path: root/doc/info/mds.texinfo

\input texinfo   @c -*-texinfo-*-

@c %**start of header
@setfilename mds.info
@settitle mds
@afourpaper
@documentencoding UTF-8
@documentlanguage en
@finalout
@c %**end of header


@dircategory Graphics environment
@direntry
* mds: (mds).                        The avant-garde micro-display server
@end direntry


@copying
Copyright @copyright{} 2014, 2015 Mattias Andrée

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts. A copy of the license is included in the section entitled
``GNU Free Documentation License''.
@end quotation
@end copying

@ifnottex
@node Top
@top mds -- The avant-garde micro-display server
@insertcopying
@end ifnottex

@titlepage
@title mds
@subtitle The avant-garde micro-display server
@author by Mattias Andrée (maandree)

@page
@center `To me, writing a monolithic system in 1991 is a truly poor idea.'
@c  Well, here we are 23 years later and we are still
@c  doing, but where it is even easier not to.
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents



@menu
* Overview::                        Brief overview of @command{mds}.
* Architecture::                    Architectural overview of @command{mds}.
* Application Design::              Guildlines for your applications.
* Protocol::                        The @command{mds} procotol.
* Utilities::                       About @command{mds} utilities.
* Servers::                         About @command{mds} servers.
* Protocols::                       @command{mds} procotols.
* libmdsserver::                    Overview of @command{libmdsserver}.
* mds-base.o::                      Overview of @file{mds-base.o}.
* Keyboard Codes::                  Scancodes and keycodes.
* Keyboard Layouts::                Writing and compiling keyboard layouts.
* Accessibility::                   Dealing with disabilities.
* Discussion::                      Discussion on display server-architecture.
* GNU Free Documentation License::  Copying and sharing this manual.
@end menu



@node Overview
@chapter Overview

@command{mds}@footnote{mds stands for micro-display server.}
is a display server protocol and an implementation of said
protocol. What makes @command{mds} stand out is its core
design choice: it is desigend just like a microkernel.
Rather than one, possibly modular, process --- a monolithic
process --- @code{mds} is comprised of many small servers,
each exchangable and responsible for one thing.

@command{mds}'s goal is neither security, performance nor
a perfect graphical experience. @command{mds} is all
about flexibility and freedom 0@footnote{The freedom to run
the program as you wish, for any purpose.}.

The reason for having a display server architectured as a
microkernel is so that components can be added, removed,
updated and replaced online. Additionally, the message passing
between the servers makes it easy to design a system that
lets you make clients that can listen on messages between
the servers and perhaps modify them. This enables you to
do so much more with your display server. Moreover, if
a single part of the system crashes it does not bring down
the whole system, and the crashed server can be respawned
with minor side effects. @command{mds} is architectured
in three layers: a microkernel, a master server and a
collection of servers. And clients are actually located
on the same layer as the servers, because there is no
actual difference, the only thing that separates a server
from a client is for what purpose you run it. @command{mds}'s
kernel is a minimal program that do initialisation of the
display, such as giving it an index and create runtime
files and directories for servers and other programs
to use. Then the kernel creates a domain socket for the
master server and spawns the master server and respawns
it if it crashes. Because of this, if the master server
crashes it will not lose its socket when it is respawned.
The master server than, on its initial spawn, starts
the all servers and other programs that the user have
choosen and then starts accepting connections to it and
coordinates messages between servers and clients. Further,
separating all components into separate processes enables
us to only give the servers the privileges they actually
need, rather than having one program with root privileges
that takes care of everything even things that do not do
require any privileges.

All @command{mds}'s servers, that is all running parts of
@command{mds} except the kernel, are designed so that they
can re-execute themself so that they can be updated online
without any side effects. Servers serialises their state,
saves it to RAM (in a directory created by the kernel),
re-execute themself and loads their serialised state. The
kernel cannot do this because when it has spawned the
master server it has no reason to re-execute, its only mission
is to respawn the master server it if would happen to crash.
It would technically be possible to enable the kernel to
re-execute but it is not worth it as it as no reason to
re-execute, and doing so puts the display server at risk
of crashing.



@node Architecture
@chapter Architecture

@menu
* Layers::                          The layers of the display server.
* Interprocess Communication::      How servers and clients communicate.
@end menu



@node Layers
@section Layers

The @command{mds} display server in architectured in
three layers. The first layer is called the kernel.
The kernel is responsible for acquiring a display
server index@footnote{As with any display server,
the system can have multiple instances of
@command{mds} running at the same time.}, set up
environment variables to indicate which display
server and display server instance is being used,
create a domain socket for the display server and
start the master server and restart it if it crashes,
and then clean up the system when the display server
closes. The kernel only responsible for creating
the domain socket for communication with the display
server, it is not responsible for using it, that
mission falls to the master server.

The second layer is the master server. The master
server has two responsibilities: coordinating
message passing between other servers and clients
@footnote{In @command{mds} their is no functional
distinction between servers and clients, the
distinction is purely semantic.} and starting
other servers.

The third layer is the other servers and clients.
protocolwise there is no specification on how
they are started. But in the reference
implementation of the master server, this is
done by starting a shell script with the
pathname @file{$@{XDG_CONFIG_HOME@}/mdsinitrc}
and the user is responsible for providing the
logic in that shell script.@footnote{Moonstruck
users are allowed to implement this in C
or any other language of their choosing.}
@c Which is better: cray-cray users, lunatic users,
@c moonstruck users, insane users, ballers, madmen,
@c loony tunes?
These servers implements the actual functionality
of the display server.



@node Interprocess Communication
@section Interprocess Communication

Intrinsic to @command{mds} is a powerful
interprocess communication mechanism. Servers
and clients connect to the display server by
connecting to a domain socket served by the
master server. A server or client that has
connected to the display server can do three
things:

@itemize
@item
Request assignment of a unique ID.

@item
Multicast a message.

@item
Join or leave a multicast groups.
@end itemize

Upon assignment of an ID the master server
will automatically place the client in a
multicast group for that specific client.
This automatically multicast group assignment
is done by the master server simply so you
as a debugger do not forget to do so. When
a client is disconnected it will and out a
message to a specific multicast group that
the client, refered to by it's ID, have closed.

A message in the @command{mds} protocol is
comprised of two parts: headers and a payload.
When a client joins a multicast group it is
actually say that it is interested and receiving
broadcasts containing a specific header or a
specific header--value pair, or that it is
interesting in all messages@footnote{This
could be used for logging, possibly spying and
networking.}. Thus a message is automatically
multicasted to groups indicated by its headers.

The multicast groups and receiving of groups
is called interceptions. The interesting
property of interceptions is that they may
be modifying. When a server registers for
message interception it can say that it wants
to be able to modify messages. If this is done
and the server receives a message for which it
has said it want to be able to modify it,
the master server will wait for that server
to respond before it send the message to
the next server in the interception list.
The server can choose to do three things
with a message that it has opted in for
modification of: leave the message as-is,
modify the message, or consume the message.
A message consumption is done by modify
the message to make it empty. A consumed
message will not be send to any further
clients or servers in the interception list.

To make this mechanism sensible, a server or
client can set a priority when it registers
for interception (does not need to be
modifying.) When a message is broadcasted it
will be received by all servers in the
interception except the original sender,
unless it gets consumes. The order in which
the master server sends the message to the
recipients is determined by priority the
servers registed with. The message first sent
to the recipients with highest priority and
last to the recipients with lowestr priority,
and orderd by the priority between those
priorities. Of two or more servers have the
same priority the order in which they will
receive the message, of those recipients,
is arbitrary.

An interesting property of this machanism
is demonstrated in the @command{mds-vt}
server. Unlike most servers @command{mds-vt}
maintains two concurrent connections to
the display. Once @command{mds-vt} receives
a signal from the OS kernel requesting to
switch virtual terminal, @command{mds-vt}
will from one of its connections send
out a message and wait for it to be
received in its other connection and the
let the OS kernel switch virtual terminal.
The secondary connection to the display
has registered interception with lower
priority of the message that the primary
connection broadcasts. This message will
be received by other servers that will
let the message continue to the next
server in the interception list once that
server is ready for the OS kernel to switch
virtual terminal. All of these server has
registered modifying interception of the
message but none will actually modify or
consume the message; it is only used a
mechanism for letting @command{mds-vt} know
when all servers are ready for the switch
without having to know how many they are
and wait for a reply from all of them.



@node Application Design
@chapter Application Design

When creating graphical @command{mds} applications,
there are some guildlines you should follow.

@itemize @bullet
@item
@b{Do not create client side decoration}. Some
users do not want decorations or wants minimal
decorations. Windows should look similar, server
side decoration helps ensure this. Your client side
decorations may not meet the requirements the users
have. For example, your client side decoration may
only support minimise, maximise and close, whilst the
user may also want, as provided by her decoration
server, stick, shade and always on top. And it should
be sufficient to configure your decorations once
rather once for every toolkit. Additionally, because
of oversight from developers, client side decoration
tends to work poorly with tiling window managers.

@item
@b{Do not remember size and position}. Some users
do not want their programs to remember their size
and position. There is also a risk that your
mechanism for implementing this does not account
for the possibility that outputs may have been
removed, resized or relocated.
@command{mds-posmem} can be used if the user wants
programs to start where they were closed the last
time they were closed.
@end itemize



@node Protocol
@chapter Protocol

@menu
* Environment Variables::           Identifying the active display server.
* Signals::                         Signalling individual servers.
* Filesystem::                      The display server's footprint on the filesystem.
* Message Passing::                 Sending messages between servers and clients.
* Interception::                    Implementing protocols and writing unanticipated clients.
* Portability::                     Restrictions for portability on protocols.
@end menu



@node Environment Variables
@section Environment Variables

A crucial of any display server is letting child
processes know which display server they should
connect to. @command{X.org} does by setting the
environment variable @env{DISPLAY} to
@code{<host>:<display index>}, where @code{<host>}
is empty if the display is one the local machine.
In this tradition @command{mds} does the same thing
with the environment variable @env{MDS_DISPLAY}.

@command{mds} also creates a new process group and
export the new process group ID to the environment
variable @command{MDS_PGROUP}. This process group
can be used to send signals to all @command{mds}
servers collectively.



@node Signals
@section Signals

@command{mds} servers can re-execute into an
updated version of their binary. This can be
used to update display server online after
a new version has been installed. To do this
send the signal @command{SIGUSR1} to the server
you want update. If a server does not support
online updating it will ignore this signal.
If the operating system defines a signal named
@command{SIGUPDATE}, this signal is used
instead of @command{SIGUSR1}.

If you need servers to free up allocated
memory that they do not use, send the signal
@command{SIGDANGER}, or if not defined
@command{SIGRTMIN + 1}. Unimportant servers may
choose to die on @command{SIGDANGER}.



@node Filesystem
@section Filesystem

The @command{mds} kernel creates two directories
for the @command{mds} servers to use: one for
runtime data and one for temporary data.
These directories are named by
@code{MDS_RUNTIME_ROOT_DIRECTORY} and
@code{MDS_STORAGE_ROOT_DIRECTORY}, respectively,
by the header file @file{<libmdsserver/config.h>}.
If the systems runtime data directory is @file{/run}
and transient temporary data directory is @file{/tmp},
and the package name of @command{mds} is @command{mds},
these directories will be @file{/run/mds} and
@file{/tmp/.@{system-directory@}.mds}, respectively.
In @file{/tmp/.@{system-directory@}.mds} the kernel
will create a directory for the display server instance
named @file{.data} prefixed by the display server index.
For example if the display server index is zero,
temporary data may be stored in
@file{/tmp/.@{system-directory@}.mds/0.data}

As defined by @code{SHM_PATH_PATTERN} by
@file{<libmdsserver/config.h>}, when a server
re-executes itself it will marshal its state to
the POSIX shared memory unit named
@file{/.proc-pid-%ji}, where @file{%ji}
@footnote{@code{%ji} is the pattern in @code{*printf}
functions for the data type @code{intmax_t}.} is
replaced with the process ID of the server. This
file will be bound to the pathname
@file{/dev/shm/.proc-pid-%ji} if POSIX shared
memory is stored in @file{/dev/shm} by the
operating system.

In @code{MDS_RUNTIME_ROOT_DIRECTORY} the kernel
will create two files. @file{.pid} and @file{.socket},
both prefixed with the display server index
@footnote{@file{0.pid} and @file{0.socket} if
the display server index is 0.}. The @file{.pid}
file contains the process ID of the display server
and is used by the kernel to figure out whether
an display server index is still in use or just
not properly cleaned up. Of course it can be used
by any program to find the process ID of the
kernel process of a display server instance.
The @file{.socket} is the domain socket used
for communication with the display server and
its servers and clients.



@node Message Passing
@section Message Passing

Message passing over domain sockets is the
underlaying technique for communicating with
the display server. To communicate with the
display server in the local machine a process
must connect to the domain socket created by
the display server kernel as named in
@ref{Filesystem}.

Clients should request a unique ID when it
connects to the display server.@footnote{There
is seldom a reason for servers to do this.}
To do this the client sends

@example
Command: assign-id\n
Message ID: 0\n
\n
@end example

where @code{\n} is an LF-line break.
The value on the @code{Message ID} line
does not need to be 0, but servers and
clients often start with 0 and count
upwards. The value is however bound to
an unsigned 32-bit integer. All message
must contain this @code{Message ID} header,
otherwise the message is considered corrupt
and is ignored.

The empty line signifies the end of the
header list, and in this case the end of
the message. But a message may contain
payload beneath this empty line. To
include a payload, add the header
@code{Length} that says how many bytes
the payload is comprised of.

A header must contain a header name and
header value without any trailing or
leading spaces, and `: ' (colon, one
regular blank space) exactly delimits
the name and the value.

When the master server receives this
@code{Command: assign-id} message it
will assign the client a unique ID
and send it to the client.@footnote{The
master server is the only server than
can address the client uniquely before
it has an ID, so this part can only
be implement in the master server.}
If the client already has an ID, it
will send back that ID to the client.
This response consists of two headers
@code{ID assignment} and @code{In
response to}, containing the client's
new (or possibly already assigned) ID
and the value that was in the
@code{Message ID} header, respectively.
For example:

@example
ID assignment: 0:1\n
In response to: 0\n
\n
@end example

Notice that the master server never
includes @code{Message ID} in message
originating from it.

As seen in this example, the client ID
consists of two integers delimited by
a colon (`:'). Both of these integers
are unsigned 32-bit integers. This is
done this way because unsigned 64-bit
integers are forbidden because it is
not supportable natively be some
programming languages.

Before a has gotten a unique client ID
assigned to it, it will be `0:0'.

If a client gets disconnected from the
master server, the master server will
sends out a signal header message.
This header will be @code{Client closed}
and contain ID of the client that closed.
For example:

@example
Client closed: 0:1\n
\n
@end example

Be aware that if a server or client
closes and does not have a unique client
ID, this message will be:

@example
Client closed: 0:0\n
\n
@end example

Once a client has an unique client ID
assigned to it, it should always include
the header @code{Client ID} in its
messages. The value of @code{Client ID}
should be the client's ID. If a server
wants to address this client, it should
include the header @code{To} with the
value set to the recipient's client ID.
Be aware that such message may not be
sent to that recipient uniquely, any
server or client is free to sign up
for receive of such message, any messages
or message contain any other header or
header--value pair that may also be
included in the header.



@node Interception
@section Interception

As discussed in @ref{Interprocess Communication},
interception in the primary feature of
@command{mds}'s message passing system.
Not only does it enable servers to select
which message it wants to receive in order
to provide it's service. It also enables
clients to do anything, things that was
never anticipated. As an exaple of its
power, @command{mds} does not provide any
protocol for taking screenshots or recording
a session. Instead, a screenshot application
signs up for messages pass between the
compositor and presentation servers, and
simply requests that the compositor resends
the screen, a feature intended for the
presentation servers. A screen recoding
application would do the same and just
hang on and record all message passed
between the servers.

If you want your server or client to
receive all messages passed around in
the display server, simply sign up for
all messages:

@example
Command: intercept\n
Message ID: 0\n
\n
@end example

But if you only want messages contain
the header @code{Command}, include
that header in the payload of the message:

@example
Command: intercept\n
Message ID: 0\n
Length: 8\n
\n
Command\n
@end example

It is allowed to include multiple headers.
You can also be more strict, and require
a specific value for a header, for example:

@example
Command: intercept\n
Message ID: 0\n
Length: 16\n
\n
Command: get-vt\n
@end example

You may mix these two types of requirements
freely. Your client will receive any message
that satisfies at least one of the requirements,
these requirements may be split into multiple
message or coalesced into one message; but
you cannot request to include receive a message
if multiple requirements are satisfied.

Alternatively you can choose to stop receiving
message that satisfies requirements. For example:

@example
Command: intercept\n
Stop: yes\n
Message ID: 1\n
Length: 16\n
\n
Command: get-vt\n
@end example

Or stop receiving all messages:

@example
Command: intercept\n
Stop: yes\n
Message ID: 1\n
\n
@end example

Note that this will stop you from receiving
messages contain the @code{To} header addressed
to you until you request to receiving such
messages again.

When you sign up for message you may request
to be able to modify them before that are
send to the next client in the list of client
that should receive them. To do this include
the header--value pair @code{Modifying: yes}:

@example
Command: intercept\n
Modifying: yes\n
Message ID: 0\n
Length: 30\n
\n
Command: keyboard-enumeration\n
@end example

It is up to the client to keep track of
which message that it may modify. When
you receive a message that you can modify
you must respond when you are done with
the message.

For example, if you have signed up
for @code{Command: keyboard-enumeration}
with the ability to modify such messages
and the message

@example
Command: keyboard-enumeration\n
To: 0:1\n
In response to: 2\n
Message ID: 1\n
Length: 7\n
\n
kernel\n
@end example

is send from a server, you may receive
it as

@example
Command: keyboard-enumeration\n
To: 0:1\n
In response to: 2\n
Message ID: 1\n
Length: 7\n
Modify ID: 4\n
\n
kernel\n
@end example

Be aware that the @code{Modify ID} may
be included even if you have not signed
up to be able to modify the message,
it is enough that one client before you
has or it was originally included
@footnote{You may however not include
this header when you send out an
orginal message.}.

If you receive the message as such
and want to add the line
@code{on-screen-keyboard-20376} to
the payload should send out:
@footnote{The first line containing
starting with @code{Message ID} is an
example, it should be whatever is
appropriate for your client.}

@example
Modify ID: 4\n
Message ID: 2\n
Modify: yes\n
Length: 127\n
\n
Command: keyboard-enumeration\n
To: 0:1\n
In response to: 2\n
Message ID: 1\n
Length: 32\n
Modify ID: 4\n
\n
kernel\n
on-screen-keyboard-20376\n
@end example

If you however decide not to modify
the message send out

@example
Modify ID: 4\n
Message ID: 2\n
Modify: no\n
\n
@end example

There is also a third option:
to consume to the message. This
stops any further clients from
receiving the message. This is
done by modifying the message
into an empty message:

@example
Modify ID: 4\n
Message ID: 2\n
Modify: yes\n
\n
@end example

You may choose to include the
header--value pair @code{Length: 0},
it is however redundant and
discouraged.

This mechanism of being able to
modify message does not make much
sense unless you can control in
the order the clients receive
messages. This is done with what
is called priority. The higher
priority you have, the earlier
you will receive the message. The
default priority is zero, and the
priority is bound to a signed
64-bit integer. If you want to
be able to list yourself in
@code{Command: keyboard-enumeration}
message, you should sign up
with a positive priority since
the final recipient or requested
the enumeration will receive it
with priority zero. Therefore
you should sign up for such message
with a message like:
@footnote{4611686018427387904 is
halfway to the maximium value.}

@example
Command: intercept\n
Modifying: yes\n
Priority: 4611686018427387904\n
Message ID: 0\n
Length: 30\n
\n
Command: keyboard-enumeration\n
@end example



@node Portability
@section Portability

For optimal portability, there are some restrictions
on protocols.

@itemize @bullet{}
@item
Because C allows unsigned integers to be encoded as
either sign and magnitude, one's complement or two's
complement@footnote{GCC only allows two's complement},
the minimum value of any signed value with a fixed
bit-size is the negative of its maximum value, that
is, the minimum value @code{int16_t} is to be assumed
to be @code{-INT16_MAX} (@math{-32767}) rather than
@code{INT16_MIN} (@math{-32768} with two's complement.)

@item
Integer that are not especially encoded must not be
larger than 64-bits if they use fixed bit-size. If,
for example, @code{size_t} is 128-bits on your
platform but you are using a language that only have
native integers up to 64-bits you must use arbitrary
size integers or otherwise make sure that the value
can be properly stored and used.

@item
Integer 64-bits that are not especially encoded
must not be unsigned if the bit-size is fixed.

@item
Native endianness when a endianness is choosen.
Do not assume big endianness, but the same
endianness that appear on the same machine when
using C.

@item
All strings musts be encoded in UTF-8 without
any NUL-character unless expressive permission
is given. NUL-character may be encoded either
using a zero byte or using Modified UTF-8, where
it is encoded using two bytes. Which is used is
selected in the protocol, however headers and
their values must not include NUL-characters.
No character may be encoded with more bytes than
necessary.

