		repaint the entire window.  If no refresh hook is
		provided, the init hook will be called when a refresh
		is needed.

	o change  This hook is called when the user requests a change.
		In the current implementation, this is when the user
		clicks the middle mouse button.  This hook is currently
		only used by the random mode, it means to move on to
		the next mode (random mode runs other modes as slaves).
		A mode is free to interpret a change request in any
		way it likes.  The logical thing is to start over,
		change colors, etc.


Calling Conventions

   The prototype for a mode hook is defined in mode.h, and looks like
this:

	void mode_hook (ModeInfo *mode_info)

   The argument, a pointer to a ModeInfo structure, contains
all the context information a mode needs to run.  Writers of new modes
are strongly encouraged to acquire all information they need through
this handle.

   A ModeInfo handle is provided to every hook type.  The information
in this structure is current ONLY AT THE TIME THE HOOK IS CALLED.  The
structure it points to is volatile and the pointer should not be cached
by the mode.  It is also important to note that xlock may be locking
several screens simultaneously.  The window information may not be the
same across subsequent calls to the same callback function.  Use the
information provided, don't look at globals, and don't stash the pointer.


The Init Hook

   A mode's init hook will be called whenever the mainline code wants
to inform a mode it is about to run on a particular window.  The mainline
xlock code will only run one mode at a time, but it may be running that
mode on several screens at once.  It is therefore possible for the init
hook to be called several times, each with a different window context,
before its callback hook is run.  An init hook should not assume the
window provided is the window to be used by the next call to the
callback hook.  Depending on the nature of the mode, it may be
necessary to maintain several sets of state information, one for each
screen.

   The number of active screens, and the maximum possible number of
screens are provided in the ModeInfo struct.  Modes are encouraged
to look at this information and allocate data structures dynamically.
The number of active screens will not change during an xlock run.
A global symbol, MAXSCREENS, is defined in xlock.h but programmers
are strongly urged not to use this or any other fixed value.  If
you use only the information passed to the hooks, your code will
always be correct.

   An init hook should also be prepared to be called at any time.  The
mainline xlock code changes window configuration in response to user
input, and may call the init hook in place of a missing refresh hook.
An init hook should therefore not expect to be balanced with a call
to the release hook.  The init hook should not allocate resources on
each call, it should track its allocations to make sure they are only
done once.

   Neither should an init hook depend on the callback hook ever being
called.  It's possible a call to the init hook may be followed by another
call to the init hook, or a call to the release hook without an intervening
call to the callback hook.

   An init hook may be called twice in a row, and will be if more than
one screen is being locked.  To avoid unexpected glitches on-screen,
it is recommended that you don't draw anything to the screen during
the init hook.  Save the drawing for the callback hook, with appropriate
status information if an initial screen paint is needed.

   Be careful not to blindly do dynamic allocations in your init hook,
keep track of your allocations and only do the necessary state reset
each time.  Track your allocations so they can be undone by the release
hook at a later time.

   The init hook will be called for each screen before the callback
hook is called for any screen.


The Callback Hook

   The callback hook is the method by which a mode runs.  The mainline
code calls a mode's callback hook periodically, usually on a fixed time
schedule, and checks for user input between each call.

   The time interval between calls to the callback hook is set by a
field in the LockStruct entry for the mode (see mode.h and mode.c).
This value may also be set by the user on the command line, or via
an X resource.  The mainline code attempts to keep the time interval
between the *beginning* of each call constant.  The time spent executing
the mode's callback hook is subtracted from the interval to keep the
ticks as constant as possible.  This is hardly perfect, but an effort
is made to remain as accurate as is possible on a multi-tasking system.

   A mode should therefore not spend a large amount of time executing 
in the callback hook.  While in the callback, the mainline code cannot
respond to external events from the user.  It is preferable for a
callback hook to do a little bit of the work on each call, rather than
a complete update each time.

   A callback should pay attention to the context information passed
to it.  On multi-headed displays, the callback may be called successively
for each screen on the display.  It may be necessary to maintain
state information which is indexed by the given screen number, rather
than simply using local static variables.