@item
The new line-character is always LF (@code{'\n'},
10, line feed) and never a combination of LF and
any other character, or multiple LF:s.
@end itemize



@node Utilities
@chapter Utilities

@menu
* mds-respawn::                     The server immortality protocol.
* mds-reg::                         The registry control command.
* mds-clip::                        The clipboard control command.
* mds-screenshot::                  The screenshot utility.
* mds-slay::                        The process killing utility.
* mds-chvt::                        Utility for switching virtual terminal.
* mds-kbdc::                        The keyboard layout compiler.
* External Utilities::              Suggestion on utilities you can utilise.
@end menu



@node mds-respawn
@section @command{mds-respawn}

@command{mds-respawn} is a utility intended to be used
in @file{$@{XDG_CONFIG_HOME@}/mdsinitrc}. It will spawn
a selected set of servers. If a server it spawns exits
with a bad status, @command{mds-respawn} will respawn it.
@command{mds-respawn} supports two options in the command
line:

@table @option
@item --alarm=SECONDS
Schedule @command{mds-respawn} to die in @var{SECONDS}
seconds. At most 1 minute.

@item --interval=SECONDS
Spawned servers that die twice with @var{SECONDS}
seconds should stop respawning until the signal
@code{SIGUSR2} is send to @command{mds-respawn}.
At most 1 minute.
@end table

Commands for servers to spawn are specified within
curly braces. Each of the braces must be alone its
its own argument. For example:

@example
mds-respawn --interval=5       \
  @{ mds-foo --initial-spawn @}  \
  @{ mds-bar --initial-spawn @}  &
@end example

will spawn and supervise the servers @command{mds-foo}
and @command{mds-bar}. Both spawned with the
argument @option{--initial-spawn}. When a server is
respawed by @command{mds-respawn}, @option{--initial-spawn}
in its argument list will be replaced by
@option{--respawn} to let the server know it is being
respawned.

A server is considered to exit with a failure status
unless it exits with the return value 0 or is terminated
by the signal @code{SIGTERM}.



@node mds-reg
@section @command{mds-reg}

@command{mds-reg} is a utility that can be used to list,
available protocols provided by running servers. It can
also wait for a set of protocols to become available. To
list all available protocols run @command{mds-reg --list}.
And to wait for the protocol @code{foo} run
@command{mds-reg --wait=foo}. To also wait for the protocol
@code{bar} run @command{mds-reg --wait=foo,bar} or
@command{mds-reg --wait=foo --wait=bar}. Both of these
styles can be mixed if you want to wait for even more
protocols.



@node mds-clip
@section @command{mds-clip}

@command{mds-clip} is a utility that can be used to
review the clipboards on the display and manipulate them.
@command{mds-clip} recognises the following options:

@table @option
@item --push
Push non-option arguments from the command line into
the clipboard.

@item --expire=SECONDS
Can be used with @option{--push}. The clip will not
removed after @var{SECONDS} seconds.

@item --pop
Pop items from the clipboard whose indices are listed
in the command line as non-option arguments. The first
index is 1.

@item --clear
Pop all items in the clipboard.

@item --list
List items in the clipboard whose indices are listed
in the command line as non-option arguments. The first
index is 1. If no indicies are specified, all clips
will be listed.

@item --size
Print the size of the clipboard, the number of clips
in the clipboard.

@item --capacity
Print the capacity of the clipboard, the number of
clips the clipboard can hold. If both @option{--size}
and @option{--capacity} is used, the size will be
printed on the first line and the capacity will be
printed on the second line.

@item --resize=CAPACITY
Change the capaciy of the clipboard to @var{CAPACITY}
clips.

@item --stdin
Can be used with @option{--push}. If used, the clip
that should be placed on the top of the clipboard
stack should be read from stdin.

@item --delimiter=DELIMITER
Can be used with @option{--stdin} or @option{--list}.
If used with @option{--stdin}, an line containing
only @var{DELIMITER} will delimit two values that
should be placed in the clipboard. If used with
@option{--list}, a line containing only @var{DELIMITER}
will delimit two values in the output. The default
delimiter for @option{--list} is an empty line.

@item -1
Use the primary clipboard, that is, the text copy
clipboard. This is the default clipboard.

@item -2
Use the secondary clipboard, that is, the text
selection clipboard.

@item -3
Use the tertiary clipboard, that is, the non-text
copy clipboard.
@end table



@node mds-screenshot
@section @command{mds-screenshot}

@command{mds-screenshot} is a simple utility, and
reference implementation thereof, that can take a
screeenshot of either the display, a monitor, or a
window with or without its decorating window. It can
also include or exclude the rat cursor or gamma
ramps. @command{mds-screenshot} recognises the
following options:

@table @option
@item --monitor
Take screenshot of the monitor. The rat will
be used to select monitor.

@item --monitor=WINDOW_ID
Take screenshot of the monitor whose root window's
window ID is @var{WINDOW_ID} or has another window
in it whose window ID is @var{WINDOW_ID}.

@item --embed
Take a screenshot of an embedded window.
The rat will be used to select window.

@item --embed=WINDOW_ID
Take a screenshot of an embedded window whose
window ID is @var{WINDOW_ID}.

@item --window
Take a screenshot a window.
The rat will be used to select window.

@item --window=WINDOW_ID
Take a screenshot of a window whose
window ID is @var{WINDOW_ID}.

@item --decoration
Include the window's decoration, if used together
with @option{--window}. Ignored if used without
@option{--window}.

@item --cursor
Include the rat cursor in the screenshot.

@item --gamma
Include the effects of gamma ramps in the screenshot.

@item --low-gamma=LOW_PRIORITY
Include the effects of gamma ramps with a priority
above @var{LOW_PRIORITY} in the screenshot.

@item --high-gamma=HIGH_PRIORITY
Include the effects of gamma ramps with a priority
below @var{HIGH_PRIORITY} in the screenshot. If used
together with @option{--low-gamma=LOW_PRIORITY},
the range [@var{LOW_PRIORITY}, @var{HIGH_PRIORITY}]
will be used.
@end table

Optionally, you can add a non-option argument that
specifies the pathname of the saved file.

If neither @option{--monitor}, @option{--embed} or
@option{--window} is used, a screenshot will be
taked of the display. That is, all monitors.

In case of mirrored outputs, one of the potential
outputs will be selected arbitrarily if
@option{--gamma}, @option{--low-gamma} or
@option{--high-gamma} is used. If neither is used,
the screenshot will be identical for all mirrored
outputs.



@node mds-slay
@section @command{mds-slay}

@command{mds-slay} a utility that can be used to
kill a process by it window or identify the window
ID of a window. @command{mds-slay} recognises the
following options:

@table @option
@item --embed
Kill an embedded window.
The rat will be used to select window.

@item --embed=WINDOW_ID
Kill an embedded window whose
window ID is @var{WINDOW_ID}.

@item --window
Kill a window.
The rat will be used to select window.

@item --window=WINDOW_ID
Kill a window whose
window ID is @var{WINDOW_ID}.

@item --signal=SIGNAL
Send the signal @var{SIGNAL} to the
process owning the selected window.

@item --no-signal
Do not send a signal; only identify the
window.

@item --keep-cursor
Do not change the cursor to a kill cursor.

@item --print
The the ID of the selected window.
@end table



@node mds-chvt
@section @command{mds-chvt}

@command{mds-chvt} is a utility similar to the command
@command{chvt} from the @command{kbd} project. However,
@command{mds-chvt} has setuid and therefore does not
require root permissions, but it will only request a
virtual terminal switch if the display server's virtual
terminal is in the foreground. @command{mds-chvt}
recognises the following options:

@table @option
@item --switch=VT
Switch to the virtual terminal with the index @var{VT}.
@end table



@node mds-kbdc
@section @command{mds-kbdc}
@command{mds-kbdc} is the program used to compile
keyboard layouts and compose tables.

TODO how to use mds-kbdc



@node External Utilities
@section External Utilities

Servers let you use the option @command{--on-init-fork}
to put the process in the background when it has been
initialised. This can used to spawn that depend on each
other in linear order. For example, if @command{mds-bar}
requires that @command{mds-foo} is initialised before it
can be initialised, you can in
@file{$@{XDG_CONFIG_HOME@}/mdsinitrc} write:

@example
mds-foo --on-init-fork
mds-bar &
@end example

This will start @command{mds-bar} when @command{mds-foo}
has been initialised. However if one of them crashes,
that server will not respawn; to fix this @command{mds-respawn}
can be used, but use of @command{mds-respawn} hinders
the use of @option{--on-init-fork}. Instead you can use
@option{--on-init-sh} and global semaphores. The packages,
and commands, @command{cmdipc} and @command{ipcmd} can be
used for this purpose. We will use @command{cmdipc} in an
example:

@example
S=$(cmdipc -Scx set 1 | cut -d ' ' -f 2)
# Create a System V semaphore with the value 1.

mds-respawn @{ mds-foo --on-init-sh="cmdipc -Sk $S p" @} &
# Spawn `mds-foo` and decrease the semaphore with 1 when initialised.

cmdipc -Sk $S z            # Wait for the semaphore's value to become 0.
cmdipc -Srk $S             # Remove the semaphore.
mds-respawn @{ mds-bar @} &  # Spawn `mds-bar`.
@end example

This is however seldom necessary as @command{mds-reg} can
often be used instead, with more abstraction as you would
only need to specify what servers need to wait for, not
what they provide.

Another useful command (and package) is @command{setpgrp}.
@command{mds} puts itself an all its children in a new
process group. However you may want to put processes that
are not @command{mds} servers or @command{mds} utilities
in a separate process group. @command{setpgrp} can be used
to starta process in a new process group.



@node Servers
@chapter Servers

An @command{mds} display server instance is comprised of
multiple small servers that each implements a small part
of the display server's functionallity. This chapter will
include all servers but the master sever, @command{mds-server}
and the kernel, @command{mds}, the latter of which is not
actually a server.

@menu
* mds-echo::                        The @command{mds-echo} server.
* mds-registry::                    The @command{mds-registry} server.
* mds-vt::                          The @command{mds-vt} server.
* mds-clipboard::                   The @command{mds-clipboard} server.
* mds-drag::                        The @command{mds-drag} server.
* mds-kkbd::                        The @command{mds-kkbd} server.
* mds-kkbdrate::                    The @command{mds-kkbdrate} server.
* mds-kbd::                         The @command{mds-kbd} server.
* mds-keytrans::                    The @command{mds-keytrans} server.
* mds-keystick::                    The @command{mds-keystick} server.
* mds-keybounce::                   The @command{mds-keybounce} server.
* mds-slowkey::                     The @command{mds-slowkey} server.
* mds-keycue::                      The @command{mds-keycue} server.
* mds-kbdbind::                     The @command{mds-kbdbind} server.
* mds-multikey::                    The @command{mds-multikey} server.
* mds-rat::                         The @command{mds-rat} server.
* mds-ratbarrier::                  The @command{mds-ratbarrier} server.
* mds-ratbind::                     The @command{mds-ratbind} server.
* mds-gestures::                    The @command{mds-gestures} server.
* mds-kbd2rat::                     The @command{mds-kbd2rat} server.
* mds-hwcursor::                    The @command{mds-hwcursor} server.
* mds-swcursor::                    The @command{mds-swcursor} server.
* mds-cursorshadow::                The @command{mds-cursorshadow} server.
* mds-cursorgamma::                 The @command{mds-cursorgamma} server.
* mds-hwgamma::                     The @command{mds-hwgamma} server.
* mds-swgamma::                     The @command{mds-swgamma} server.
* mds-coopgamma::                   The @command{mds-coopgamma} server.
* mds-dcvs::                        The @command{mds-dcvs} server.
* mds-colour::                      The @command{mds-colour} server.
* mds-retro-crt::                   The @command{mds-retro-crt} server.
* mds-state::                       The @command{mds-state} server.
* mds-focus::                       The @command{mds-focus} server.
* mds-kill::                        The @command{mds-kill} server.
* mds-screensaver::                 The @command{mds-screensaver} server.
* mds-compositor::                  The @command{mds-compositor} server.
* mds-crtc::                        The @command{mds-crtc} server.
* mds-dri::                         The @command{mds-dri} server.
* mds-fb::                          The @command{mds-fb} server.
* mds-mds::                         The @command{mds-mds} server.
* mds-meta::                        The @command{mds-meta} server.
* mds-seat::                        The @command{mds-seat} server.
* mds-nest::                        The @command{mds-nest} server.
* mds-host::                        The @command{mds-host} server.
* mds-remote::                      The @command{mds-remote} server.
* mds-xmds::                        The @command{mds-xmds} server.
* mds-wmds::                        The @command{mds-wmds} server.
* mds-mmds::                        The @command{mds-mmds} server.
* mds-mdsx::                        The @command{mds-mdsx} server.
* mds-mdsw::                        The @command{mds-mdsw} server.
* mds-mdsm::                        The @command{mds-mdsm} server.
* mds-posmem::                      The @command{mds-posmem} server.
* mds-decorator::                   The @command{mds-decorator} server.
* mds-tile::                        The @command{mds-tile} server.
* mds-stack::                       The @command{mds-stack} server.
* mds-workspace::                   The @command{mds-workspace} server.
* mds-tray::                        The @command{mds-tray} server.
@end menu



@node mds-echo
@section @command{mds-echo}

@command{mds-echo} is a server that echos message that
contain the header--value pair @command{Command: echo}.
This server can be used for debugging and testing as
well as to enable network heartbeats.



@node mds-registry
@section @command{mds-registry}

@command{mds-registry} is a server that keeps a registry
of all protocols that are supported they the sum of all
active servers. It can also be used by other servers to
wait until a protocol has become available.



@node mds-vt
@section @command{mds-vt}

@command{mds-vt} is the server that acquires a virtual
terminal for the display, manages virtual terminal
switches and enables other servers to get access to
the virtual terminal's TTY and informs them of which
virtual terminal the display is located on. It also
enables other servers to switch the virtual terminals
mode to graphical mode or text mode.

By default @command{mds-vt} will select the next
available virtual terminal for the display server.
You can override this behaviour by exporting a value
to the environment variable @env{XDG_VTNR}. The value
must be a decimal integer of a valid virtual terminal
index@footnote{Which is the same thing as a valid TTY
index.}. To select the virtual terminal the display
was started from you can use the following code in
your @file{~/.mdsinitrc}:

@example
export XDG_VTNR="$(fgconsole)"
@end example

@command{fgconsole} is a part of the @command{kbd}
package.



@node mds-clipboard
@section @command{mds-clipboard}

@command{mds} has three clipboards, one for copied
text, one for selected text, and one for non-textual
data. Each of these clipboards are stacks, just
like in GNU Emacs. @command{mds-clipboard} implements
these clipboards and automatic removal of outdated
clips. Clips can be configured to expire based on
time or when its originator closes.



@node mds-drag
@section @command{mds-drag}

@command{mds-drag} is the server that implements
drag-and-drop support.



@node mds-kkbd
@section @command{mds-kkbd}

@command{mds-kkbd} implements access to the kernel-based
keyboard. It does not however implement delay and rate
configurations for the kernel-based keyboard as that
requires root privileges. The kernel-based keyboard is
a keyboard that can be accessed by reconfiguring
stdin in a TTY using @code{ioctl} and then read from
stdin. @command{mds-kkbd} does not implement any keyboard
layout, rather it broadcasts scancodes and keycode.
However it can remap keycodes, but not scancodes.



@node mds-kkbdrate
@section @command{mds-kkbdrate}

@command{mds-kkbdrate} is a complemental server to
@command{mds-kkbd}, it implements rate and delay
control for the kernel-based keyboard.



@node mds-kbd
@section @command{mds-kbd}

@command{mds-kbd} is an alternative to
@command{mds-kkbd} and @command{mds-kkbdrate}.
In contrast to @command{mds-kkbd}, @command{mds-kbd}
implements control over individual keybroads rather
than utilising the kernels keyboard drivers to
treats all keyboards a one keyboard. This server
is only useful for multiseat sessions and if you
otherwise actually want to handle the keyboards
individually.



@node mds-keytrans
@section @command{mds-keytrans}

@command{mds-keytrans} is the server than translates
keycodes from @command{mds-kkbd} and @command{mds-kbd},
and third-party alternatives, to characters and other
attributes. It implements the keyboard's layouts including
modifiers, letters, other symbols, dead keys and compose.



@node mds-keystick
@section @command{mds-keystick}

@command{mds-keystick} is a server that can be used
to enable sticky keys. See @ref{Sticky Keys} for more
information.



@node mds-keybounce
@section @command{mds-keybounce}

@command{mds-keybounce} is a server that can be used
to enable bounce keys. See @ref{Bounce Keys} for more
information.



@node mds-slowkey
@section @command{mds-slowkey}

@command{mds-slowkey} is a server that can be used
to enable slow keys. See @ref{Slow Keys} for more
information.



@node mds-keycue
@section @command{mds-keycue}

@command{mds-keycue} is a server that can be used
to enable loud keys. See @ref{Loud Keys} for more
information.




@node mds-kbdbind
@section @command{mds-kbdbind}

@command{mds-kbdbind} is a server similar to
@command{xbindkeys}. It can be used to run commands
upon selected key combinations, for example starting
@code{dmenu} or change keyboard layout.
@command{mds-kbdbind} can distinguish keyboards
from eachother.



@node mds-multikey
@section @command{mds-multikey}

@command{mds-multikey} is a server that can
bind a key, key combination, or sequence their
of to a sequence of keys or key combinations.
For example, you could bind `<super>x, y' to
simulate that a key `Faux1' is pressed, a key
that does not exist, this key press could be
picked up by @command{mds-kbdbind} to enable
@command{mds-kbdbind} to respond to squences
rather than single keys and single key
combinations. alternatively you could bind
`<super>x' to press `x' a selected number of
times with a short selectable delay between
them; or `<super>x, 5' to press `x' five times.



@node mds-rat
@section @command{mds-rat}

@command{mds-rat} is a server that implements
support of rat (also known as mouse) devices.



@node mds-ratbarrier
@section @command{mds-ratbarrier}

@command{mds-ratbarrier} is a server that
lets you set up barriers for the rat pointer,
for example at the screen edges.

A barrier requires that the rat be moved a
bit extra move it can move on to the next
pixel. A barrier can be directional. A barrier
can also be infinite, which blocks the rat
pointer fully and it cannot pass through at all.



@node mds-ratbind
@section @command{mds-ratbind}

@command{mds-ratbind} is a server similar to
@command{mds-kbdbind}. However, @command{mds-ratbind}
respons to rat and rat cursor actions rather
than keyboard actions. It can for example be
used to implement hotcorners.



@node mds-gestures
@section @command{mds-gestures}

@command{mds-gestures} is a server similar to
@command{mds-ratbind}. However it is specialised
to respond to gestures.



@node mds-kbd2rat
@section @command{mds-kbd2rat}

If you do not have a rat or rather use your keyboard,
the server @command{mds-kbd2rat} can be used to
bind keyboard actions to simulate rat actions.
This server is a specialisation of @code{mds-kbdbind},
rather than spawning generic commands it broadcasts
messages without the display server to move the
rat cursor and click on rat buttons. @code{mds-kbdbind}
could be used to do this, but @command{mds-kbd2rat}
will not spawn a new process for each action.
For more information see @ref{Mouse Keys}.



@node mds-hwcursor
@section @command{mds-hwcursor}

@command{mds-hwcursor} is a server that draws
the rat cursor to the monitor on a plane
separate from all other content on the display.
In less esoteric terms, it implements a hardware
cursor.



@node mds-swcursor
@section @command{mds-swcursor}

@command{mds-swcursor} is a server that draws
the rat cursor to the monitor on the same plane
as all other content on the display. In less
esoteric terms, it implements a software cursor.



@node mds-cursorshadow
@section @command{mds-cursorshadow}

@command{mds-cursorshadow} is a server that
can be used to decorate the rat cursor with
a configurable shadow.



@node mds-cursorgamma
@section @command{mds-cursorgamma}

@command{mds-cursorgamma} is a server you can
use if you use @command{mds-hwcursor} to, if
not done by the graphics driver, correct the
gamma correction on the hardware cursor using
software gamma ramps. This of courses works
whether you are using hardware or software
gamma ramps for your monitor's gamma correction.
If can even be used if you do not use gamma
correction, in such case, only the cursor
will have its gamma corrected.



@node mds-hwgamma
@section @command{mds-hwgamma}

To enable gamma correction, use the server
@command{mds-hwgamma}. It implements hardware
gamma ramps, that is, gamma ramps assisted
by hardware acceleration.



@node mds-swgamma
@section @command{mds-swgamma}

If your graphics driver does not support
@command{mds-hwgamma}, you can instead use
@command{mds-swgamma}. It implements software
gamma ramps, that is, it will modify each
pixel according to the selected gamma
correction before it is send to the presentation
sever. To accelerate this process,
@command{mds-swgamma} can tell programs how
to modify its colours before sending it; the
programs can then tell @command{mds-swgamma}
not to apply its correction. Programs such
as video players can also use this to tell
the server not to apply gamma correction as
that may cause the video to be played back
to slowly.



@node mds-coopgamma
@section @command{mds-coopgamma}

@command{mds-coopgamma} is a server that
can be used to enable multiple clients to
manipulate the gamma ramps without stepping
on eachothers toes. It does this by letting
clients tell which priority their corrections
has and use this data to chain together there
modifications. For example if one program
wants to apply a red filter to the display
and another program wants to correct the
monitors' gamma, the red filter program will
send lookup tables for the gamma with high
priority and the correction program will
send its lookup tables with low priority.
@command{mds-coopgamma} will then apply the
latter lookup tables on top of the red filter.
The clients can tell @command{mds-coopgamma}
whether it should remove their changes when
they close, or even keep them and wait for
the client to restart.



@node mds-dcvs
@section @command{mds-dcvs}

@command{mds-dcvs} is a server than can be
used to simulate defective colour vision.
That is, it can for example turn the display
greyscale (colour blindness) or add a filter
the simulates deuteranopia or deuteranomaly.
This server is intended for testing that
interfaces are suitable for people with
defective colour vision.



@node mds-colour
@section @command{mds-colour}

@command{mds-colour} is a server that implements
colour names, such as system colours and generic
names, for example `red', whose exact colour can
be configured by the user. A terminal written for
@command{mds} whould look up colours such as `red'
and `light red' and get the colours the terminal
should use by default. Nothing is to be assumed
for such colours, not even that `light red' is in
fact lighter than `dark red', or that `red' is in
fact `red', only that it is the colour the user
wants to see when a colour is supposed to be `red'.
@command{mds-colour} will notify clients when a
colour has been reconfigured, added or removed.

@command{mds-colour} is also responsible for
informing clients on which two colours clients
should use and how to dither them (by percent, not
by pattern). This is useful if only 16-bit colours
can be used, or if only 24-colour can used but
gradients between for example sRGB(255, 255, 255)
and sRGB(254, 254, 254) is to be drawn.

@command{mds-colour} is gamma ramp-aware. For
example, if for the red channel, 0 is mapped to
0, 1 is mapped to 3, 2 is mapped 2 and 3 is mapped
to 1, but 1 and 3 requires dithering, then if
3 is requested, @command{mds-colour} will tell
the client to dither 0 and 2 with 50 %, which
should generate 1, but 1 and 3 has been swapped.



@node mds-retro-crt
@section @command{mds-retro-crt}

@command{mds-retro-crt} is a server that applies
filters used in the terminal emulator
@command{cool-retro-term} to the whole display.



@node mds-state
@section @command{mds-state}

@command{mds-state} is the server that keeps tracks
of the windows' state.



@node mds-focus
@section @command{mds-focus}

@command{mds-focus} is the server focuses windows
and windows' components.



@node mds-kill
@section @command{mds-kill}

@command{mds-kill} is a server that can be used to
send signals to processes by identifying them by
their windows. This server can also be used to
simply identify the process that owns a window.



@node mds-screensaver
@section @command{mds-screensaver}

@command{mds-screensaver} is a server that can be
used to start a screensaver or deactive monitors when
the input devices has not be used for a period of
time provided that no client has disabled this. It
is capable of deactiving single monitors or start
a screensaver on single monitors rather than all
monitors.



@node mds-compositor
@section @command{mds-compositor}

@command{mds-compositor} is the server that composes
the output. It takes output of all windows and arranges
it to one image per monitor and sends it to the presentation
servers, such as @command{mds-dri} and @command{mds-fb}.



@node mds-crtc
@section @command{mds-crtc}

@command{mds-crtc} is the server that identifies
CRTC:s and provide access to them.



@node mds-dri
@section @command{mds-dri}

@command{mds-dri} is a server that displays
content using the Direct Rendering Infrastructure.



@node mds-fb
@section @command{mds-fb}

@command{mds-fd} is a server that displays
content using framebuffers.



@node mds-mds
@section @command{mds-mds}

@command{mds-mds} is a server that displays
content using another @command{mds} window.
It creates a window that emulates a monitor.



@node mds-meta
@section @command{mds-meta}

@command{mds-meta} is a meta-display server.
It creates or joins a named meta-display server,
and creates and alternative values for
@env{MDS_DISPLAY}. Any server connecting to
this alternative @env{MDS_DISPLAY} connects
to this meta-display server. This can be used
to make servers shared between display server
instances.

@command{mds-meta} uses the environment variable
@env{MDS_METADISPLAY} to acquire the name of the
meta-display server instance it should join or
create. If @env{MDS_METADISPLAY} has not been set
it is treated as having an empty string for its
value which is a valid meta-display server
instance name.

@command{mds-meta} works by connecting to the
running display server instance, the display,
and create a new display server instance, the
metadisplay. Messages passed via the metadisplay's
socket is forward to the display, and messages
passed to via the display to @command{mds-meta}
is send to the appropriate server. @command{mds-meta}
manages interception in the same way as
@command{mds-server} and @command{mds-remote}.

If @command{mds-meta} creates a new metadisplay,
rather than joining an existing metadisplay, it
will spawn @file{~/.mdsmetainitrc} to let you
start the shared servers.

An interesting property of @command{mds-meta} is
that it can be used to share servers across display
servers on multiple computers. For example, if you
start @command{mds-host} and @command{mds-clipboard}
inside the metadisplay on your central computer,
displays started on other servers can run
@command{mds-remote} to connect to the metadisplay
so that all computers share the same clipboard.
However, this network will be centralised and not
distributed, so it is not perfect.



@node mds-seat
@section @command{mds-seat}

@command{mds-seat} is a server that enables
seat-sandboxing. It can be used to place
two users on the same machine without them
interfering with each others monitors and
input devices. Servers started below
@command{mds-seat} become shared and servers
started above @command{mds-seat} become
seat-private. @command{mds-seat} can filter
messages from shared servers so only the
appropriate seat receives them.



@node mds-nest
@section @command{mds-nest}

@command{mds-nest} is a server that creates
a new @command{mds} instance inside another
@command{mds} instance. A display server
inside another display server.



@node mds-host
@section @command{mds-host}

@command{mds-host} is a server that enables
servers like @command{mds-remote} running
on remote machines to connect to the local
machine and its display server.



@node mds-remote
@section @command{mds-remote}

@command{mds-remote} is a server that enables
you to connect extend a remote @command{mds}
with your local machine. This can be used to
make a display server instance span multiple
computers including its monitors and input
devices.



@node mds-xmds
@section @command{mds-xmds}

@command{mds-xmds} is a server that translates
X.org calls to @command{mds} calls. It can be used
to run X.org-only programs inside @command{mds}.


@node mds-wmds
@section @command{mds-wmds}

@command{mds-wmds} is a server that translates
Wayland calls to @command{mds} calls. It can be used
to run Wayland-only programs inside @command{mds}.


@node mds-mmds
@section @command{mds-mmds}

@command{mds-mmds} is a server that translates
Mir calls to @command{mds} calls. It can be used
to run Mir-only programs inside @command{mds}.



@node mds-mdsx
@section @command{mds-mdsx}

@command{mds-mdsx} is a server that translates
@command{mds} calls to X.org calls. It can be used
to enable @command{mds} specific programs to run
inside the X.org display servers.



@node mds-mdsw
@section @command{mds-mdsw}

@command{mds-mdsw} is a server that translates
@command{mds} calls to Wayland calls. It can be used
to enable @command{mds} specific programs to run
inside the Wayland display servers.



@node mds-mdsm
@section @command{mds-mdsm}

@command{mds-mdsm} is a server that translates
@command{mds} calls to Mir calls. It can be used
to enable @command{mds} specific programs to run
inside the display server Mir.



@node mds-posmem
@section @command{mds-posmem}

@command{mds-posmem} is a server that remembers where
window's were positioned and their size, and moves
and resizes them accordingly when they are created.



@node mds-decorator
@section @command{mds-decorator}

@command{mds-decorator} is a server that provides a
simple, reference implementation of a, window decorator.



@node mds-tile
@section @command{mds-tile}

@command{mds-tile} is a server that provides a
simple, reference implementation of a, tiling window manager.



@node mds-stack
@section @command{mds-stack}

@command{mds-stack} is a server that provides a
simple, reference implementation of a, stack window manager.



@node mds-workspace
@section @command{mds-workspace}

@command{mds-workspace} is a server that provides
simple, reference implementation of, workspaces.



@node mds-tray
@section @command{mds-tray}

@command{mds-tray} is a server that provides a
simple, reference implementation of a, status icon tray.



@node Protocols
@chapter Protocols

@menu
* Infrastructure Protocols::        Infrastructure protocols.
* Virtual Terminal Protocols::      Virtual terminal protocols.
* Keyboard Protocols::              Keyboard protocols.
* Clipboard Protocols::             Clipboard protocols.
* Status Icon Protocols::           Status icon protocols.
* Colour Protocols::                Colour protocols.
* Miscellaneous Protocols::         Miscellaneous protocols.
@end menu



@node Infrastructure Protocols
@section Infrastructure Protocols

@menu
* assign-id::                       Assign new ID to client, or fetch current ID.
* intercept::                       Sign up for reception of message.
* register::                        Register availability of a command for which you implement a service.
* reregister::                      Request for reregistration for available commands.
* error::                           Notify a client about a request failure.
@end menu



@node assign-id
@subsection assign-id

@table @asis
@item Identifying header:
@code{Command: assign-id}

@item Action:
Assign new ID to client, or fetch current ID.

@item Purpose:
Assigning ID to clients so server can respond to that client.

@item Compulsivity:
Manditory, part of the core infrastructure.

@item Reference implementation:
@command{mds-server}
@end table



@node intercept
@subsection intercept

@table @asis
@item Identifying header:
@code{Command: intercept}

@item Action:
Sign up for reception of message.

@item Optional header: @code{Stop}
Stop reception of messages if the value for
the header @code{Stop} is @code{yes}.

@item Optional header: @code{Priority}
Signed 64-bit integer of reception priority
(reversed of order).

@item Optional header: @code{Modifying}
Send message asynchronously and await
modification if the value for the header
@code{Modifying} is @code{yes}.

@item Optional header: @code{Length}
Length of the message.

@item Message:
List of headers and header--value-pairs that
qualifies a message for reception, all messages
qualifies if this list is empty.

@item Purpose:
Filter received message for clients and servers.

@item Purpose:
Assigned interception order for modification of messages.

@item Compulsivity:
Manditory, part of the core infrastructure.

@item Reference implementation:
@command{mds-server}
@end table



@node register
@subsection register

@table @asis
@item Identifying header:
@code{Command: register}

@item Action:
Register availability of a command for which you implement a service.

@item Required header: @code{Client ID}
Your ID, provided by the @code{ID assignment} header
in response to a @code{Command: assign-id} header.

@item Conditionally required header: @code{Length}
Length of the message.
Required if @code{Action: list} is included in the headers.

@item Optional header: @code{Action}
@table @code
@item remove
Remove availability from registry if the value
of the header @code{Action} is @code{remove}.
@item wait
Wait until listed commands are available if the
value of the header @code{Action} is @code{wait}.
However if a protocol becomes unavailable during this
wait period it will still be counted as available for
this wait action.
@item list
Send a list of availability commands if the value
of the header @code{Action} is @code{list}.
@end table

@item Conditionally optional header: @code{Time to live}
The maximum number of seconds to wait.
Available and optional if @code{Action: wait}
is included in the headers.

@item Message:
List of values for the header @code{Command}
that you implement.

@item Purpose:
Identify supported display server operations.

@item Purpose:
Initialisation process synchronisation.

@item Compulsivity:
Highly recommended, programs may stall a bit from
time to time without it, or at initialisation
depending on the program's implementation.

@item Reference implementation:
@command{mds-registry}
@end table



@node reregister
@subsection reregister

@table @asis
@item Identifying header:
@code{Command: reregister}

@item Action:
Request that all servers resends @code{Command: register}
with either @code{Action: add} or without the @code{Action}
header (does the same thing.)

@item Purpose:
Rebuild registry created with @code{Command: register}
if the registry server crashes.

@item Compulsivity:
Highly recommended, programs may think a protocol is not
supported of the registry server crashes if you do not
implement this in your server.

@item Reference implementation:
@command{mds-registry}.
@end table



@node error
@subsection error

@table @asis
@item Identifying header:
@code{Command: error}

@item Action:
Notify a client about a request failure.

@item Required header: @code{To}
The ID of the client that send a request that failed.

@item Required header: @code{In response to}
The ID of the message whose request failed.

@item Required header: @code{Error}
The errno number of the error, 0 on success if the
message was not an information query. The string
``custom'' can be used if there is not errno number,
optionally followed by a blank space and a number
that identifies the error, this number must be
positive (not zero).

@item Conditionally optional header: @code{Length}
The length of the message.
Available and optional if ``custom'' is used in
the header @code{Error}.

@item Message:
Description of the error, single line, mid-sentence case,
no punctuation in the end, must not be question but rather
it must be a statement.

@item Purpose:
Enable keyboard layout servers to automatically set active
locks when the server starts based on currently active LED:s.

@item Compulsivity:
Optional.
@end table



@node Virtual Terminal Protocols
@section Virtual Terminal Protocols

@menu
* get-vt::                          Get the index of the virtual terminal the server is display on.
* configure-vt::                    Reconfigure the virtual terminal the server is display on.
* switching-vt::                    Notify servers about an ongoing virtual terminal switch.
@end menu



@node get-vt
@subsection get-vt

@table @asis
@item Identifying header:
@code{Command: get-vt}

@item Action:
Get the index of the virtual terminal the server is display on.

@item Required header: @code{Client ID}
Your ID, provided by the @code{ID assignment} header
in response to a @code{Command: assign-id} header.

@item Response:
The server will response with the header @code{VT index}
and the index of the virtual terminal the server is
display on in decimal format. Additionally the server
will respond with the header @code{Active} with the value
@code{yes} if the VT is in the foreground or the value
@code{no} if the VT is in the background.

@item Purpose:
Allow programs to be aware of whether the display is in the
foreground or the background.

@item Purpose:
Allow programs to be aware of which VT the server is running on.

@item Purpose:
Allow programs to gain access of the TTY associated with the
VT such that they can use ioctl and similar calls on that TTY.

@item Compulsivity:
Required.

@item Reference implementation:
@code{mds-vt}
@end table



@node configure-vt
@subsection configure-vt

@table @asis
@item Identifying header:
@code{configure-vt}

@item Action:
Reconfigure the virtual terminal the server is display on.

@item Required header: @code{Client ID}
Your ID, provided by the @code{ID assignment} header
in response to a @code{Command: assign-id} header.

@item Optional header: @code{Graphical}
@table @code
@item yes
Set the TTY graphical mode if the value of the header
@code{Graphical} is @code{yes}.
@item no
Set the TTY text mode if the value of the header
@code{Graphical} is @code{no}.
@end table
The server implementing this protocol should not set the
TTY to text mode temporarily when switching TTY. It is
up to the server that set the request for graphical mode
to temporarily switch to text mode when switching TTY.

@item Optional header: @code{Exclusive}
@table @code
@item yes
The server may block other process from opening the TTY
if the value of the header @code{Exclusive} is @code{yes}.
@item no
The server may not block other process from opening the TTY
if the value of the header @code{Exclusive} is @code{no}.
@end table
The server implementing this protocol should keep a counter
for how many servers have requested non-exclusive mode and
only switch back to exclusive mode when that counter reaches
zero

@item Response:
The server will response with a @code{Command: error}.

@item Purpose:
Allow presentation servers to enter and leave graphical mode.

@item Purpose:
Allow programs to gain access of the TTY associated with the
VT such that they can use ioctl and similar calls on that TTY.

@item Compulsivity:
Required.

@item Reference implementation:
@code{mds-vt}
@end table



@node switching-vt
@subsection switching-vt

@table @asis
@item Identifying header:
@code{Command: switching-vt}

@item Action:
Notify servers about an ongoing virtual terminal switch.

@item Required header: @code{Status}
@table @code
@item deactivating
The kernel wants to place the display in the background
if the value of the header @code{Status} is @code{deactivating}.
@item activating
The kernel wants to place the display in the foreground
if the value of the header @code{Status} is @code{activating}.
@end table

@item Instructions:
When a virtual terminal switch is requested the
server implementing control VT switching involving
the display's virtual terminal will get signaled by
the kernel. Upon this signal the server should
broadcast this command. All servers that need to
release or acquire resouces should intercept this
message with the possibility of modifying it. Once
a server is ready for the VT to switch it should
let the message pass to the next server by telling
the master server that it is no modification to do.
Once all servers are read for the switch the server
that emitted this message should signal the kernel
that it may switch VT. The server should detect
this by setting up secondary contection to the
display that intercepts this message. This connection
should intercept this message with priority @math{-2^{62}},
all servers that need to perform actions before the
switch takes place must have a priority higher than
@math{-2^{62}}, preferably 0.

@item Purpose:
Allow servers to release resources when the user switch
virtual terminal before the terminal actually changes and
to reacquire resources when the virtual terminal become
active again.

@item Compulsivity:
Required.

@item Reference implementation:
@code{mds-vt}
@end table



@node Keyboard Protocols
@section Keyboard Protocols

@menu
* key-sent::                        Announce a keyboard input event.
* enumerate-keyboards::             List available keyboards.
* keyboard-enumeration::            Response to @code{Command: enumerate-keyboards}.
* set-keyboard-leds::               Activate and deactivate LED:s on a keyboard.
* get-keyboard-leds::               List exisiting LED:s on a keyboard and their state.
* keycode-map::                     Remap keyboard keycodes and query current mapping.
* new-keyboard::                    Announce the existance of a new keyboard.
* old-keyboard::                    Announce the removal of an old keyboard.
@end menu



@node key-sent
@subsection key-sent

@table @asis
@item Identifying header:
@command{Command: key-sent}

@item Action:
Announce a keyboard input event.

@item Required header: @code{Keyboard}
Any string that uniquely identifies the keyboard.
@table @asis
@item Purpose:
Enable multi-keyboard aware programs and give at
least on keyboard per seat in a multi-seat environment.
@item Note:
mds-kkbd uses @code{kernel} to indicate that it uses
the kernel and thus lumps together all keyboards.
@end table

@item Required header: @code{Released}
@table @code
@item yes
The value of the header @code{Released} will
be @code{yes} if the key was released.
@item no
The value of the header @code{Released} will be
@code{no} otherwise, that is, held down or pressed.
@end table
Note: pause/break is automatically released directly after
it has been pressed. This is feature built into keyboards
and servers should not try to circumvent this.

@item Required header: @code{Keycode}
An unsigned 14-bit integer identifying the key, may be remapped.

@item Optional header: @code{Scancode}
Either an unsigned 7-bit integer or a single blank space
separated trio of unsigned 7-bit integers, identifying the key.
This is the scancode sent from the keyboard and optionally
unified by the keyboard driver, however with the typed/released
bit zeroed out. This may not be remapped.

@item Optional header: @code{Modifiers}
Single blank space separated list of active modifiers:
@table @code
@item shift
Shift (level 2)
@item ctrl
Control
@item alt
Alternative/Option
@item altgr
Alternative Graphic (level 3)
@item lvl*
@code{*} may be any @math{2^n + 1} integer with
@math{1 < n < 20}.
@item super
Super
@item hyper
Hyper
@item ultra
Ultra
@item caps
Caps (usually a lock key)
@item num
Num (usually a lock key)
@item scrl
Scroll (usually a lock key)
@item top
Top (historical)
@item front
Front (historical)
@item greek
Greek (historical)
@item compose
Compose (rare, it is usually a dead key)
@end table
Any key that has been locked should be prefix with
@code{+}, if the key has been locked by nullified
with non-lock modifier it should be prefixed with
a @code{-}. If no modifier is active or has been
nullified, @code{none} should be used.

@item Optional header: @code{Key}
A textual representation of the key that has been typed or
released, as mapped by the keyboard layout.
@table @code
@item esc
Escape
@item f*
F@code{*} where @code{*} is any integer.
@item sysrq
System Request/Print Screen
@item scrl
Scroll (lock)
@item break
Break/Pause
@item backspace
Backspace
@item tab
Tab
@item return
Return/Enter
@item space
Blank Space
@item menu
Application Menu
@item ins
Insert
@item home
Home
@item pgup
Page Up
@item del
Delete
@item end
End
@item pgdown
Page Down
@item up
Up Arrow
@item left
Left Arrow
@item down
Down Arrow
@item right
Right Arrow
@item shift
Shift (level 2)
@item begin
Begin (keypad 5 in nagivation mode)
@item ctrl
Control
@item alt
Alternative/Option
@item altgr
Alternative Graphic (level 3)
@item lvl*
@code{*} may be any @math{2^n + 1} integer
with @math{1 < n < 20}.
@item super
Super
@item hyper
Hyper
@item ultra
Ultra
@item caps
Caps (usually a lock key)
@item num
Num (usually a lock key)
@item scrl
Scroll (usually a lock key)
@item top
Top (historical)
@item front
Front (historical)
@item greek
Greek (historical)
@item compose
Compose (usually a dead key)
@item hexcompose
Hex-Compose (usually a dead key)
(Used to create aribitrary characters.)
@item longhexcompose
Long Hex-Compose (usually a dead key)
(Variant of hexcompose for longer codepoints.)
@item modelock
Mode Lock
@item letter *
@code{*} may be any UTF-8 encoded letter.
@end table
Keys that lock/unlock a modifer should be suffixed with a
blank space and a @code{lock}. If the key is a dead key
(even the compose key) should use @code{dead} instead. A
position, either @code{left}, @code{right}, @code{keypad}
or an index, followed by a blank space, should prefix any
key that occurs on multiple position on the keyboard,
unless it only appears on the keypad once and once not on
the keypad. Keys without any meaning should be identified
as @code{unknown}. Modifiers and dead keys should not
affect the value.

@item Optional header: @code{Characters}
UTF-8 encoded string that has been written.

@item Purpose:
Enable the user to use a keyboard, physical or on-screen.

@item Purpose:
Enable programs to send keys as part of a
script or a reply of a recorded session.