   The screen number is provided by the ModeInfo argument and will
range from 0 to n, where n is the number of active screens minus one.
There will always be a screen 0.  A mode wishing to paint the same
imagery one each screen should do the same thing each time the callback
is called, and advance its state when the screen number is 0.  However,
the window may not be the same size on every screen.  Don't assume it is.

   A call to the callback hook is not guaranteed following an init call
for a given screen, but at least one init call is guaranteed before the
first callback call.  If the window size changes, as when the icon
window is presented to prompt for the password, an init call
will be made with the new window information.  A mode should
always use the window information passed to it rather than caching
information passed to the init hook, but it can use the information
passed to the init hook to setup its own data structures and rely
on the information matching the next callback call, *for that screen*.


The Release Hook

   This hook is new to release 3.8.  The release hook will be called by
the mainline code when it is about to call the init hook of another mode,
and your init hook has been called at least once since the last call
to your release hook.

   The release hook should undo any dynamic allocations done by the
init hook, or anything else that needs to be done to make the mode
inactive.  Once the release hook returns, the mode is marked as not
initialized.  If your mode is never initialized again, no further
calls to any of its hooks will be made.  The release hook is where
you must surrender any resources that only your mode knows about.

   The release hook is called ONLY ONCE.  It will not be called
for each screen like most of the other hooks.  A mode should not
access any of the window information, other that the display handle
and number of active screens.

   Once the release hook has been called the mode is considered
to be inactive, the same as if it had never been run at all.

  < Final call at shutdown?? >


The Refresh Hook

   This hook is new to release 3.8.  The refresh hook is called when
the mainline suspects the window may have been damaged.

   When running in "inwindow" mode (xlock runs in a plain window, not locking)
this may happen when windows are shuffled by the user.  It may also happen
when in normal full-screen mode and some new window appears on the display.
In this case, xlock immediately pushes itself to the top, to cover the
new window.  However, the temporary appearance of the new window may
have left a "hole" on the display.  The refresh hook should take steps
to repaint the entire display whenever it is called.

   It will also be called when the window is first mapped.  However,
a mode should not depend on a refresh call to do its initial screen
paint.  When running random mode, other modes will be stopped and started
on the same window, with no intervening refresh call.  A mode should
usually do a full paint on the first callback following an init call.
It is a good strategy to have the refresh hook simply set a status flag
(or whatever) to cause the next callback call to do a full repaint.  This
would be similar to an init, but internal state would not be reset.

   If no refresh hook is provided for your mode (configured in mode.c),
then your init hook will be called in place of the refresh hook.  The
refresh hook is provided so your mode can repair window damage
without losing the internal state of the mode.

   As of this writing, there is a hack in place which will prevent a
second call to the init hook (in place of a refresh) if the callback
hook has not been called since the last init call for that screen.
This causes undesirable behavior in some naive modes.  It is expected
this hack will be removed.  Modes should be prepared for their init
hooks to be called at any time, even repeatedly.


The Change Hook

   This hook is new to release 3.8.  It is called when the user requests
a change.  This is currently in response to a click on the middle mouse
button.

   In the case of random mode, which runs other modes, it means to
move on to the next mode without waiting for the time to expire.  Other
modes are free to interpret this call in way they choose.  If no change
hook is provided for a mode, no action will be taken when the middle mouse
button is clicked.

   This hook will be called once for each active screen when a change
request is made.


Hook Calling Sequence

   A typical sequence of calls when running on two screens would be:

	init [screen 0]
	init [screen 1]
	refresh [screen 0]	(caused by first mapping the window)
	refresh [screen 1]
	callback [screen 0]
	callback [screen 1]
	callback [screen 0]
	callback [screen 1]
	...
	refresh [screen 0]	(caused by window damage)
	refresh [screen 1]
	callback [screen 0]
	callback [screen 1]
	...
	init [screen 0]		(switch to icon screen)
	callback [screen 0]
	callback [screen 1]
	...