@item Compulsivity:
Highly-recommended, a computer is as good
as useless without a keyboard.

@item Reference implementation:
@command{mds-kkbd}, @command{mds-kbd} and @command{mds-keytrans}
@end table



@node enumerate-keyboards
@subsection enumerate-keyboards

@table @asis
@item Identifying header:
@command{Command: enumerate-keyboards}

@item Action:
List available keyboards.

@item Required header: @code{Client ID}
Your ID, provided by the @code{ID assignment} header
in response to a @code{Command: assign-id} header.

@item Instructions:
This message must be consumed by the first server that
receives it and implements support for it, and then send
out a @code{Command: keyboard-enumeration} populated with
the keyboard it provide as named in the @code{Keyboard}
header for protocols such as @code{Command: key-sent}.

@item Purpose:
Make it possible for clients to list all available
keyboards so that can be configured individually.

@item Compulsivity:
Optional.

@item Reference implementation:
@command{mds-kkbd} and @command{mds-kbd}
@end table



@node keyboard-enumeration
@subsection keyboard-enumeration

@table @asis
@item Identifying header:
@command{Command: keyboard-enumeration}

@item Action:
Response to @code{Command: enumerate-keyboards}.

@item Required header: @code{To}
The ID received under @code{Client ID} header in
the @code{Command: enumerate-keyboards} message
that triggered this message to be broadcasted

@item Required header: @code{In response to}
The ID received under the @code{Message ID} header
in the @code{Command: enumerate-keyboards} message
that triggered this message to be broadcasted.

@item Required header: @code{Length}
Length of the message.

@item Message:
New line separated list of available keyboards.

@item Instructions:
All keyboard servers should listen for this message
and append all keyboards it implement to the message
once recieved.

@item Purpose:
Make it possible for clients to list all available
keyboards so that can be configured individually

@item Compulsivity:
Required if you implement @command{Command: enumerate-keyboards}.

@item Reference implementation:
@command{mds-kkbd} and @command{mds-kbd}
@end table



@node set-keyboard-leds
@subsection set-keyboard-leds

@table @asis
@item Identifying header:
@command{Command: set-keyboard-leds}

@item Action:
Activate and deactivate LED:s on a keyboard.

@item Required header: @code{Active}
LED:s that should be turned on. If a LED is listed here
but not in @code{Mask} that LED should be turned on if
it is off, and turned off if it is on.

The value is a single blank space separated list of LED:s:
@table @code
@item num
Num lock
@item caps
Caps lock
@item scroll
Scroll lock
@item compose
Compose
@end table
Unsupported LED:s should be silently ignored.

@item Required header: @code{Mask}
LED:s listed here that do not appear in @code{Active}
should be turned off. The value of this header follows
the same rules as for @code{Active}.

@item Optional header: @code{Keyboard}
A string that identifies the keyboard that should be
affected. If omitted all keyboard are affected.

@item Purpose:
Enable keyboard layout servers to activate and deactive
LED:s on the keyboard to indicate active locks.

@item Compulsivity:
Optional.

@item Reference implementation:
@command{mds-kkbd}, @command{mds-kbd} and @command{mds-keytrans}
@end table



@node get-keyboard-leds
@subsection get-keyboard-leds

@table @asis
@item Identifying header:
@command{Command: get-keyboard-leds}

@item Action:
List exisiting LED:s on a keyboard and their state.

@item Required header: @code{Client ID}
Your ID, provided by the @code{ID assignment} header
in response to a @code{Command: assign-id} header.

@item Required header: @code{Keyboard}
A string that identifies the keyboard that
should be affected.

@item Response:
The server implementing support for
@code{Command: get-keyboard-leds} for the keyboard
indicated by @code{Keyboard} should send a message
back to the client indicated by rge @code{Client ID}
header (using the @code{To} header) with the headers:
@table @code
@item Active
List of currently turned on LED:s.
@item Present
List of LED:s that the server believes
to be present on the keyboards.
@end table
Both of these headers followes the rules of the
@code{Active} header under @code{Command: set-keyboard-leds}.

@item Purpose:
Enable keyboard layout servers to automatically
set active locks when the server starts based on
currently active LED:s

@item Compulsivity:
Recommended. Required if you implement support for
@code{Command: set-keyboard-leds}. If you do not
support this protocol servers and clients and stall
when they try to get the active LED:s

@item Reference implementation:
@command{mds-kkbd}, @command{mds-kbd} and @command{mds-keytrans}
@end table



@node keycode-map
@subsection keycode-map

@table @asis
@item Identifying header:
@command{Command: keycode-map}

@item Action:
Remap keyboard keycodes and query current mapping.

@item Required header: @code{Action}
@table @code
@item remap
Remap keys if the value of the header @code{Action}
is @code{remap}.
@item reset
Reset all mappings to identity mapping if the value
of the header @code{Action} is @code{reset}.
@item query
Query mapping if the value of the header @code{Action}
is @code{query}.
@end table
Each affected server will send a message format
like that of @code{Action: remap} with current
mapping that are not identity mappings.

@item Optional header: @code{Keyboard}
A string that identifies the keyboard that should be
affected. If omitted all keyboard are affected.

@item Conditionally required header: @code{Client ID}
Your ID, provided by the @code{ID assignment} header
in response to a @code{Command: assign-id} header.
Required if @code{Action: query} is included in the headers.

@item Conditionally optional header: @code{Length}
The length of the message.
Available and optional if @code{Action: remap}
is included in the headers.

@item Message:
Each line contains contains two single space delimited numbers,
the first number is the keycode as determined by the scancode,
the second number is keycode that scancode should generate.
For example, @code{1 1} resets Escape to be mapped to Escape,
and @code{1 59} remaps Escape to F1, while
@example
1 59
59 1
@end example
swaps Escape and F1.

@item Purpose:
Enable the user to swap or replace keys on the keyboard.

@item Purpose:
Enable the user manually correct an incorrectly mapped keyboard.

@item Compulsivity:
Optional.

@item Reference implementation:
@command{mds-kbd} and @command{mds-kkbd}
@end table



@node new-keyboard
@subsection new-keyboard

@table @asis
@item Identifying header:
@command{Command: new-keyboard}

@item Action:
Announce the existance of a new keyboard.

@item Required header: @code{Length}
The length of the message.

@item Message:
List of strings that identifies the keyboards
that have been added.

@item Purpose:
Enable servers and clients to detect new keyboards.

@item Compulsivity:
Recommended.

@item Reference implementation:
@command{mds-kbd} and @command{mds-kkbd}
@end table



@node old-keyboard
@subsection old-keyboard

@table @asis
@item Identifying header:
@command{Command: old-keyboard}

@item Action:
Announce the removal of an old keyboard.

@item Required header: @command{Length}
The length of the message.

@item Message:
List of strings that identifies the keyboards
that have been removed.

@item Purpose:
Enable servers and clients to detect removal of keyboards.

@item Compulsivity:
Recommended.

@item Reference implementation:
@command{mds-kbd}
@end table



@node Clipboard Protocols
@section Clipboard Protocols

@menu
* clipboard::                       Read or manipulate a clipboard.
* clipboard-info::                  Clipboard event announcements.
@end menu



@node clipboard
@subsection clipboard

@table @asis
@item Identifying header:
@command{Command: clipboard}

@item Action:
Read or manipulate a clipboard.

@item Required header: @code{Level}
The clipboard level, an [1, 3] integer:
@table @code
@item 1
Text copied/pasted using the keyboard or a menu item.
(This level is called `primary'.)
@item 2
Text copied/pasted using the rat.
(This level is called `secondary'.)
@item 3
Data to begin with a line describing the data type.
(This level is called `tertiary'.)
@end table

@item Required header: @code{Action}
What to do with the clipboard:
@table @code
@item add
Write the message to the clipboard if the value of
the header @code{Action} is @code{add}.
@item read
Read the clipboard if the value of
the header @code{Action} is @code{read}.
@item clear
Clear all entries on the selected level on the clipboard
if the value of the header @code{Action} is @code{read}.
@item set-size
Shrink/grow the clipstack if the value of
the header @code{Action} is @code{set-size}.
@item get-size
Read the size of the clipstack if the value of
the header @code{Action} is @code{get-size}.
In the reply, the server will send a message
containing the headers:
@table @code
@item Size
The configured maximum size of the clipstack.
@item Used
The number of elements currently in the clipstack.
@end table
@end table

@item Conditionally required header: @code{Length}
Length of the message.
Required if @code{Action: add} is included in the headers.

@item Conditionally required header: @code{Size}
The maximum number of elements in the clipstack.
Required if @code{Action: set-size} is included in the headers.

@item Conditionally required header: @code{Client ID}
Your ID, provided by the @code{ID assignment} header
in response to a @code{Command: assign-id} header.
Required if @code{Action: read} or @code{Action: read} is
included in the headers, or if @code{Action: add} is
included in the headers and if the header @code{Time to live}
is included and has a value starting with @code{until-death}.

@item Conditionally optional header: @code{Index}
The index of the item in the clipstack, starting at 0.
Available and optional if the @code{Action: read} is
included in the headers.

@item Conditionally optional header: @code{Time to live}
The number of seconds the entry should be available before
it is removed by the server, or:
@table @code
@item until-death
Remove entry when the client closes.
@item until-death #
Remove entry when the client closes,
or @code{#} seconds have elapsed.
@item forever:
Never remove it. (This is the default.)
@end table
The server will always remove the entry when either:
@enumerate 1
@item
it is at the bottom of the clipstack and a new
entry is added to the clipstack, or
@item
@code{Action: clear} is issued for the clipstack.
@end enumerate
The entry will also be removed, unless
@code{Time to live: forever} is used, if the server
crashes or is re-executed.

It is up to the implementation to choose when the
removal actually takes place. For example, the reference
implementation will pop entries that have timed out when
a new entry is added, the reading on the clipstack is
requested or the server is reexecuted, but another
implement may choose to pop entires asynchronously using
another thread or an alarm an pop when @code{SIGARLM} is
received.

Available and optional if the @code{Action: add} is
included in the headers.

@item Message:
The content to add to the clipboard.

@item Purpose:
Enable the user to duplicate content from one process
into another process without requiring those processes
to be aware of eathother to any extent.

@item Compulsivity:
Optional.

@item Reference implementation:
@command{mds-clipboard}
@end table



@node clipboard-info
@subsection clipboard-info

@table @asis
@item Identifying header:
@command{Command: clipboard-info}

@item Action:
The clipboard server sends out some information about
what it is doing, such as automatically removing entires.

@item Included header: @code{Event}
@table @code
@item pop
The value of the header @code{Event} is @code{pop}
when an item in the clipstack has been removed.
If the value header--value-pair is used the following
headers will also be included in the message:
@table @code
@item Level
The clipboard level that has been affected.
@item Popped
The index of the item in the clipstack that has been removed.
@item Size
The configured maximum size of the clipstack.
@item Used
The number of elements currently in the clipstack.
@end table
@item crash
The value of the header @code{Event} is @code{crash} when
the clipboard has been reset because of a software crash.
@end table

@item Purpose:
Enable clients to get notification about changes
to the clipboard, that cannot trivially derived
from @command{Command: clipboard}

@item Compulsivity:
Optional add-on to the clipboard's functionallity.

@item Reference implementation:
@command{mds-clipboard}
@end table



@node Status Icon Protocols
@section Status Icon Protocols

@menu
* add-tray-icon::                   Add a status icons to the status icon tray.
* update-tray-icon::                Change the status of a status icon.
* tray-update::                     Send updates about the status tray to the status icon.
* new-tray::                        Announce the existence of a new status icon trays.
@end menu



@node add-tray-icon
@subsection add-tray-icon

@table @asis
@item Identifying header:
@command{Command: add-tray-icon}

@item Action:
Add a status icons to the status icon tray.

The client should keep in mind that there can be any number of
trays available on the system: zero, one, two or three, …,
and that it will get a response once from every tray.

@item Required header: @code{Client ID}
Your ID, provided by the @code{ID assignment} header
in response to a @code{Command: assign-id} header.

@item Required header: @code{Package}
The name of the package to which the program announced the icon
belongs.

@item Required header: @code{Icon ID}
An ID of the icon that can be used identify the icon, icon ID:s
are not unique, but the combination of a package and a icon ID
should be unque and can be used to ignore already added icons
and hide icons that the user has been configured to be hidden.

@item Required header: @code{Title}
A title describing the icon for the user, used to configured
when icons should be hidden and shown among other configuration.

@item Required header: @code{Icon}
The name or pathname of an icon to use together with the title.

@item Response:
Recipients will respond with a message containing the headers:
@table @code
@item To
Will contain the value of the @code{Client ID} from the
message that triggered this response.
@item In response to
Will contain the value of the @code{Message ID} from the
message that triggered this response.
@item Message ID
Will contain a value as described in @ref{Message Passing}.
@item Socket
Will contain an ID to where the icon should be embeded.
@item Will send update
The value of this header will be @code{yes} if this
message will be followed by a @code{Command: tray-update}
message. Otherwise the value will be @code{no}.
@end table

@item Purpose:
Enable clients to add a small icon that displays the status
of programs, particularly minimised programs and services.

@item Compulsivity:
Optional.
@end table



@node update-tray-icon
@subsection update-tray-icon

@table @asis
@item Identifying header:
@command{Command: update-tray-icon}

@item Action:
Change the status of a status icon.

@item Required header: @code{Status}
@table @code
@item hide
Hide the icon if the value of the
@code{Status} header is @code{hide}.
@item show
Show the icon if the value of the
@code{Status} header is @code{show}.
@item active
The icon is active if the value of
the @code{Status} header is @code{active}.
@item inactive
The icon is inactive if the value of
the @code{Status} header is @code{inactive}.
@end table

@item Purpose:
Enable status trays to automatically hide inactive icons.

@item Purpose:
Hide icons without actually removing them.

@item Compulsivity:
Required if supporting @code{Command: add-tray-icon},
only @code{Status: hide} and @code{Status show} is required.
@end table



@node tray-update
@subsection tray-update

@table @asis
@item Identifying header:
@command{Command: tray-update}

@item Action:
Send updates about the status tray to the status icon.

@item Required header: @code{Socket}
Where the icon is embedded, used to identify the
affected tray.

@item Conditionally required header: @code{Max colour}
The maximum colour component value, for example, if using 24-bit
colour, which component will be 8-bit and the maximum value will
be 255, this also applies to the alpha component.
Required if either for the @code{Colour}-, @code{Foreground}-
or @code{Alpha}-header are used.

@item Conditionally required header: @code{Size}
The width and height, in pixels, the icon should have.
Required if the @code{Length}-header is used,
otherwise this header is optional.

@item Conditionally required header: @code{Has alpha}
@table @code
@item yes
The message contains an alpha channel if the
value of the @code{Has alpha} header is @code{yes}.
@item no
The message does not contain an alpha channel if the
value of the @code{Has alpha} header is @code{no}.
@end table
Required if the @code{Length}-header is used.

@item Conditionally required header: @code{Bytes}
The number of bytes the subpixels used, for example,
24-bit colours will have this set to 1 because each
subpixel has 8 bits, 48-bit colours will have this
set to 2 because each subpixel has 16 bits
Allowed values are: 1, 2, 4 and 8. These values are
used used so that CPU:s with any endianness can be
trivially used as the words sizes are guaranteed to
be supported in C, and mixed/middle-endiannes gets
complicated if we go outside this.
Required if the @code{Length}-header is used.

@item Conditionally optional/required header: @code{Colour}
Single blank space-separated [0, @code{<Max colour>}]
sRGB 3-tuple.
Available and optional if the @code{Length}-header
is not used.
Required if the @code{Foreground}-header but not
@code{Length}-header is used.

@item Conditionally optional header: @code{Foreground}
Single blank space-separated [0, @code{<Max colour>}] sRGB 3-tuple.

@item Optional header: @code{Alpha}
The opacity of the tray.

@item Optional header: @code{Length}
Length of the message.

@item Optional header: @code{Use urgency}
@table @code
@item yes
The icon tray may blink if the value of the
@code{Use urgency} header is @code{yes}.
@item no
The icon tray may not blink if the value of
the @code{Use urgency} header is @code{no}.
@end table

@item Message:
Raw binary encoding of the background image, bytes
are orders: row, pixel, channel (alpha, red, green,
blue), subpixel value (native CPU encoding). The
Alpha channel should be included but ignored if
@code{Has alpha: no}, in such as it is best to set
it to full.

Example image with @code{Bytes: 2},
@code{Has alpha: no} and @code{Size: 3}:
@example
sRGB(x0102, 0, 0), sRGB(0, x0304, 0), sRGB(0, 0, x0506)
sRGB(x0708, 0, 0), sRGB(0, x090A, 0), sRGB(0, 0, x0B0C)
sRGB(x0D0E, 0, 0), sRGB(0, x0F10, 0), sRGB(0, 0, x1112)
@end example

Encoding of example image in hexadecimal representation:
@example
FFFF 0102 0000 0000 FFFF 0000 0304 0000 FFFF 0000 0000 0506
FFFF 0708 0000 0000 FFFF 0000 090A 0000 FFFF 0000 0000 0B0C
FFFF 0D0E 0000 0000 FFFF 0000 0F10 0000 FFFF 0000 0000 1112
@end example

Note that on a big-endian system this would be:
@footnote{x86_64 computers are big-endian.}
@example
FF FF 02 01 0 0 0 0 FF FF 0 0 04 03 0 0 FF FF 0 0 0 0 06 05
FF FF 08 07 0 0 0 0 FF FF 0 0 0A 09 0 0 FF FF 0 0 0 0 0C 0B
FF FF 0E 0D 0 0 0 0 FF FF 0 0 10 0F 0 0 FF FF 0 0 0 0 12 11
@end example

It is up to the networking servers to translate
the encoding between machines.@footnote{The host
translates to big-endian unless they can confirm
that they have the same endianness.}

@item Purpose:
Enable clients to be aware of the appearance of the tray,
such as colours, transparency and background image.

@item Purpose:
Enable clients to be aware of how the user wants
status icons to behave.

@item Compulsivity:
Optional.
@end table



@node new-tray
@subsection new-tray

@table @asis
@item Identifying header:
@command{Command: new-tray}

@item Action:
Announce the existence of a new status icon trays.

@item Purpose:
Allow clients to add their status icons to status icon
trays that have been added to the display after those
programs have started and attempted to add their icons.

@item Compulsivity:
Required if supporting @code{Command: add-tray-icon}.
@end table



@node Colour Protocols
@section Colour Protocols

@menu
* get-gamma-info::                  Query gamma ramp information.
* get-gamma::                       Query gamma ramps.
* set-gamma::                       Modify gamma ramps.
@end menu



@node get-gamma-info
@subsection get-gamma-info

@table @asis
@item Identifying header:
@command{Command: get-gamma-info}

@item Action:
Query gamma ramp information.

@item Required header: @code{Client ID}
Your ID, provided by the @code{ID assignment}
header in response to a @code{Command: assign-id}
header.

@item Required header: @code{CRTC}
The output name for the CRTC of interest.

@item Response:
The server will response with a @code{Command: error}
on error, unsuccess the server will respond with a
message contain the headers:
@table @code
@item To
Will contain the value of the header @code{Client ID}
in the message that was received by the server.
@item In response to
Will contain the value of the header @code{Message ID}
in the message that was received by the server.
@item Cooperative
Whether a server like @command{mds-coopgamma} is
running. That is, if priorities and classes are
respected. The value with be either @code{yes},
for cooperative, or @code{no}, for non-cooperative.
@item Depth
The bit-depth of the gamma ramps. Possible values
are: 8, 16, 32 och 64.
@item Red size
The number of stops in the red gamma ramp.
@item Green size
The number of stops in the green gamma ramp.
@item Blue size
The number of stops in the blue gamma ramp.
@item Gamma support
Will have one of the following values:
@table @code
@item yes
It is known that gamma ramps are supported.
@item no
It is known that gamma ramps are not supported.
@item maybe
It is now known whether gamma ramps are supported.
@end table
@end table
If @code{Gamma support: no} is send in the response,
the headers @code{Depth}, @code{Red size},
@code{Green size} and @code{Blue size} may be omitted.

@item Purpose:
Enable performance optimisation when manipulating
gamma ramps.

@item Compulsivity:
Optional. Required if your implement support
for @command{Command: get-gamma}
or @command{Command: set-gamma}.

@item Reference implementation:
@command{mds-hwgamma}, @command{mds-swgamma},
@command{mds-coopgamma} and @command{mds-cursorgamma}.
@end table



@node get-gamma
@subsection get-gamma

@table @asis
@item Identifying header:
@command{Command: get-gamma}

@item Action:
Query gamma ramps.

@item Required header: @code{Client ID}
Your ID, provided by the @code{ID assignment}
header in response to a @code{Command: assign-id}
header.

@item Required header: @code{CRTC}
The output name for the CRTC of interest.

@item Required header: @code{Coalesce}
Whether the received the full gamma ramp
filter list, of the value is @code{yes},
rather than the result of them, of the
value is @code{no}.

@item Required header: @code{High priority}
The upper bound of the priority range of
the gamma ramps to received. This is a
signed 64-bit integer.

@item Required header: @code{Low priority}
The lower bound of the priority range of
the gamma ramps to received. This is a
signed 64-bit integer.