HANDS OFF THOSE GLOBALS

   All the environmental information a mode needs is provided to the
hook functions via the ModeInfo passed as an argument.  But prior to
the restructuring done in xlockmore-3.8, much of this information
had to be accessed directly from global variables.  Listed here are
the globals which correspond to the information passed in ModeInfo.
You should not access these variables directly (they will go away),
nor should you use these names for local variables.  The first column
is the global name, the second column is the macro to use to get
the same information from the ModeInfo argument (see mode.h):

   These variables pertain to the X screen
	dsp		MI_DISPLAY	handle to the X server display.
	screen		MI_SCREEN	Current screen number
	Scr		MI_PERSCREEN	perscreen struct ptr for curr screen
	Scr[n].gc	MI_GC		gc handle for current screen
	Scr[n].npixels	MI_NPIXELS	num available pixels for curr screen
	Scr[n].cmap	MI_CMAP		colormap handle for current screen
	Scr[n].pixels	MI_PIXELS	pixel array for current screen
	Scr[n].pixels[i] MI_PIXEL	a given pixel in the pixel array

   These variables control execution, set by cmd line or resources
	delay		MI_DELAY	time (microsecs) between callbacks
	batchcount	MI_BATCHCOUNT	batchcount value
	cycles		MI_CYCLES	cycles value
	saturation	MI_SATURATION	colormap saturation value

   These variables are booleans, usually set by cmd line:
	mono		MI_WIN_IS_MONO	use only B&W (can be set for color)
	inwindow	MI_WIN_IS_INWINDOW  running in regular window
	inroot		MI_WIN_IS_INROOT  running in the root window

   The MI_WIN_IS_MONO flag will be true if the global "mono" is set
(which can be specified on the command line for color displays) or
if the screen is a monochrome device.  It IS possible to have both
color and monochrome screens at the same time.  Use the passed-in
information on a screen-by-screen basis, don't assume they are all
the same.

   There are several other global booleans in resource.c.  These will
probably be eliminated in future releases.  Don't access them directly.
They shouldn't be of interest to a mode anyway, but be careful not to
use those names in your own code.


PLUGGING A NEW MODE INTO XLOCK

   The code making up a mode should be self-contained.  A mode should
hide all of its internal variables and functions.  Only the hook functions
and one configuration variable should be visible outside the module
the mode is defined in.  Because there are some many code modules
compiled into xlock, written by many different people, the chance
of naming conflicts is quite high.  Keep all your local stuff local.

   The nexus where the mainline xlock code connects to the individual
modes is in the file mode.c.  It contains an array of pre-initialized
LockStruct structures named LockProcs.  This struct is currently
defined as:

	typedef struct LockStruct_s {
		char       *cmdline_arg;	/* mode name */
		ModeHook   *init_hook;		/* func to init a mode */
		ModeHook   *callback_hook;	/* func to run (tick) a mode */
		ModeHook   *release_hook;	/* func to shutdown a mode */
		ModeHook   *refresh_hook;	/* tells mode to repaint */
		ModeHook   *change_hook;	/* user wants mode to change */
		ModeHook   *unused_hook;	/* for future expansion */
		ModeSpecOpt *msopt;		/* this mode's def resources */
		int         def_delay;		/* default delay for mode */
		int         def_batchcount;
		int         def_cycles;
		float       def_saturation;
		char       *desc;		/* text description of mode */
		int         flags;		/* state flags for this mode */
		void       *userdata;		/* for use by the mode */
	} LockStruct;

   Of these fields, the hooks and msopt are defined in the mode itself.
The hooks are names of functions which are called as previously described.
Init and callback hooks are required, all others are optional.
Any hooks not provided are specified as NULL.  The field "msopt" is
a pointer to a ModeSpecOpt struct (xlock.h).  Every mode must define
one of these structures and make it globally visible to mode.c.  This
structure provides a handle to X resource information that allows
for parsing command line arguments unique to your mode and setting
static variables private to your mode.
	<< unfinished, see random.c for an example >>

   The remaining fields of the LockStruct struct are defined directly
in the array in mode.c.  The fields with the names def_* are the default
values to be used when running this mode, if not overridden by command
line arguments of resources.

   The field def_delay controls how often the callback hook is called
(specified in microseconds).