@item Response:
The server will response with a @code{Command: error}
on error, unsuccess the server will respond with a
message contain the headers:
@table @code
@item Depth
The bit-depth of the gamma ramps. Possible values
are: 8, 16, 32 och 64.
@item Red size
The number of stops in the red gamma ramp.
@item Green size
The number of stops in the green gamma ramp.
@item Blue size
The number of stops in the blue gamma ramp.
@item Tables
The number of gamma ramp lookup tables that
is included in the respone's message. This header
will not necessarily be included if @code{Coalesce: yes}
was used in the query, rather reference implementations
will exclude it.
@end table
These headers are included so you can make sure
the no metadata for gamma ramps have changed,
which could happen if the user switches between
hardware and software gamma ramps.
The response will also contain a @code{Length}
header and a message formatted in the same manner
as for @command{Command. set-gamma} messages. That
is, assuming as an example that the gamma ramp depth
is 16 bits, @code{Coalesce: yes} was used in the
query, the red ramp is (1, 2, 3, 4, 5, 6), the green
ramp is (17, 18, 19, 20, 21, 22, 23) and the blue
ramp is (33, 34, 35, 36, 37, 38, 39, 40) then the
message will be (hexadecimal representation):
@example
0001 0002 0003 0004 0005 0006
0011 0012 0013 0014 0015 0016 0017
0021 0022 0023 0024 0025 0026 0027 0028
@end example

On a big-endian system this would be:
@example
01 00 02 00 03 00 04 00 05 00 06 00
11 00 12 00 13 00 14 00 15 00 16 00 17 00
21 00 22 00 23 00 24 00 25 00 26 00 27 00 28 00
@end example

However if @code{Coalesce: no} was used in the query,
the message will include multiple gamma ramps lookup
tables. These will be in the order they are applied,
that is, highest priority first. The tables will be
encoded in the same way as for @code{Coalesce: yes}
and they with be included without any delimiter.
However, each table will be prefixed with the priority
and the class. The priority will be encoded in native
binary format as an @code{int64_t} and the class will
be encoded as a NUL-terminated UTF-8 string

@item Purpose:
Enable analysis and readings of the current
gamma ramps.

@item Compulsivity:
Optional. Required if your implement support
for @command{Command: get-gamma-info}
or @command{Command: set-gamma}.

@item Reference implementation:
@command{mds-hwgamma}, @command{mds-swgamma},
@command{mds-coopgamma} and @command{mds-cursorgamma}.
@end table



@node set-gamma
@subsection set-gamma

@table @asis
@item Identifying header:
@command{Command: set-gamma}

@item Action:
Modify gamma ramps.

@item Required header: @code{Client ID}
Your ID, provided by the @code{ID assignment}
header in response to a @code{Command: assign-id}
header.

@item Required header: @code{CRTC}
The output name for the CRTC of interest.

@item Required header: @code{Priority}
A signed 64-bit integer of the priority for the filter.
gamma correction should use zero priority. It is
preferable that search logical adjustment is sent with
different priorities so other programs can insert filters
between them.

@item Required header: @code{Class}
A UTF-8 string that identifies the filter. It should
be formatted as @code{pkg::cmd::role}. @code{pkg} should
be the package name the package was installed with on
the system. @code{cmd} should be the basename of the
command for the program.

@item Required header: @code{Lifespan}
The value may be one of the following:
@table @code
@item until-removal
Remove the filter when @command{Lifespan: remove}
is sent.
@item until-death
Remove the filter when the client dies.
@item remove
Remove the filter now.
@end table

@item Conditionally required header: @code{Length}
The length of the message.
Available and required if @code{Lifespan: remove}
is not included in the message.

@item Message:
The gamma ramps in binary encoding. As an example,
assume the gamma ramp depth is 16 bits, the red ramp
is (1, 2, 3, 4, 5, 6), the green ramp is (17, 18, 19,
20, 21, 22, 23) and the blue ramp is (33, 34, 35, 36,
37, 38, 39, 40) then the message will be (hexadecimal
representation):
@example
0001 0002 0003 0004 0005 0006
0011 0012 0013 0014 0015 0016 0017
0021 0022 0023 0024 0025 0026 0027 0028
@end example

Note that on a big-endian system this would be:
@footnote{x86_64 computers are big-endian.}
@example
01 00 02 00 03 00 04 00 05 00 06 00
11 00 12 00 13 00 14 00 15 00 16 00 17 00
21 00 22 00 23 00 24 00 25 00 26 00 27 00 28 00
@end example

It is up to the networking servers to translate
the encoding between machines.@footnote{The host
translates to big-endian unless they can confirm
that they have the same endianness.}

The use of binary rather than text here is chosen
to increase performance for programs that try the
change the adjustments fluently. For programs
similar to @command{xgamma} that sets the ramps
once this is however unnessary. However it does
simplify the program code as one would only need
to write the ramps to the message without creating
a string with all stops converted and then measure
the length of that string.

@item Response:
The server will response with a @code{Command: error}.

@item Instructions:
For optimal flexibility a system may run a server such
as @command{mds-hwgamma} that applies the gamma ramps,
and a server such as @command{mds-coopgamma} to let
multiple programs adjust the output with their open
filters that stack up. In a configuration like this,
@command{mds-coopgamma} will keep track of all filters
and when a modification is made it sends the grand result
to @command{mds-hwgamma}, that is, what the filters together
produce. To do this, @command{mds-coopgamma} listens for
@command{Command: set-gamma} with priority @math{2^{62}}
and modifies the message so the payload is filled with the
result rather than to single filter. This modified message
is then received by @command{mds-hwgamma} that listens with
priority zero and applies the gamma ramps.
@command{mds-hwgamma} will ignore the @code{Priority} and
the @code{Class} header, but it will respect the @code{Lifespan}
header, therefore @command{mds-coopgamma} will always modify
the value of the @code{Lifespan} header to @code{until-removal}.

@item Purpose:
Enable colour output correction such as gamma correction.

@item Purpose:
Enable colour output filters such colour temperature
adjustments, colour invertion and dimming.

@item Compulsivity:
Optional. Required if your implement support
for @command{Command: get-gamma-info}
or @command{Command: get-gamma}.

@item Reference implementation:
@command{mds-hwgamma}, @command{mds-swgamma},
@command{mds-coopgamma} and @command{mds-cursorgamma}.
@end table



@node Miscellaneous Protocols
@section Miscellaneous Protocols

@menu
* echo::                            Echo back a message.
* kill::                            The window killing protocol.
@end menu



@node echo
@subsection echo

@table @asis
@item Identifying header:
@command{Command: echo}

@item Action:
Echo back a message.

@item Required header: @code{Client ID}
Your ID, provided by the @code{ID assignment} header
in response to a @code{Command: assign-id} header.

@item Optional header: @code{Length}
Length of the message.

@item Message:
Message to echo.

@item Purpose:
Debugging and testing.

@item Purpose:
Network heartbeat.

@item Compulsivity:
Recommended for network enabled servers.

@item Reference implementation:
@command{mds-echo}
@end table



@node kill
@subsection kill

@table @asis
@item Identifying header:
@command{Command: kill}

@item Action:
Kill and identify processes based on the their windows.

@item Required header: @code{Client ID}
Your ID, provided by the @code{ID assignment} header
in response to a @code{Command: assign-id} header.

@item Required header: @code{Window ID}
The ID of the window whose owning process should
be identified or signaled.

@item Required header: @code{Signal}
A numerical value of the signal to send to the
process. It is up to networking protocols to
translate these numbers of the display spans
multiple operating systems. The signal zero
can usually be used if no signal is to be sent,
this is however dependent on he operating system.

@item Response:
The server will respond with a
@command{Command: error} message.
In this message the server all include an
ad-hoc header: @code{Process ID}. Its value
will be the ID of the process that owns the
window.

@item Purpose:
Identify and send signal to processes by refering
to them by one of their windows.

@item Compulsivity:
Optional.

@item Reference implementation:
@command{mds-kill} and @command{mds-slay}
@end table



@node libmdsserver
@chapter libmdsserver

libmdsserver is library written for the reference
implementation of the @command{mds} servers.
llibmdsserver does not contain support or any
protocols, rather it contains auxiliary functions,
macros, data structures such as linked lists and
hash tables, and support the basics of the message
passing protocol: receiving message and decode it
into headers and payloads.

@menu
* Macros::                          Writing macroscopic systems.
* Auxiliary Functions::             Auxiliary functions for servers.
* Data Structures::                 Data structures available in libmdsserver.
@end menu



@node Macros
@section Macros

The header file @file{<libmdsserver/macros.h>}
contains macros for readability and code reduction,
it also contains macros and definitions for portability;
they may either provide portability by nature, or
provide one place to do modifications to port the
system.

@table @asis
@item @code{xasprintf} [(@code{char* buffer, ...}) @arrow{} @code{int}]
This is a wrapper for @code{asprintf} that has two
important properties, the buffer is guaranteed to
be @code{NULL} on failure, and it will return zero
on and only on success. Unlike @code{asprintf},
@code{xasprintf} takes the buffer's variable as
its first argument rather than the address of that
variable.

@item @code{xsnprintf} [(@code{char buffer[], ...}) @arrow{} @code{int}]
This is a wrapper for @code{snprintf} that allows you
to forget about the buffer size. When you know how long
a string can be, you should use @code{sprintf}. But when
you cannot know for sure you should use @code{xsnprintf}.
@code{xsnprintf} works exactly as @code{sprintf}, but
it will require that the first argument is defined
using @code{[]} rather than @code{*} because it will use
this to find out how large the buffer is so it can call
@code{snprintf} with that size.

@item @code{eprint} [(@code{const char* format}) @arrow{} @code{int}]
A wrapper for @code{fprintf} that prints a string prefixed
with the value value of @code{*argv} to @code{stderr}.
Because @code{eprintf} naïvely wraps @code{fprintf}, all
`%':s in the string must be duplicated.

@item @code{eprintf} [(@code{const char* format, ...}) @arrow{} @code{int}]
@code{eprint} extends @code{eprint} with variadic arguments
that can be used to insert values into the format string
just like you can do in @code{fprintf}.

@item @code{with_mutex} [(@code{pthread_mutex_t mutex, instructions})]
Wraps @code{instructions} with @code{errno = pthread_mutex_lock(mutex);}
and @code{errno = pthread_mutex_unlock(mutex);}, so a set of
instructions can be invoked inside mutex protection.

@item @code{with_mutex_if} [(@code{pthread_mutex_t mutex, condition, instructions})]
An alternative to @code{with_mutex} where @code{instructions}
is wrapped around @code{if (condition)} which in turn is
wrapped inside the mutex protection.

@item @code{max} [(@code{a, b})]
Returns the higher value of @code{a} and @code{b}.

@item @code{min} [(@code{a, b})]
Returns the lower value of @code{a} and @code{b}.

@item @code{buf_cast} [(@code{char* buffer, type, size_t index})]
Casts @code{buffer} to a @code{type} buffer and
subscripts to the @code{index}:th element. You
can either use this function as a getter or a
setter.

@item @code{buf_set} [(@code{char* buffer, type, size_t index, type variable}) @arrow{} @code{type}]
Wrapper for @code{buf_cast} that sets the addressed
element to the value of @code{variable}.

@item @code{buf_get} [(@code{const char* buffer, type, size_t index, type variable}) @arrow{} @code{type}]
Wrapper for @code{buf_cast} that sets the value of
@code{variable} to the value of the addressed element.

@item @code{buf_next} [(@code{char* buffer, type, size_t count}) @arrow{} @code{char*}]
Increases the pointer @code{buffer} by the size of
@code{type} @code{count} types.

@item @code{buf_prev} [(@code{char* buffer, type, size_t count}) @arrow{} @code{char*}]
Decreases the pointer @code{buffer} by the size of
@code{type} @code{count} types.

@item @code{buf_set_next} [(@code{char* buffer, type, type variable}) @arrow{} @code{type}]
@example
buf_set(buffer, type, 0, variable),
buf_next(buffer, type, 1);
@end example

@item @code{buf_get_next} [(@code{char* buffer, type, type variable}) @arrow{} @code{type}]
@example
buf_get(buffer, type, 0, variable),
buf_next(buffer, type, 1);
@end example

@item @code{strequals} [(@code{const char* a, const char* b}) @arrow{} @code{int}]
Evaluates whether the strings @code{a} and @code{b}
are equals, neither may be @code{NULL}.

@item @code{startswith} [(@code{const char* haystack, const char* needle}) @arrow{} @code{int}]
Evaluates whether the string @code{haystack}
starts with the string @code{needle}, neither
may be @code{NULL}.

@item @code{drop_privileges} [(void) @arrow{} @code{int}]
Sets the effective user to the real user and the
effective group to the real group. This is used
by most servers and ensure that they are not
running with unnecessary privileges. Returns zero
on and only on success.

@item @code{monotone} [(@code{struct timespec* time_slot}) @arrow{} @code{int}]
Stores the time of an unspecified monotonic clock
into @code{time_slot}. Returns zero on and only on
success.

@item @code{close_files} [(@code{condition}) @arrow{} @code{void}]
Closes all file descriptors named by a variable
@code{fd} for which @code{condition} evalutes
to non-zero.

@item @code{xfree} [(@code{void** array, size_t elements}) @arrow{} @code{void}]
Calls @code{free} on the first @code{elements}
elements in @code{array}, and than calls
@code{free} on @code{array}. This macro
requires @code{size_t i} is declared.

@item @code{xmalloc} [(@code{type* var, size_t elements, type}) @arrow{} @code{int}]
Allocates a @code{type*} with @code{elements}
elements and stores the allocated pointer to
@code{var}. Returns zero on and only on success.

@item @code{xbmalloc} [(@code{type* var, size_t bytes}) @arrow{} @code{int}]
Allocates @code{bytes} bytes and stores the allocated
pointer to @code{var}. Returns zero on and only on success.

@item @code{xcalloc} [(@code{type* var, size_t elements, type}) @arrow{} @code{int}]
Allocates a zero-initialised @code{type*} with
@code{elements} elements and stores the allocated
pointer to @code{var}. Returns zero on and only
on success.

@item @code{xbcalloc} [(@code{type* var, size_t bytes}) @arrow{} @code{int}]
Allocates and zero-initialises @code{bytes} bytess
and stores the allocated pointer to @code{var}.
Returns zero on and only on success.

@item @code{xrealloc} [(@code{type* var, size_t elements, type}) @arrow{} @code{int}]
Reallocates @code{var} and updates the variable
@code{var} accordingly. @code{var} will be
allocated to have @code{elements} elements
of the type @code{type}. If @code{var} is
@code{NULL} a new allocation is created. If
@code{elements} is zero, @code{var} will
be deallocated. Returns zero on and only
on success. On failure, @code{var} will be
@code{NULL}, so you must store the @code{var}
into another variable in case this macro
fails.

@item @code{xxrealloc} [(@code{type* old, type* var, size_t elements, type}) @arrow{} @code{int}]
Variant of @code{xrealloc} that will
return with @code{old} set to @code{NULL}
on success, and @code{old} set to @code{var}
on error. Like @code{xrealloc}, @code{xxrealloc}
returns zero on and only on success.

@item @code{yrealloc} [(@code{type* tmp, type* var, size_t elements, type}) @arrow{} @code{int}]
Variant of @code{xrealloc} that will
store @code{var} to @code{tmp} before
reallocating @code{var} and then restore
@code{var} if the reallocation failed.
Like @code{xrealloc}, @code{yrealloc}
returns zero on and only on success.

@item @code{growalloc} [(@code{type* old, type* var, size_t elements, type}) @arrow{} @code{int}]
When using this macro @code{var} should
be a @code{type*} pointer allocated for
@code{elements} elements of the type
@code{type}. This macro will reallocate
@code{var} to contain twice as many elements
and update @code{elements} accordingly.
On failure nothing changes. You must specify
an auxiliary @code{type*} variable and
specify it in as the @code{old} parameter.
Returns zero on and only on success.

@item @code{xstrdup} [(@code{char* var, const char* original}) @arrow{} @code{int}]
Wrapper for @code{strdup} that
returns zero on and only on success.
@code{original} is duplicate and the
duplicate is stored in the variable
@code{var}. If @code{original} is
@code{NULL}, @code{var} is set to
@code{NULL} and zero is returned.

@item @code{xmemdup} [(@code{void* var, const void* original, size_t elements, type}) @arrow{} @code{int}]
Allocates a pointer of the type @code{type*}
with room for @code{elements} elements and
stores the pointer to @code{var}. If successful,
the content of @code{original} (@code{elements}
elements of size @code{sizeof(type*)}) is
copied to @code{var}, and zero is returned.
On failure, a non-zero value is returned.

@item @code{xperror} [(@code{const char* str}) @arrow{} @code{void}]
Invokes @code{perror(str)} if and only if
@code{errno} is non-zero and then sets
@code{errno} to zero. @code{str} should
unless you have a specific reason be
@code{*argv}.

@item @code{fail_if} [(@code{condition}) @arrow{} @code{void}]
If @code{condition} is satisfied, a jump
is made to the label @code{fail}.
Additionally the location of failure will
be printed to standard error.

@item @code{exit_if} [(@code{condition, instructions}) @arrow{} @code{void}]
If @code{condition} is satisfied,
@code{instructions} is invoked and
@code{1} is @code{return}:ed.
@end table

Additionally, @file{<libmdsserver/macros.h>}
defines any missing signal name:
currenly @code{SIGDANGER} and
@code{SIGUPDATE}, and by inclusion of
@file{<libmdsserver/macro-bits.h>}, variants
of @code{atoi} for portability and
convenience:

@table @code
@item atoz
Parse a human readable @code{const char*}
10-radix integer to a @code{size_t}.

@item atosz
Parse a human readable @code{const char*}
10-radix integer to a @code{ssize_t}.

@item atoh
Parse a human readable @code{const char*}
10-radix integer to a @code{short int}.

@item atouh
Parse a human readable @code{const char*}
10-radix integer to an @code{unsigned short int}.

@item atou
Parse a human readable @code{const char*}
10-radix integer to an @code{unsigned int}.

@item atoul
Parse a human readable @code{const char*}
10-radix integer to an @code{unsigned long int}.

@item atoull
Parse a human readable @code{const char*}
10-radix integer to an @code{unsigned long long int}.

@item ato8
Parse a human readable @code{const char*}
10-radix integer to an @code{int8_t}.

@item atou8
Parse a human readable @code{const char*}
10-radix integer to an @code{uint8_t}.

@item ato16
Parse a human readable @code{const char*}
10-radix integer to an @code{int16_t}.

@item atou16
Parse a human readable @code{const char*}
10-radix integer to an @code{uint16_t}.

@item ato32
Parse a human readable @code{const char*}
10-radix integer to an @code{int32_t}.

@item atou32
Parse a human readable @code{const char*}
10-radix integer to an @code{uint32_t}.

@item ato64
Parse a human readable @code{const char*}
10-radix integer to an @code{int64_t}.

@item atou64
Parse a human readable @code{const char*}
10-radix integer to an @code{uint64_t}.

@item atoj
Parse a human readable @code{const char*}
10-radix integer to an @code{intmax_t}.

@item atouj
Parse a human readable @code{const char*}
10-radix integer to an @code{uintmax_t}.
@end table



@node Auxiliary Functions
@section Auxiliary Functions

In the header file @file{<libmdsserver/util.h>},
libmdsserver defines common functions to help
write servers more concisely.

@table @asis
@item @code{parse_client_id} [(@code{const char* str}) @arrow{} @code{uint64_t}]
Convert a client ID string into a client ID integer.

@item @code{getenv_nonempty} [(@code{const char* var}) @arrow{} @code{char*}]
Read an environment variable, return @code{NULL} if
the variable's value is an empty string.

@item @code{prepare_reexec} [(@code{void}) @arrow{} @code{int}]
Prepare the server so that it can re-execute into
a newer version of the executed file.

This is required for two reasons:

@enumerate 1
@item
We cannot use @code{argv[0]} as @env{PATH}-resolution
may cause it to reexec into another pathname, and
maybe to wrong program. Additionally @code{argv[0]}
may not even refer to the program, and @code{chdir}
could also hinter its use.

@item
The kernel appends ` (deleted)' to
@file{/proc/self/exe} once it has been removed,
so it cannot be replaced.
@end enumerate

The function will should be called immediately, it
will store the content of @file{/proc/self/exe}.
Return zero on success and @code{-1} on error.

@item @code{reexec_server} [(@code{int argc, char** argv, int reexeced}) @arrow{} @code{void}]
Re-execute the server. If @code{prepare_reexec}
failed or has not been called, @code{argv[0]}
will be used as a fallback. This functions
has three input parameters:

@table @code
@item argc
The number of elements in @code{argv}.
@item argv
The command line arguments.
@item reexeced
Whether the server has previously been re-executed
@end table

This function only returns on failure.

@item @code{xsigaction} [(@code{int signo, void (*function)(int signo)}) @arrow{} @code{int}]
@code{sigaction} with the same parameters as @code{signal}.
This function should only be used for common @command{mds}
signals and signals that does not require any special settings.
This function may choose to add additional behaviour depending
on the signal, such as blocking other signals. Returns zero
on success and @code{-1} on error.

@item @code{send_message} [(@code{int socket, const char* message, size_t length}) @arrow{} @code{size_t}]
Send the message @code{messsage}, of length @code{length}
over the socket that is access with the file descriptor
@code{socket}. Returns the number of bytes that have been
sent, even on error.

@item @code{strict_atoi} [(@code{const char* str, int* value, int min, int max}) @arrow{} @code{int}]
A version of @code{atoi} that is strict about the syntax
and bounds. Parses the string @code{str} into an @code{int}
and stores it in @code{*value}. If the string is not a
10-radix integer or has a value outside [@code{min},
@code{max}], @code{-1} is returned, otherwise zero is
returned.

@item @code{full_write} [(@code{int fd, const char* buffer, size_t length}) @arrow{} @code{int}]
Send the buffer @code{buffer}, with the length @code{length},
into the file whose file descriptor is @code{fd} and ignores
interruptions. Returns zero on success and @code{-1} on error.

@item @code{full_read} [(@code{int fd, size_t* length}) @arrow{} @code{char*}]
Read the file whose file descriptor is @code{fd} completely
and ignore interruptions. If @code{length} if not @code{NULL},
the length of the read file is stored in @code{*length}.
On success, the read content is retured, on error @code{NULL}
is returned.

@item @code{startswith_n} [(@code{const char*, const char*, size_t, size_t}) @arrow{} @code{int}]
Check whether a string begins with a specific string,
where neither of the strings are necessarily NUL-terminated.
The parameters are:

@table @code
@item const char* haystack
The string that should start with the other string.
@item const char* needle
The string the first string should start with.
@item size_t haystack_n
The length of @code{haystack}.
@item size_t needle_n
The length of @code{needle}.
@end table

Returns 1 if @code{haystack} beings with @code{needle},
otherwise zero is returned.

@item @code{uninterruptable_waitpid} [(@code{pid_t pid, int* restrict status, int options}) @arrow{} @code{pid_t}]
Wrapper around @code{waitpid} that never returns on an
interruption unless it is interrupted one hundred times
within the same clock second. The parameters and return
value are exactly those of @code{waitpid}.

@item @code{verify_utf8}[(@code{const char* string, int allow_modified_nul}) @arrow{} @code{int}]
Checks whether a NUL-terminated string's encoding
matches UTF-8. This function will reject the string
if it does not use the shorted possible
byte-combination for each character. However, if
@code{allow_modified_nul} is set, it will allow
@code{192 128} in place of @code{0} for a
NUL-character.@footnote{Remember @code{0} is used
to terminated the string, but @code{192 128} is not.}
This function returns zero if the @code{string} is
properly formatted, and @code{-1} otherwise.
@end table



@node Data Structures
@section Data Structures

libmdsserver provides a small set of datastructures
that are used by the @command{mds} servers. All of
these are written with marshal-functionallity.

@table @asis
@item @code{client_list_t} @{also known as @code{struct client_list}@}
In the header file @file{<libmdsserver/client-list.h>},
libmdsserver defines a dynamic list for storing
client ID:s.

@item @code{linked_list_t} @{also known as @code{struct linked_list}@}
In the header file @file{<libmdsserver/linked-list.h>},
libmdsserver defines a linear array sentinel doubly
linked list.

@item @code{hash_table_t} @{also known as @code{struct hash_table}@}
In the header file @file{<libmdsserver/hash-table.h>},
libmdsserver defines a hash table.

@item @code{fd_table_t} @{also known as @code{struct fd_table}@}
In the header file @file{<libmdsserver/fd-table.h>},
libmdsserver defines a lookup table for small
positive integer keys, intended as an alternative
to hash tables for file descriptors as keys.

@item @code{mds_message_t} @{also known as @code{struct mds_message}@}
In the header file @file{<libmdsserver/mds-message.h>},
libmdsserver defines a data structure for message
between the server or client and the master server,
with the capability of reading for a socket.
@end table

These data structures share a common set of associated
function. However, they do not use the same functions;
they are identical except they are are named with the
associated data structure. We will use @code{X_t}
as an example.

@table @asis
@item @code{X_destroy} [(@code{X_t* restrict this}) @arrow{} @code{void}]
Releases all resouces in @code{*this},
@code{this} itself is however not @code{free}:d.

However, @code{hash_table_destory} and
@code{fd_table_destory} have another signature.

@item @code{X_clone} [(@code{const X_t* restrict this, X_t* restrict out}) @arrow{} @code{int}]
Create a deep duplicate of @code{*this} and store
it in @code{*out}.

@item @code{X_marshal_size} [(@code{const X_t* restrict this}) @arrow{} @code{size_t}]
Calculates the exact allocate size needed for
the parameter @code{data} in the function
@code{X_marshal} if called with the same
@code{this} parameter.

@item @code{X_marshal} [(@code{const X_t* restrict this, char* restrict data}) @arrow{} @code{void}]
Marshal the state of @code{*this} into
@code{data}. The number of bytes that
will be stored (contiguously) in @code{data}
can be calculated with @code{X_marshal_size}.

@item @code{X_unmarshal} [(@code{X_t* restrict this, char* restrict data)}) @arrow{} @code{int}]
Unmarshal a @code{X_t} from
@code{data} into @code{*this}. Returns
zero on success and @code{-1} on error.
The number of bytes read from @code{data}
should, if required, have been precalculated
with @code{X_marshal_size} and stored in an
earlier location of @code{data}.

However, @code{hash_table_unmarshal} and
@code{fd_table_unmarshal} have another signature.
@end table

@menu
* Client List::                     The @code{client_list_t} data structure.
* Linked List::                     The @code{linked_list_t} data structure.
* Tables::                          The @code{fd_table_t} and @code{hash_table_t} data structures.
* Message Structure::               The @code{mds_message_t} data structure.
@end menu



@page
@node Client List
@subsection Client List

To create a client list, allocate a
@code{client_list_t*} or otherwise obtain
a @code{client_list_t*}, and call
@code{client_list_create} with that
pointer as the first argument, and
the @code{0} as the second argument,
unless you want to tune the initialisation.
@code{client_list_create} will return
zero on and only on successful initialisation.
@code{client_list_create}'s second parameter
--- @code{size_t capacity} --- can be used
to specify how many element the list should
initially fit. It will grow when needed, but
it is a good idea to tell it how many elements
you are planning to populate it with.

@code{client_list_t} has two associated
functions for manipulating its content:

@table @asis
@item @code{client_list_add} [(@code{client_list_t* restrict this, uint64_t client}) @arrow{} @code{int}]
This function will add the element @code{client}
to the list @code{*this}, and return zero on
and only on success.

@item @code{client_list_remove} [(@code{client_list_t* restrict this, uint64_t client}) @arrow{} @code{void}]
This function will remove exactly one occurrence,
provided that there is at least on occurrence,
of the element @code{client} for the list @code{*this}.
@end table

The retrieve the number elements stored in
a list, reads its variable @code{size_t size}.
The variable @code{uint64_t* clients} is
used to retrieve stored elements.

@example
void print_elements(client_list_t* this)
@{
  size_t i;
  for (i = 0; i < this->size; i++)
    printf("Element #%zu: %" PRIu64 "\n", i, this->elements[i]);
@}
@end example



@node Linked List
@subsection Linked List

@code{linked_list_t} is a linear array sentinel
doubly linked list. This means that is implemented
using arrays rather than node references. More
specifically, since it is doubly linked@footnote{And
not using XOR-linking.}, it is implemented using
three arrays:

@table @asis
@item @code{values} [@code{size_t*}]
The value stored in each node.

@item @code{next} [@code{ssize_t*}]
The next node for each node, @code{edge} if the current
node is the last node, and @code{LINKED_LIST_UNUSED} if
there is no node on this position.

@item @code{previous} [@code{ssize_t*}]
The previous node for each node, @code{edge} if the current
node is the first node, and @code{LINKED_LIST_UNUSED} if
there is no node on this position.
@end table

The linked list has a sentinel node that joins
boths ends of the list. The index of this node
is stored in the variable @code{edge}.

Because the list is implemented using arrays, if the
number of elements in it shinks considerably, it will
not be able to automatically free unused space. Instead
you must call @code{linked_list_pack}:

@table @asis
@item @code{linked_list_pack} [(@code{linked_list_t* restrict this}) @arrow{} @code{int}]
Pack the list so that there are no reusable positions,
and reduce the capacity to the smallest capacity that
can be used. Note that values (nodes) returned by the
list's methods will become invalid. Additionally (to
reduce the complexity) the list will be defragment so
that the nodes' indices are continuous. This method has
linear time complexity and linear memory complexity.
@end table

To create a linked list list, allocate a
@code{linked_list_t*} or otherwise obtain
a @code{linked_list_t*}, and call
@code{linked_list_create} with that
pointer as the first argument, and
the @code{0} as the second argument,
unless you want to tune the initialisation.
@code{linked_list_create} will return
zero on and only on successful initialisation.
@code{linked_list_create}'s second parameter
--- @code{size_t capacity} --- can be used
to specify how many element the list should
initially fit. It will grow when needed, but
it is a good idea to tell it how many elements
you are planning to populate it with.

There are five functions adding and removing
items to and from a linked list:

@table @asis
@item @code{linked_list_insert_after} [(@code{this, size_t value, ssize_t predecessor}) @arrow{} @code{ssize_t}]
Create a new node with the value @code{value} and add it
to the list @code{*this} after the node @code{predecessor}.
On success, the new node is returned, on failure
@code{LINKED_LIST_UNUSED} is returned.

@item @code{linked_list_insert_before} [(@code{this, size_t value, ssize_t successor}) @arrow{} @code{ssize_t}]
Create a new node with the value @code{value} and add it
to the list @code{*this} before the node @code{successor}.
On success, the new node is returned, on failure
@code{LINKED_LIST_UNUSED} is returned.

@item @code{linked_list_remove_after} [(@code{this, ssize_t predecessor}) @arrow{} @code{ssize_t}]
Remove and return the node in the list @code{*this}
directly after the node @code{predecessor}.

@item @code{linked_list_remove_before} [(@code{this, ssize_t successor}) @arrow{} @code{ssize_t}]
Remove and return the node in the list @code{*this}
directly before the node @code{predecessor}.

@item @code{linked_list_remove} [(@code{this, ssize_t node}) @arrow{} @code{void}]
Remove the node @code{node} from the list @code{*this}.
@end table

The data type for @code{this} is @code{linked_list_t*}
with the @code{restrict} modifier for these and all
other @code{linked_list_t} functions.

Note that if the node @code{this->edge} is removed,
the list become circularly linked and the sentinel
will become missing which renders invokation of all
macros undefined in behaviour. Further note that
removing the sentinel while it is the only node in
the list invokes undefined behaviour. Also note that
addressing non-existing nodes invokes undefined
behaviour.

@file{<libmdsserver/linked_list.h>} defines two
macros for inserting nodes at the edges of a linked
list and two macros for removing nodes from the
edges of a linked list:

@table @asis
@item @code{linked_list_insert_beginning} [(@code{linked_list_t* this, size_t value}) @arrow{} @code{ssize_t}]
Create a new node with the value @code{value} in
insert it to the beginning of the list @code{*this}.
On success, the new node is returned, on failure
@code{LINKED_LIST_UNUSED} is returned.

@item @code{linked_list_insert_end} [(@code{linked_list_t* this, size_t value}) @arrow{} @code{ssize_t}]
Create a new node with the value @code{value} in
insert it to the end of the list @code{*this}.
On success, the new node is returned, on failure
@code{LINKED_LIST_UNUSED} is returned.

@item @code{linked_list_remove_beginning} [(@code{linked_list_t* this}) @arrow{} @code{ssize_t}]
Remove and return the first node in the
list @code{*this}.

@item @code{linked_list_remove_end} [(@code{linked_list_t* this}) @arrow{} @code{ssize_t}]
Remove and return the node node in the
list @code{*this}.
@end table

Additionally the library defines a macro that
wrappes the @code{for} keyword to iterate over
all nodes (except the sentinel node) the a
linked list:

@table @asis
@item @code{foreach_linked_list_node} [(@code{linked_list_t this, ssize_t node})]
Wrapper for `for` keyword that iterates over each
element in the list @code{this}, and store the
current node to the variable named by the parameter
@code{node} for each iterations.

@example
void print_linked_list_values(linked_list_t* list)
@{
  ssize_t node;
  foreach_linked_list_node (*list, node)
    printf("%zi\n", list->values[node]);
@}
@end example

Note that the data type for @code{this} in the
macro is not a pointer.
@end table

There is also a function intended for debugging:

@table @asis
@item @code{linked_list_dump} [(@code{linked_list_t* restrict this, FILE* restrict output}) @arrow{} @code{void}]
The all internal data of the list @code{*this}
into the stream @code{output}.
@end table



@node Tables
@subsection Tables

libmdsserver defines two similar data structures:
@code{fd_table_t} and @code{hash_table_t}. Whenever
a function exists for both data structures we will
write @code{X_table} instead of @code{fd_table} and
@code{hash_table}. Additionally, unless otherwise
stated, a function's parameter named @code{this}
will be of the type @code{hash_table_t*} if the
function's name start with @code{hash_table} and
@code{fd_table_t*} if the function's name start
with @code{fd_table}, with the @code{restrict}
modifier.

@table @asis
@item @code{X_table_create} [(@code{this}) @arrow{} @code{int}]
Initialises @code{*this} so it can be used as a
table. Returns zero on and only on success.

These functions are defined as macros.

@item @code{X_table_create_tuned} [(@code{this, size_t initial_capacity}) @arrow{} @code{int}]
Initialises @code{*this} so it can be used as a
table, and makes its initial capacity at least
@code{initial_capacity}. Returns zero on and only
on success.

@code{hash_table_create_tuned} is defined as a macro.

@item @code{hash_table_create_tuned} [(@code{this, size_t initial_capacity, float load_factor}) @arrow{} @code{int}]
Initialises @code{*this} so it can be used as a
table, and makes its initial capacity at least
@code{initial_capacity} and its load factor
@code{load_factor}. Returns zero on and only
on success.

@item @code{X_table_destroy} [(@code{this, free_func* key_freer, free_func* value_freer}) @arrow{} @code{void}]
Release all resources in the table @code{*this},
but do not @code{free} @code{this} itself.
Should be called even if construction fails.
If @code{keys_freer} is not @code{NULL}, this
function will be called for each key.
If @code{values_freer} is not @code{NULL}, this
function will be called for each value.

@item @code{X_table_contains_value} [(@code{const this, size_t value}) @arrow{} @code{int}]
Check whether the value @code{value} is stored
in the table @code{*this}.

@item @code{X_table_contains_key} [(@code{const this, key}) @arrow{} @code{int}]
Check whether the key @code{code} is used in the
table @code{*this}.

The data type for the parameter @code{key} is
@code{size_t} for @code{hash_table} and @code{int}
for @code{fd_table}.

@item @code{X_table_get} [(@code{const this, key}) @arrow{} @code{size_t}]
Look up a value by its key @code{key} in the
table @code{*this}. Zero will be returned if
the key was not used.

@item @code{hash_table_get_entry} [(@code{const this, size_t key}) @arrow{} @code{hash_entry_t*}]
Look up an entry by its key @code{key} in the
table @code{*this}. @code{NULL} will be returned
if the key was not used.

@item @code{X_table_put} [(@code{this, key, size_t value}) @arrow{} @code{size_t}]
Map the value @code{value} to the key @code{key}
in the talbe @code{*this}. If a value was already
mapped to the key, that value will be returned,
otherwise zero will be returned. Zero will also
be returned on error. @code{errno} will be set to
zero on and only on success.

The data type for the parameter @code{key} is
@code{size_t} for @code{hash_table} and @code{int}
for @code{fd_table}.

@item @code{X_table_remove} [(@code{this, key}) @arrow{} @code{size_t}]
Unmaps the key @code{key} for the table @code{*this}.
If a value was mapped to the key, that value will
be returned, otherwise zero will be returned.

The data type for the parameter @code{key} is
@code{size_t} for @code{hash_table} and @code{int}
for @code{fd_table}.

@item @code{X_table_clear} [(@code{this}) @arrow{} @code{void}]
Unmaps all keys in the table @code{*this}.

@item @code{X_table_unmarshal} [(@code{this, char* restrict data, remap_func* remapper}) @arrow{} @code{int}]
As described in @ref{Data Structures} but with one
additional parameter: @code{remapper}. If this
parameter is not @code{NULL} this function is used
to edit values. It will be called once for each
value and the output of the function will be used
inplace of the input value.
@end table

@file{<libmdsserver/hash-table.h>} also defines
as wrapper macro for the @code{for} keyword:

@table @asis
@item @code{foreach_hash_table_entry} [(@code{hash_table_t this, size_t i, hash_entry_t* entry})]
Iterates over entry element in the hash table
@code{*this}. On each iteration, the entry will
be stored to the variable @code{entry} and the
bucket index will be stored to the variable
@code{i}.

@example
void print_hash_table(hash_table_t* table)
@{
  hash_entry_t* entry;
  size_t i;
  foreach_hash_table_entry (*table, i, entry)
    printf("%zu --> %zu\n", entry->key, entry->value);
@}
@end example

Note the the data type for the parameter @code{this}
is not a popinter.
@end table

The structures @code{hash_table_t} and @code{fd_table_t}
contain the variable @code{value_comparator} which by
default is @code{NULL}. If this variable is set to @code{NULL},
two values will be considered equal if and only if they are
numerically identical; otherwise two values will be considered
equal if and only if @code{value_comparator} returned a
non-zero value if those two values are used for the function's
arguments. The data type for @code{value_comparator} is
@code{compare_func*}.

@code{hash_table_t} also contains two other variables:

@table @asis
@item @code{key_comparator} [@code{compare_func*}]
Identical to @code{value_comparator}, except it is used for
keys rather the values.

@item @code{hasher} [@code{hash_func*}]
By default, the hash value for key is identical to the key
itself. However, if this variable is not @code{NULL}, it
will be used to calculate the hash value for keys.
@end table

There is a secondary data structure defined for hash tables:
@code{hash_entry_t} @{also known as @code{struct hash_entry}@}.
It is the data structure used for entries in a hash table.
@code{hash_entry_t} contain three variables you may be interested in:

@table @asis
@item @code{key} [@code{size_t}]
The key.

@item @code{value} [@code{size_t}]
The value associated with the key.

@item @code{hash} [@code{size_t}]
The hash value of the key.
@end table

By inclusion of @file{<libmdsserver/table-common.h>},
@file{<libmdsserver/hash-table.h>} and @file{<libmdsserver/fd-table.h>}
defines four @code{typedef}:s for function signatures:

@table @asis
@item @code{compare_func} [(@code{size_t a, size_t b}) @arrow{} @code{int}]
A function that performs a comparison of two objects.
Should return non-zero if and only if @code{a} and
@code{b} are to be considered equal in the given
context.

@item @code{hash_func} [(@code{size_t value}) @arrow{} @code{size_t}]
A function that hashes an object or a value.
Should return the hash value for @code{value}.

@item @code{free_func} [(@code{size_t obj}) @arrow{} @code{void}]
A function that, to the extent that is appropriate,
releases the object @code{obj}'s resources and
@code{free}:s it.

@item @code{remap_func} [(@code{size_t obj}) @arrow{} @code{size_t}]
A function that translates a object into a new object.
The function should return new object that should replace
the object @code{obj}.
@end table

If you are working with strings, you may consider
including the header file @file{<libmdsserver/hash-help.h>}.
It defines to useful functions:

@table @asis
@item @code{string_hash} [(@code{const char* str}) @arrow{} @code{size_t}]
Calculate and returns the hash value of the string @code{str}.

@item @code{string_comparator} [(@code{char* str_a, char* str_b}) @arrow{} @code{int}]
Returns non-zero if either both @code{str_a} and @code{str_b}
are @code{NULL} or neither are @code{NULL} but are identical
strings by content upto their first NUL characters (or by address).
@end table

These functions are defined as pure and @code{static inline}.



@node Message Structure
@subsection Message Structure

Apart from internal data @code{mds_message_t} contains four
variables:

@table @asis
@item @code{headers} [@code{char**}]
The headers in the message, each element in this list
as an unparsed header, it consists of both the header
name and its associated value, joined by `: '. A header
cannot be @code{NULL} (unless its memory allocation failed,)
but @code{headers} itself is @code{NULL} if there are
no headers. The `Length' header should be included in
this list.

@item @code{header_count} [@code{size_t}]
The number of headers in the message.

@item @code{payload} [@code{char*}]
The payload of the message, @code{NULL} if
none (of zero-length).

@item @code{payload_size} [@code{size_t}]
The length of the message's payload.
This value will be the same as the value
of the `Length' header.
@end table

There are six functions specific to @code{mds_message_t}.
The @code{this}-parameter's data type for this functions
are @code{mds_message_t*} with the @code{restrict} modifier.

@table @asis
@item @code{mds_message_initialise} [(@code{this}) @arrow{} @code{int}]
Initialises @code{*this} so that it can be used by
@code{mds_message_read}. Returns zero on and only on
success. On failure you should destroy @code{*this}
using @code{mds_message_destroy}.

@item @code{mds_message_zero_initialise} [(@code{this}) @arrow{} @code{void}]
This function is similar to @code{mds_message_initialise},
however it cannot fail and thus have no return value.
The difference it is action is that it will not allocate
an internal buffer.

@item @code{mds_message_extend_headers} [(@code{this, size_t extent}) @arrow{} @code{int}]
Ensures that @code{extent} additional headers can
be stored in the @code{*this}. Returns zero on
and only on success.

@item @code{mds_message_read} [(@code{this, int fd}) @arrow{} @code{int}]
Reads the next message from the socket file descriptor
@code{fd} and stores it in @code{*this}. Returns zero
on success and non-zero on error or interruption. @code{*this}
should be destroyed using @code{mds_message_destroy} on
error but not on interruption. If @code{-2} is returned
@code{errno} will not have been set; @code{-2} indicates
that the message is malformated, which is a state that
cannot be recovered from.