   The floating point number def_saturation controls the saturation of
the colormap to allocate.  This controls how the color ramp is 
populated (<<unfinished: how?>>)

   The other two default values, def_batchcount and def_cycles, are
for use by the mode.  They can be used to control how many thingys
to draw at once, how often to restart, etc.  These values can be
specified at run time which allows the user to affect how a mode runs.

   The text pointers cmdline_arg and desc are used when printing
command line help.  They provide the simple name and a more verbose
short description.

   The flags field should always be set to zero, it is used internally
to keep track of state information.

   The last field, userdata, is for use by the mode.  The mode may use
this generic pointer for any purpose it likes.  The mainline code will
never touch it and it will be available to all subsequent hook calls.
This value will survive init - release - init cycles.


GETTING INFORMATION FROM ModeInfo

   The ModeInfo structure is defined in mode.h.  It contains several
types of information and is actually made up of several other structs.
This structure is likely to undergo major revision in future releases,
so macros are provided to access all the fields.  Use the macros,
things are guaranteed to change.

   Of the fields available in ModeInfo, most are copies of the same
information available in the globals described above.  But some are new.
Most notably, the window dimensions and black/white pixel values are
now provided, so there is no need to make direct X library calls to
get this information yourself.

   There is also some provision for a future debugging facility to
fake multiple screens by using multiple regular windows.  This
code is not yet implemented.  When it is, the number returned by
the MI_SCREEN macro may not correspond to a real X screen.  As of
this writing, MI_SCREEN and MI_REAL_SCREEN always contain the same
value.  Use MI_SCREEN as an index to track which window you are
drawing to, use MI_REAL_SCREEN when calling X Windows functions
which require actual screen numbers.  The MI_SCREENPTR pointer
will always be valid, but identical for all faked screens.

   The ModeInfo structure also provides a pointer to the LockStruct
record that corresponds to the mode.  DO NOT MODIFY THIS STRUCTURE.
This is provided so that you can see what your compiled in defaults
where for delay, batchcount, cycles and saturation.  You can also
get your own name and description and access the userdata field (it's
ok to modify userdata, don't change anything else, use the macros).

   All fields should be considered read-only, except MI_USERDATA
and MI_PAUSE.  MI_USERDATA is not used by the mainline code, you
can use any way you like and its value will be preserved.  However,
the MI_PAUSE field is special.  MI_PAUSE is examined up upon return
from your callback hook.  If it is not zero (it will be set zero
before the call) it is interpreted as a time, in microseconds, to
delay before making the next callback.

   The MI_PAUSE mechanism is somewhat of a hack, and it is expected
to change in future revisions of xlock.  Most probably it will be
moved out of the ModeInfo struct and the callback return value will
specify the delay value.  This one-time pause mechanism is also
broken for multiple screens.  It is only noticed on the highest
numbered screen.   Future revisions of xlock will (hopefully) fix this,
but for now you can see how it works by looking at the code for maze
or triangle.


EXAMPLE

   <<unfinished>>

   The eyes mode (eyes.c) was written by the same author that did
the majority of the restructuring for the new mode interface.  It
should (hopefully) serve as a good example of a properly written
mode.  It makes use of the new refresh and release hooks.  The
random mode (random.c) will also illustrate the change hook and
private resources.  The triangle mode (triangle.c) has also been
updated to use the new scheme.  It uses the MI_PAUSE mechanism to
sleep between scenes.

------------------------

   The official xlockmore maintainer is David Bagley.  He can be
reached at bagleyd@megahertz.njit.edu.  The current release of xlockmore
is available by anonymous ftp at ftp.x.org in /contrib/applications.

   The restructuring of the calling mechanism for mode hooks was
done by Ron Hitchens <ron@idiom.com>.

------------------------

   This document written by Ron Hitchens <ron@idiom.com>
It is still very rough and incomplete.  What you see here is
basically the first draft, brain-dump version.  It needs to be
polished to make it more readable, condensed to make it less
redundant and organized to make it more cogent.  But it's a start.
Hopefully, this will eventually be converted to LaTeX.  When
I get some time...


Last Update:
Mon Mar 18 03:46:16 MST 1996