@item @code{mds_message_compose_size} [(@code{const this}) @arrow{} @code{size_t}]
This function is to @code{mds_message_compose} as
@code{mds_message_marshal_size} is to
@code{mds_message_marshal}.

@item @code{mds_message_compose} [(@code{const this, char* restrict data}) @arrow{} @code{void}]
This function is similar to @code{mds_message_marshal}.
The only difference is that it will not store internal
data and instead create a message that can be broadcasted
in the display server message passing system.
@end table



@node mds-base.o
@chapter @file{mds-base.o}

@file{mds-base.c} and @file{mds-base.h} as an object
filepair whose purpose is similar to libmdsserver.
@file{mds-base} is compiled into all @command{mds}
servers and implements common procedures including
@code{main}. It also complements procedures that are
weakly defined, that is, if the server implementation
also defines them, the server implementations procedure
replaces @file{mds-base}'s implementation at
compile-time.

@file{mds-base} defines one function that you can
call from threads you create and functions that should
be implement depending on specified conditions:

@table @asis
@item @code{trap_signals} [(@code{void}) @arrow{} @code{int}]
Set up signal traps for all especially handled signals.
Returns zero on and only on success.
@end table

@file{mds-base} weakly defines functions that you can
replace if they do not suit your needs:

@table @asis
@item @code{parse_cmdline} [(@code{void}) @arrow{} @code{int}]
Parses command line arguments.
Returns zero on and only on success.

This function will parse the following options:

@table @option
@item --initial-spawn
It is the first time the server is spawn by its
spawner process.

@item --respawn
The server was respawned.

@item --re-exec
The server is re-executing.

@item --alarm=SECONDS
Kill the process after @var{SECONDS} seconds.
At most one minute.

@item --on-init-fork
Fork the process to detach it from its parent when
the server has been initialised.

@item --on-init-sh=COMMAND
When the server has been initialised, run the
command @var{COMMAND}.

@item --immortal
The server should to its best not to die. For example
do not die if @code{SIGDANGER} is received even if that
is the server's default action.
@end table

@item @code{connect_to_display} [(@code{void}) @arrow{} @code{int}]
Connects to the display.
Returns zero on and only on success.

@item @code{server_initialised} [(@code{void}) @arrow{} @code{int}]
This function should be called when the server has
been properly initialised but before initialisation
of anything that is removed at forking is initialised.
Returns zero on and only on success.

@item @code{signal_all} [(@code{int signo}) @arrow{} @code{void}]
This function should be implemented by the actual server
implementation if the server is multi-threaded. It sends
the singal @code{signo} to all threads except the current
thread.

@item @code{received_danger} [(@code{int signo}) @arrow{} @code{void}]
This function is called when a signal that signals the
system is running out of memory has been received. The exact
received signal is specified by the parameter @code{signo}.
When this function is invoked, the server should free up
all memory it can spare. When this function is invoked, it
should set the variable @code{danger} to a non-zero value.
If @code{server_characteristics.danger_is_deadly} is set,
this function will never be called.

@item @code{received_reexec} [(@code{int signo}) @arrow{} @code{void}]
This function is called when a signal that signals the
server to re-execute has been received. The exact
received signal is specified by the parameter @code{signo}.
When this function is invoked, it should set the variables
@code{reexecing} and @code{terminating} to a non-zero value.

@item @code{received_terminate} [(@code{int signo}) @arrow{} @code{void}]
This function is called when a signal that signals the
server to terminate has been received. The exact received
signal is specified by the parameter @code{signo}. When
this function is invoked, it should set the variable
@code{terminating} to a non-zero value.

@item @code{fork_cleanup} [(@code{int status}) @arrow{} @code{void}]
This function should be implemented by the actual server
implementation if the server has set
@code{server_characteristics.fork_for_safety} to be a
non-zero value. This function is called by the parent server
process when the child server process exits, if the server
has completed its initialisation. The parameter @code{status}
specifies the child process exit status as returned by
@code{waitpid}.
@end table

Additionally, @file{mds-base} expects the server implementation
to define and implement a set of functions:

@table @asis
@item @code{preinitialise_server} [(@code{void}) @arrow{} @code{int}]
This function will be invoked before @code{initialise_server}
(if not re-executing) or before @code{unmarshal_server}
(if not re-executing). Returns zero on and only on success.

@item @code{initialise_server} [(@code{void}) @arrow{} @code{int}]
This function should initialise the server. It not invoked
after a re-execution. Returns zero on and only on success.

@item @code{postinitialise_server} [(@code{void}) @arrow{} @code{int}]
This function will be invoked after @code{initialise_server}
(if not re-executing) or after @code{unmarshal_server} (if
re-executing). Returns zero on and only on success.

@item @code{marshal_server_size} [(@code{void}) @arrow{} @code{size_t}, pure]
Calculate and returns the number of bytes that will be stored
by @code{marshal_server}. On failure the server should call
@code{abort} or exit with failure status by other means.
However it should not be possible for this function to fail.
@code{marshal_server_size} must be pure.@footnote{That is,
define with and conforming to @code{__attribute__((pure))}.}.

@item @code{marshal_server} [(@code{char* state_buf}) @arrow{} @code{int}]
Marshal server implementation specific data into the buffer
@code{state_buf}. Returns zero on and only on success.

@item @code{unmarshal_server} [(@code{char* state_buf}) @arrow{} @code{int}]
Unmarshal server implementation specific data from the
buffer @code{state_buf} and update the servers state
accordingly. Returns zero on and only on success.

On critical failure the program should call @code{abort}
or exit with failure status by other means. That is, do not
let @code{reexec_failure_recover} run successfully, if it
unrecoverable error has occurred or one severe enough that
it is better to simply respawn.

@item @code{reexec_failure_recover} [(@code{void}) @arrow{} @code{int}]
Attempt to recover from a re-execution failure that has been
detected after the server successfully updated it execution
image. Returns zero on and only on success.

@item @code{master_loop} [(@code{void}) @arrow{} @code{int}]
Perform the server's mission. Returns zero on and only on success.
@end table

@file{mds-base} also defines a number of global variables.

@table @asis
@item @code{argc} [@code{int}]
Number of elements in @code{argv}.

@item @code{argv} [@code{char**}]
Command line arguments.

@item @code{is_respawn} [@code{int}]
Whether the server has been respawn rather than this
being the initial spawn. This will be at least as true
as @code{is_reexec}.

@item @code{is_reexec} [@code{int}]
Whether the server is continuing from a self-reexecution.

@item @code{is_immortal} [@code{int}]
Whether the server should do its best to resist event
triggered death.

@item @code{on_init_fork} [@code{int}]
Whether to fork the process when the server has been
properly initialised.

@item @code{on_init_sh} [@code{char*}]
Command the run (@code{NULL} for none) when the server
has been properly initialised.

@item @code{master_thread} [@code{pthread_t}]
The thread that runs the master loop.

@item @code{terminating} [@code{volatile sig_atomic_t}]
Whether the server has been signaled to terminate.

@item @code{reexecing} [@code{volatile sig_atomic_t}]
Whether the server has been signaled to re-execute.

@item @code{danger} [@code{volatile sig_atomic_t}]
Whether the server has been signaled to free unneeded memory.

@item @code{socket_fd} [@code{int}]
The file descriptor of the socket that is connected
to the server.
@end table

@file{mds-base} expects the server implementation to define
a variable that specifies how @file{mds-base} should behave:

@table @asis
@item @code{server_characteristics} [@code{server_characteristics_t}]
This variable should declared by the actual server
implementation. It must be configured before @code{main}
is invoked. That is, it should be configured by a
constructor. If it is configured at its definition,
it is configured by a constructor; that is normally
how you want to configured it.
@end table

@code{server_characteristics_t} @{also known as
@code{struct server_characteristics}@} is a packed
@footnote{That is, define with @code{__attribute__((packed))}.}
with the following fields:

@table @asis
@item @code{require_privileges} [@code{unsigned : 1}]
Setting this to zero will cause the server to drop
privileges as a security precaution.

@item @code{require_display} [@code{unsigned : 1}]
Setting this to non-zero will cause the server to connect
to the display.

@item @code{require_respawn_info} [@code{unsigned : 1}]
Setting this to non-zero will cause the server to refuse
to start unless either @option{--initial-spawn} or
@option{--respawn} is used.

@item @code{sanity_check_argc} [@code{unsigned : 1}]
Setting this to non-zero will cause the server to refuse to
start if there are too many command line arguments.

@item @code{fork_for_safety} [@code{unsigned : 1}]
Setting this to non-zero will cause the server to place
itself in a fork of itself when initialised. This can be
used to let the server clean up fatal stuff after itself
if it crashes. When the child exits, no matter how it
exits, the parent will call @code{fork_cleanup} and then
die it the same manner as the child.

@item @code{danger_is_deadly} [@code{unsigned : 1}]
Setting this to non-zero without setting a signal action
for @code{SIGDANGER} will cause the server to die if
@code{SIGDANGER} is received. It is safe to set both
@code{danger_is_deadly} and @code{fork_for_safety} to
non-zero, during the call of @code{server_initialised}
the signal handler for @code{SIGDANGER} in the parent
process will be set to @code{SIG_IGN} independently of
the value of @code{danger_is_deadly} if
@code{fork_for_safety} is set to non-zero.

This setting will be treated as set to zero if
@option{--immortal} is used.
@end table



@node Keyboard Codes
@chapter Keyboard Codes

Keyboard servers receive scancodes from keyboard
drivers. A scancode can either be comprised of
one byte or three bytes. In each byte, the most
significant bit (assuming unsigned bytes) is
ignore, however for it first byte in the scancode
it signifies whether the key was released: it is
set of the key is released, and not set if the key
was pressed or is being held down.

A scancode is comprised of three bytes if the lower
7-bits of the first byte is are all cleared, and the
highest bit in the two following bytes are set.

Ignoring the most significant bit in all bytes, the
keycode is the value of the byte if the scancode
is a single byte scancode. If the scancode is comprised
of three bytes, the first byte is ignored and the
keycode is @math{a \cdot 128 + b} where @math{a} is the
value of the second byte and @math{b} is the value
of the third byte.

@menu
* 105-keys Keycodes::               List of keycodes for 105-keys keyboards.
@end menu



@node 105-keys Keycodes
@section 105-keys Keycodes

This is a list of keyboards for the
105-keys keyboards, using QWERTY-layout
for reference.

@table @asis
@item @code{1}
@kbd{Escape} key
@item @code{2}--@code{11}
Keys: @kbd{1}, @kbd{2}, @kbd{3}, @kbd{4}, @kbd{5}, @kbd{6}, @kbd{7}, @kbd{8}, @kbd{9}, @kbd{0}
@item @code{12}
Key right of @kbd{0}.
@item @code{13}
Key left of @kbd{backspace}
@item @code{14}
@kbd{Backspace} key
@item @code{15}
@kbd{Tab} key
@item @code{16}--@code{25}
Keys: @kbd{q}, @kbd{w}, @kbd{e}, @kbd{r}, @kbd{t}, @kbd{y}, @kbd{u}, @kbd{i}, @kbd{o}, @kbd{p}
@item @code{26}
Key right of @kbd{p}, once removed
@item @code{27}
Key right of @kbd{p}, twice removed
@item @code{28}
@kbd{Return} key
@item @code{29}
Left @kbd{control} key
@item @code{30}--@code{38}
Keys: @kbd{a}, @kbd{s}, @kbd{d}, @kbd{f}, @kbd{g}, @kbd{h}, @kbd{j}, @kbd{k}, @kbd{l}
@item @code{39}
Key right of @kbd{l}, once removed
@item @code{40}
Key right of @kbd{l}, twice removed
@item @code{41}
Key left of @kbd{1}
@item @code{42}
Left @kbd{shift} key
@item @code{43}
Key right of @kbd{l}, three times removed
@item @code{44}--@code{50}
Keys: @kbd{z}, @kbd{x}, @kbd{c}, @kbd{v}, @kbd{b}, @kbd{n}, @kbd{m}
@item @code{51}
Key right of @kbd{m}, once removed
@item @code{52}
Key right of @kbd{m}, twice removed
@item @code{53}
Key right of @kbd{m}, three times removed
@item @code{54}
Right @kbd{shift} key
@item @code{55}
@kbd{Multiply} key on the keypad
@item @code{56}
@kbd{Alternative} key
@item @code{57}
@kbd{Space} key
@item @code{58}
@kbd{Caps lock} key
@item @code{59}--@code{68}
Keys: @kbd{F1} through @kbd{F10}
@item @code{69}
@kbd{Num lock} key
@item @code{70}
@kbd{Scroll lock} key
@item @code{71}--@code{73}
@kbd{7}, @kbd{8}, @kbd{9} keys on the keypad
@item @code{74}
@kbd{Minus} key on the keypad
@item @code{75}--@code{77}
@kbd{4}, @kbd{5}, @kbd{6} keys on the keypad
@item @code{78}
@kbd{Plus} key on the keypad
@item @code{79}--@code{82}
@kbd{1}, @kbd{2}, @kbd{3}, @kbd{0} keys on the keypad
@item @code{83}
@kbd{Comma} key on the keypad
@item @code{86}
Key left of @kbd{z}
@item @code{87}
@kbd{F11} key
@item @code{88}
@kbd{F12} key
@item @code{96}
@kbd{Return} key on the keypad
@item @code{97}
Right @kbd{control} key
@item @code{98}
@kbd{Divide} key on the keypad
@item @code{99}
@kbd{System Request/Print Screen} key
@item @code{100}
@kbd{Alternative graphic} key
@item @code{102}
@kbd{Home} key
@item @code{103}
@kbd{Up} arrow key
@item @code{104}
@kbd{Page up} key
@item @code{105}
@kbd{Left} arrow key
@item @code{106}
@kbd{Right} arrow key
@item @code{107}
@kbd{End} key
@item @code{108}
@kbd{Down} arrow key
@item @code{109}
@kbd{Page down} down
@item @code{110}
@kbd{Insert} key
@item @code{111}
@kbd{Delete} key
@item @code{119}
@kbd{Pause/Break} key
@item @code{125}
Left @kbd{super} key
@item @code{126}
Right @kbd{super} key
@item @code{127}
@kbd{Application menu} key
@end table



@node Keyboard Layouts
@chapter Keyboard Layouts

Keyboard layouts as compiled from one or more files.
When compiling a layout from multiple files, it is
important that the files are specified in the correct
order. The general rule is that the layout file,
for example the Swedish QWERTY-keyboard, is specified
first and is followed by add-ons such as the compose
table and layout modifiers. @command{mds-kbdc} is
used to compile layouts.

Installed keyboard layout files are located in
@file{/usr/share/mds/keyboard}.@footnote{If you are
hacking in the source tree, you will find this under
@file{res/keyboard}.} Layouts are located in the
subdirectory @file{layout}, modifiers are located in
the subdirectory @file{mods} and compose tables are
located in the subdirectory @file{compose}.
@command{mds-kbdc} prefixes @file{/usr/share/mds/keyboard}
unless the specifed files starts with @file{/}, @file{./}
or @file{../}. Dead keys are implemented by compose tables
and not in the layouts.

@menu
* Keyboard Layout Syntax::                    How to write your how layouts.
* Builtin Functions::                         Functions provided by the compiler.
@end menu



@node Keyboard Layout Syntax
@section Keyboard Layout Syntax

Similar to the C programming language, keyboard layout
files are parsed from the top down. This means that any
function or macro can only be used from lines beload
the definition of said callable. However, the order of
the mapping statements themself, in respect to each
other, does not matter. Additionally, the layout files
are parsed line by line, and leading whitespace is ignored.
Comment can be started with a #-character and end at the
end of the line. It is important to know that modifiers
like @kbd{shift} and @kbd{control} needs to be mapped from
a keycode, this and similar that many keyboards have in
common, except dead key composition and compose sequences,
is already available in the @file{layout/common} directory
and can be included from the layout file. Compositions are
implement in the @file{compose} directory and should be
selected by the user at compile-time. Keyboard layout files
must be written in UTF-8 (without UTF-8 BOM) and with
line feeds for new lines.

@menu
* Mapping Statements::                        Mapping keycodes to logical keys and text.
* Sequence Mapping::                          Implementing dead keys and compositions.
* Keyboard Layout Identification::            Specifing the layout language, country and variant.
* Layout Assumptions::                        Making assumption about the keyboard layout.
* Include Statement::                         Including base files.
* Layout Macros and Functions::               Reducing repetition.
* Escaping::                                  Backslashes have so many uses.
@end menu



@node Mapping Statements
@subsection Mapping Statements

The most fundamental part of the layout files are mapping
statements. These specify which keycode the keys have
and what happens when certain keys pressed, combined or
pressed and a sequence. If we want to map keycode 57 to
the space key we write

@example
<keycode 57> : <space>
@end example

but then we also want the space key to product a blank
space when we are writting so we add

@example
<space> : " "
@end example

giving us

@example
<keycode 57> : <space>
<space> : " "
@end example

Because the order of the mapping statements does not
matter we can just as well write

@example
<space> : " "
<keycode 57> : <space>
@end example

@code{" "} represents a text string with one blank
space, but it is possible to have multiple characters.

We want to extend this to @kbd{altgr+space} producing
a no-break space, we can add either of the lines

@example
<altgr space> : "\u00A0" # no-break space (# comment)
<altgr keycode 57> : "\u00A0" # no-break space
@end example

However, we also need a mapping to @kbd{altgr}:

@example
<keycode 100> : <altgr>
@end example

If we want to add a mapping to @kbd{ultra} from
@kbd{altgr+menu} we can write

@example
<keycode 127> : <menu>
<altgr menu> : <-altgr ultra>
@end example

@code{-altgr} means that @kbd{altgr} should
not be reported as held down.

As can be seen in these examples it is not
possible to distinguish between modifiers
and keys. It is up to the keyboard layout
server and keyboard layout compiler to
know this. However, it is defined in the
keyboard layout files whether modifiers keys
are lock keys or not. To map the keycode
58 to @kbd{caps lock} write

@example
<keycode 58> : <caps lock>
@end example

But if you do not want it be a lock key,
but instead be required to be held down,
similar to how is normal for @kbd{shift},
instead write

@example
<keycode 58> : <caps>
@end example

Any modifier may be a lock key.

Another, just as important, use of mappings
to is map letter keys. Unlike control keys
like space and shift, there are no predefined
letters@footnote{With letters with mean any
character other than space.}. Therefore the
letter is prefixed with the word `letter'.
For example:

@example
<keycode 16> : <letter q>     # The Q-key has keycode 16 (on QWERTY)
<letter q> : "q"              # The Q-key should produce a `q'
<shift letter q> : "Q"        # but `Q' when shift is used
<caps q> : "Q"                # or when caps is used
<shift caps letter q> : "q"   # but not when both are used
@end example

Special characters like simple double quotes,
backspace and, in @code{<>}-notation, greater than
sign must be escaped with a prepending backslash.

Many keyboard layouts also have dead keys.
Dead keys are keys that affect the next key-press.
For example, `´' followed by `e' may product `é'.
@kbd{compose} may be a dead key, just like it is in X.org,
but it can also be a modifer.

To define @kbd{´}, with keycode 13, @kbd{compose}, with
keycode 125, as a dead keys write

@example
<keycode 13> : <dead letter ´>
<keycode 125> : <dead compose>
@end example

Some may appear on multiple locations on the keyboard,
for example, there may be a left and a right shift key,
and a normal return key and one on the keypad:

@example
<keycode 42> : <left shift>
<keycode 54> : <right shift>
<keycode 28> : <return>
<keycode 96> : <keypad return>
@end example

Because @code{<left>} and @code{<right>} are
valid keys --- they are arrow keys --- it is
importatn to place them directly before the
key, and not after. For instance @code{<left shift>}
denotes the left @kbd{shift} key, whilst
@code{<shift left>} denotes the left-arrow key
with a @kbd{shift} key held down. Modifiers
goes first.



@node Sequence Mapping
@subsection Sequence Mapping

Compose tables use mapping statements to map
key sequences. For example the compose key
followed by two `s':es makes an `ß':

@example
<dead compose> "s" "s" : "ß"
@end example

It is also possible to map a sequence to
another sequence:

@example
<dead compose> <tab> : <tab> <tab> <tab> <tab>
@end example

Of course, the input does not need to be
a sequence:

@example
<super tab> : <tab> <tab> <tab> <tab>
@end example

En alternative to @kbd{compose} as a dead key,
is @kbd{compose} as a modifier. If you use this,
the compose table need to be written for just
that. There two ways do this this. Either you
can write for example

@example
<compose "s"> <compose "s"> : "ß"
@end example

This maps two `s':es to a `ß', but requires that
@kbd{compose} is held down during both key-presses.
The other way is to write

@example
<compose "s", "s"> : "ß" # Note the `,'
@end example

This also requires that @kbd{compose} is not
released between the key-presses.

The compose table is filled with compositions
where it does not matter in which order you
press some of the keys. For example, instead of

@example
<dead compose> "S" "|" : "$"
<dead compose> "|" "S" : "$"
@end example

you can write

@example
<dead compose> ("S" "|") : "$"
@end example

@code{( )} denotes an unordered subsequence.
You can also use @code{[ ]} for alternation.
For example, instead of

@example
<dead compose> ("S" "|") : "$"
<dead compose> ("s" "|") : "$"
@end example

you can write

@example
<dead compose> (["S" "s"] "|") : "$"
@end example

Inside an alternation you can use a dot
for specify that no key press is needed.
For example, instead of

@example
<dead compose> "|" "S" : "$"
<dead compose> "|" "|" "S" : "$"
@end example

you can write

@example
<dead compose> "|" ["|" .] "S" : "$"
@end example

It is undefined in which order alternations
and unordered subsequences are expanded;
neither sequencewise or levelwise.
Thus, there should not be side-effects where
either one is used, nor does it make since
to nest the two constructs in any other
way than alternation or unordered subsequence
inside unordered subsequence. The compiler
may however choose to discourage unordered
subsequence inside unordered subsequence
because of readability issues.

Unordered subsequences longer than 5 elements
cannot compile under normal circumstances. This
is eliminiation of unordered subsequences grows
superexponentially, and thus is probably an error
than can cause memory exhaustion and unrealistic
compilation-time. Therefore, if an unordered
subsequences longer than 5 elements is used the
compiler required that the @option{--force} flag
is used and that the unordered subsequences uses
double brackets:

@example
<dead compose> ("1" "2" "3" "4" "5") : "120"
<dead compose> (("1" "2" "3" "4" "5" "6")) : "720"
<dead compose> (("1" "2" "3" "4" "5" "6" "7")) : "5040"
<dead compose> (("1" "2" "3" "4" "5" "6" "7" "8")) : "40320"
<dead compose> (("1" "2" "3" "4" "5" "6" "7" "8" "9")) : "362880"
@end example



@node Keyboard Layout Identification
@subsection Keyboard Layout Identification

Whilst it is possible to write a comment
that states what keyboard layout a file
implements, there is a standardise way
to do this in code. The intention with
this is to make it possible for graphical
tools to easily list the layouts and
easy to understand descriptions.

There are three things a keyboard layout
file should specify: the language, the
country where it is used, and the variant.

For example the Swedish QWERTY layout used
in Sweden would have the code.

@example
information
  language "Swedish"
  country "Sweden"
  variant "QWERTY"
end information
@end example

If the layout is used multiple countries,
or even for multiple lanuages, @code{country}
and @code{language} may be specified on
multiple lines. For example:

@example
information
  language "Spanish"
  country "Argentina"
  country "Bolivia, Plurinational State of"
  country "Chile"
  country "Colombia"
  country "Costa Rica"
  country "Cuba"
  country "Dominican Republic"
  country "Ecuador"
  country "El Salvador"
  country "Guatemala"
  country "Haiti"
  country "Honduras"
  country "Mexico"
  country "Nicaragua"
  country "Panama"
  country "Paraguay"
  country "Peru"
  country "Puerto Rico"
  country "Uruguay"
  country "Venezuela, Bolivarian Republic of"
  variant "Latin American, QWERTY"
end information
@end example

or

@example
information
  language "Norwegian"
  language "Bokmål"
  language "Nynorsk"
  country "Norway"
  variant "QWERTY"
end information
@end example



@node Layout Assumptions
@subsection Layout Assumptions

When writing generic compose tables it can be helpful
to let the compiler assume that a certain set of keys
will be provided by the layout file and not making
other assumptions. This is helpful because if you want
to make an compose table that can compose all characters
given only the ASCII letters, modifiers and a compose
key, rather than written a phony layout file and select
it each time you compile to compose table you can state
in the compose table file that the compiler should as
that those keys are provided when the compose table file
is compile by itself. If this is done, the compiler can
warn when one of the compositions cannot be reached from
those basic keys.

If we want to make the compiler assume that @kbd{compose}
is available as a dead key, that @kbd{shift}, @kbd{altgr}
and @kbd{space} are available and that the ASCII letter,
digits and some basic special characters are available we
can write.

@example
assumption
  have <dead compose>
  have <shift>
  have <altgr>
  have <space>
  have_range "0" "9"
  have_range "a" "z"
  have_range "A" "Z"
  have_chars "!\"@@#$%&/@{([)]=@}?\\^~*'<>|,;.:-_"
end assumption
@end example



@node Include Statement
@subsection Include Statement

Writing layout files from scratch is probably something
you want to avoid. For instance you would we need to
create mappings for `A' to `Z' and `0' to `9' (assuming
its a latin-based language), and map up all specific key,
like modifiers, space, arrow keys, and the keypad. And
you would have to make sure do only that the keys are
mapped but that they are mapped to the text the should
product and that they word correcly with the modifiers.
These are things most keyboards have in common with many
other layouts.

For instance @file{layout/sv/qwerty} has two include
statements to implement its basics:

@example
include "../common/qwerty"
include "../common/base"
@end example



@node Layout Macros and Functions
@subsection Layout Macros and Functions

There is a lot of repetitive work in layouts, for instance
all letters need mapping for any combination of use of
@kbd{shift} and @kbd{compose}. To reduce this, you can
define macros.

For example instead of writing

@example
<letter a> : "a"
<shift letter a> : "A"
<caps letter a> : "A"
<shift caps letter a> : "a"

<letter b> : "b"
<shift letter b> : "B"
<caps letter b> : "B"
<shift caps letter b> : "b"

# and so on ...
@end example

you can use a macro and write

@example
macro letter/2
  <letter \1> : "\1"
  <shift letter \1> : "\2"
  <caps letter \1> : "\2"
  <shift caps letter \1> : "\1"
end macro

letter("a" "A")
letter("b" "B")
# and so on ...
@end example

The name of this macro is `letter/2',
but it is called with the name `letter'
and two arguments. The `/2'-suffix means
that it is invoked with exactly two arguments.
You can use this do define multiple version
of the same macro, with the same invocation
name but with different number of arguments.
For example:

@example
macro letter/2
  <letter \1> : "\1"
  <shift letter \1> : "\2"
  <caps letter \1> : "\2"
  <shift caps letter \1> : "\1"
end macro
macro letter/1
  letter(\1 \add(\sub(\1 "a") "A"))
end macro

letter("a")
letter("b")
# and so on ...
letter("å" "Å")
letter("ä" "Ä")
letter("ö" "Ö")
@end example

@code{\add( )} and @code{\sub( )} are calls to two
built-in functions named `add/2' and `sub/2'.

Alternation can be used to invoke a macro:

@example
letter(["a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m"])
letter(["n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z"])
@end example

You may use `.' in an alternation, in that case
macro is called once with the argument, causing it
to invoke for example `letter/0' instead of
`letter/1'.

A related issue are for-loops. If we for
example want to call the macro `letter/1' for
all letters betweeh and including `a' and `z'
we can just write

@example
for "a" to "z" as \1
  letter(\1)
end for
@end example

instead of using the alternation-trick.

You call also use if-statments. For example

@example
for "à" to "þ" as \1
  # times sign is not upper case of division sign
  if \not(\equals(\1 "\u00F7"))
    letter(\1 \add(\sub(\1 "à") "À"))
  end if
end for
@end example

or equivalently

@example
for "à" to "þ" as \1
  if \equals(\1 "\u00F7")
    continue # times sign is not upper case of division sign
  end if
  letter(\1 \add(\sub(\1 "à") "À"))
end for
@end example

@code{continue} can be used to stop the iteration of
the innermost for-loop and skip to the next iteration.
You can also use @code{break}, but it also has the
effect to stop the entire loop. Similarly, @code{return}
can be used to break an entire macro call, or function
call.

You can also use @code{if} for more example things,
and use @code{else if} and @code{else if}:

@example
macro latter/1
  if \not(\greater(\1 "z"))
    letter(\1 \add(\sub(\1 "a") "A"))
  else if \not(\greater(\1 "þ"))
    letter(\1 \add(\sub(\1 "à") "À"))
  else
    letter(\1 \sub(\1 1))
  end if
end macro
@end example

Note that there is no quotes around the `a'
in @code{letter(\1 \sub(\1 1))}. This means
that the argument will be than value 1 rather
than the code point of the character `1'. Note
however that values lower than zero or equals
to or greater than 2 to the power of 31 not
allowed and can either cause compile-time
error or erroneous compiled files.

Functions are similar to function macros,
the difference is that a function is called
inline and is prefixed with slash, and rather
than inline the code inside it, the evalutes
to the last value it evaluted before it returned.

For example instead of @code{\not(\greater(\1 "z"))}
you can write @code{\less_eq(\1 "z")} after you
have defined the function `less_eq/2' with the
following code:

@example
function less_eq/2
  \not(\greater(\1 \2))
end function
@end example

A final construct to make layout code less
repetitive is let-statements. This can be
used to assign values to variables.

The code

@example
macro latter/1
  if \not(\greater(\1 "z"))
    letter(\1 \add(\sub(\1 "a") "A"))
  else if \not(\greater(\1 "þ"))
    letter(\1 \add(\sub(\1 "à") "À"))
  else
    letter(\1 \sub(\1 1))
  end if
end macro
@end example

can equivalently be written using @code{let} as

@example
macro latter/1
  if \not(\greater(\1 "z"))
    let \2 : \sub("a" "A")
  else if \not(\greater(\1 "þ"))
    let \2 : \sub("à" "À")
  else
    let \2 : 1
  end if
  letter(\1 \sub(\1 \2))
end macro
@end example

It is also possible to declare arrays:

@example
let \1 : @{ "å" "ä" "ö" "à" "é" "ü" @}
@end example

Arrays may however not have arrays for
values.

Because arrays can be very large, they, but only
them, may span multiple lines. For example
you may write

@example
let \1 : @{
  "å" "ä" "ö" "à" "é" "ü"
  @}
@end example

but not

@example
let \1 :
  @{ "å" "ä" "ö" "à" "é" "ü" @}
@end example

See @ref{Builtin Functions} for how they are used.

Variable names can only be numerical and most not
start with a zero. `0' is not valid variable name,
and thus @code{\0} does not address a variable.
Macro and function names, may only include
`0'--`9', `a'--`z', `A'--`Z' and `_', but must and
not start with `0'--`9'. Additionally, when declared
macro and function names must be suffixed with `/'
follwed by the exact number of arguments the macro
or function takes.

Variable indices are constrained to the 31:th power
of 2, exclusively. Attempts to use higher variable
indices invoke undefined behaviour. Additionally
there is no guarantee that the compiler allocates
indexwise spares variables efficiently.

Like variables, the size of arrays are also
restricted to the 31:th power of 2.



@node Escaping
@subsection Escaping

Similar to most, if not all, programming language, a
backslash inside quotes can be used to parse the next
character with special meaning. For instance, `\"' is
parsed as a literal `"', and `\\' is parsed as a literal
`\'. `\>' is too parsed as a literal `>', for example
you may need to write @code{<letter \>>}. The characters
`(', `)', `[', `]', `@{', `@}', `<' and `,' also follow
this rule to make those character accesible inside
a @code{< >}. But `\' can also be used to specify
characters by their code point, for example if you want
an `æ' you can write @code{"\u00E6"} or @code{"\uE6"},
instead of @code{"æ"}. You can also write @code{"\0346"},
the difference between `\0' and `\u' is that `\0' uses
octal whereas `\u' uses hexadecimal. A noteworthy
side-effect of this is that function names cannot start
with a lower case `u'.

`\' can also be used to access variables and parameters.
For example `\1' in

@example
macro letter/2
  <letter \1> : "\1"
  <shift letter \1> : "\2"
  <caps letter \1> : "\2"
  <shift caps letter \1> : "\1"
end macro
letter("å" "Å")
@end example

is expanded to an `å', where as `\2' is expanded to an `Å'.

`\' is also used to call functions, for example if
you want to call the function `f/0' you write
@code{\f()}.

Because numerical (possibly prefixed with an `u') are of
variable length, it is possible to specify the escape's
termination point with a dot. For instance, if you
want the value of the first variable (\1) followed by
two zeroes, you do not write `\100' as that would expand
to the value of the hundredth variable. Instead you
write `\1.00'.

Use of function calls and variables inside
@code{include}-statments invokes undefined behaviour.
Escaped numerals greater than or equal to the 31th
power of 2 also invoke undefined behaviour.


@node Builtin Functions
@section Builtin Functions

To help you write meaningful functions in your keyboard
layout files, the compiler defines an almost minimal set
of basic functions:

@table @code
@item add/2
The code points in \1 plus the code points of the
corresponding characters in \2. If \1 and \2 are
not of the same length, the returned string will
be of the length of the longer of the parameters,
and modulo is used to map to the corresponing
character.
@item sub/2
Like `add/2' but subtraction.
@item mul/2
Like `add/2' but multiplication.
@item div/2
Like `add/2' but division.
@item mod/2
Like `add/2' but modulo.
@item rsh/2
Like `add/2' but rightward bitwise shift.
If a character in \2 is has a code point
greater than 30, undefined behaviour is
invoked.
@item lsh/2
Like `add/2' but leftward bitwise shift.
If a character in \2 is has a code point
greater than 30, undefined behaviour is
invoked.
@item or/2
Like `add/2' but bitwise OR.
@item and/2
Like `add/2' but bitwise AND.
@item xor/2
Like `add/2' but bitwise XOR.
@item not/1
For each character in \1, evaluate to
zero if the character is not zero, and
one if the character is zero.
@item equals/2
For each character, evalute to one if
the characters in \1 and \2 are equal
and zero otherwise.
@item greater/2
Like `equals/2' but \1 greater than \2
rather than \1 equals \2.
@item less/2
Like `equals/2' but \1 less than \2
rather than \1 equals \2.
@item set/3
Set the element with index \2, in the
array with variable index \1, to \3,
and return \3.
For example @code{\set(1 0 4)} sets the
first element in \1 to 4.
@item get/2
Return the element with index \2 in the
array with variable index \1.
For example after @code{\set(1 0 4)} or
@code{let \1 : @{ 4 3 2 1 0 @}} has been
used, @code{\get(1 0)} evaluates to 4.
@end table



@node Accessibility
@chapter Accessibility

@menu
* Sticky Keys::                               Ephemeral key-locking.
* Bounce Keys::                               Key repetition-elimination.
* Slow Keys::                                 Key press-acceptance delay.
* Loud Keys::                                 Keyboard tiggered sound cues.
* Mouse Keys::                                Controlling the mouse with the keyboard.
@end menu



@node Sticky Keys
@section Sticky Keys

Sticky keys is an accessibility feature that lets the
user process modifier keys instead of holding them down.
Sticky keys is implemented by the @command{mds-keystick}
server.

When @command{mds-keystick} is active, any key that
is mapped in @command{mds-keystick} to be a modifier
is held down once press and not released until an
non-modifier is released. It is possible for map any
key to have this feature, that do not have to be
actual modifiers, that is, modifiers according to
@command{mds-keytrans}.

If a key is press three times without any intermediate
keys, and is a modifier, that key released until it is
pressed again.

Note that @command{mds-keystick} is not aware of which
keys are considered modifiers by @command{mds-keytrans},
but it is affected of remappings in @command{mds-kbd}
and @command{mds-kkbd}, therefore it is better to swap,
for example, @kbd{control} and @kbd{caps lock} in the
keyboard servers than in @command{mds-keytrans}. Doing
so means that you do not have to configure
@command{mds-keystick} to know the original @kbd{caps lock}
is a modifier but the original @kbd{control} is not.

For greater accessibility you can, in @command{mds-keytrans},
replace a key with the @kbd{mode lock}-key. If this is done,
pressing a sequence of modifiers and then the
@kbd{mode lock}-key will caused those modifiers to be locked
until the next time you press @kbd{mode lock}. Any modifier
pressed directly before, optionally with intermediate
modifiers, the second @kbd{mode lock} will stay locked. The
action of @kbd{mode lock} is to release all modifiers it is
locked and the lock those that are currently held down; active
stickly keys are artificially held down and thus included.



@node Bounce Keys
@section Bounce Keys

Bounce keys is an accessibility feature that filters out
rapidly repeated key strokes. Bounce keys is implemented
by the @command{mds-keybounce} server.



@node Slow Keys
@section Slow Keys

Slow keys is an accessibility feature that filters out
brief key strokes. Slow keys is implemented by the
@command{mds-slowkey} server.



@node Loud Keys
@section Loud Keys

Loud keys is an accessibility feature that can emulate
key clicking sounds when a key is pressed or generate
tones when certain keys are pressed. For example if
@kbd{caps lock} is actived by a key stroke a high-pitched
tone can be generated when when it is deactived by a key
stroke a low-pitched tone can be generated. Loud keys is
implemented by the @command{mds-keycue} server.



@node Mouse Keys
@section Mouse Keys

Rat keys (also known as mouse keys) is an accessibility
and usability feature that lets the user use the keyboard
as a pointing device. This feature is implemented by the
@command{mds-kbd2rat} server, and this section covers the
details of that server.

If @command{mds-kbd2rat} is used, the keypad acts as a
pointing device when @kbd{num lock} is not active. It
is also possible to configure @command{mds-kbd2rat} to
disregard @kbd{num lock} and always be active, until
it receives a signal to toggle its state. For example,
if @command{mds-kbd2rat} is configured to disregard
@kbd{num lock}, @command{mds-kbdtrans} can be configured
to send to fauxkey @kbd{rat} when @kbd{alt+shift+num}
is pressed, and this key can be caught by
@command{mds-keybind} that sends a signal to
@command{mds-kbd2rat} to toggle.

When @command{mds-kbd2rat} is active, it maps the
keypad to the rat according to the following table:

@table @kbd
@item division
Right-hand left (primary) rat button.

@item multiplication
Middle (modifier) rat button.

@item subtraction
Right-hand (alternate) right rat button.

@item addition
Scroll up.

@item return
Scroll down.

@item comma
Cancel actions waiting by @kbd{0} and
@kbd{5}.

@item 0
Hold down the next selected rat button
until that rat button is selected again.
This is a dead key, that is, you do not
have to hold it down.

@item 1
Cursor left down.

@item 2
Cursor down.

@item 3
Cursor right down.

@item 4
Cursor left.

@item 5
Double click with the next selected
rat button. This is a dead key, that is,
you do not have to hold it down.
If pressed twice, a triple click triggered
instead, and so on. If this key is prefixed
by @kbd{0}, the last click will be replaced
by hold down action.

@item 6
Cursor right.

@item 7
Cursor left up.

@item 8
Cursor up.

@item 9
Cursor right up.
@end table

This table is not affected by mappings in
@command{mds-keytrans}, but it is affected
by remappings in @command{mds-kbd} and
@command{mds-kkbd}.

@command{mds-kbd2rat} can be configured to
permute the rat button keys. It is independent
of permutations in @command{mds-rat}.

If multiple direction keys@footnote{@kbd{1},
@kbd{2}, @kbd{3}, @kbd{3}, @kbd{4}, @kbd{6},
@kbd{7}, @kbd{8} and @kbd{9}} are used, the
average direction is used.

@command{mds-kbd2rat} is five parameters that
control with which speed it moves the cursor:

@table @asis
@item delay
The delay it takes before the first time a
mouse event is repeated when a key is held
down. The default is for the server to wait
for the keyboard to send another key press.

@item interval
The time it takes before a mouse event is
repeated since its previous repear when a
key is held down. The default is for the
server to wait for the keyboard to send
another key press.

@item max speed
The maximum speed with which the cursor can move.

@item time to max speed
The time a key has to be held down before
the maximum speed is reached. The cursor
accelerate with each mouse event repeat.

@item curve
Ramp used to reach pointer speed.
@end table

The pointer's speed, not taking caps into consideration,
is calculated by

@iftex
@math{max~speed \left ( hold~down~time ~-~ delay \over time~to~max~speed \right)^{1 ~+~ curve}}
@end iftex
@ifnottex
max speed ((hold down time - delay) / time to max speed)^(1 + curve)
@end ifnottex



@node Discussion
@chapter Discussion

@menu
* Server Architecture::                       Discussion on fundamental design choices.
* Fixing X.org Issues::                       Can we avoid the problems X.org has?
@end menu



@node Server Architecture
@section Server Architecture

This chapter aims to enumerate advantages and
disadvantages with micro-display servers,
traditional monolithic display servers and
other possible designs. Please chime in with
any insight.

@menu
* The Microserver Architecture::              The microserver architecture.
* The Monolithic Server Architecture::        The monolithic server architecture.
* The Hybrid Server Architecture::            The hybrid server architecture.
* The Megalithic Server Architecture::        The megalithic server architecture.
* The Modular Server Architecture::           The modular server architecture.
* The Modular Microserver Architecture::      The modular microserver architecture.
* The Exoserver Architecture::                The exoserver architecture.
@end menu



@node The Microserver Architecture
@subsection The Microserver Architecture

Description: The display server is implement with
multiple binaries that speak with each other using
a well defined protocol.

@noindent
Implementations: mds.

@noindent
Advantages:

@itemize @bullet{}
@item
Knowing the names of the servers you use and
their purpose makes it very easy to find where
you want to do patching in the source code.

@item
Spaghetti code to a larger extent is virtually
impossible; the microserver architecture guarantees
a certain quality of the code architecture for
the display server.

@item
If the message passing used in the display server
allows for message modification and retrieval
ordering, extending, modifying and using the display
server in unforeseen ways becomes much easier,
and will often not require any modifications to
the existing servers.

@item
Replacing the display server is easier for a
micro-display server than it is for a monolithic
display server, because the servers could be
replaced one by one and could even support running
under two distinct protocol during the transitional
period.

@item
Not as many subprotocols needs to be defined.
For example, recording the output of the display
does not require a special protocol, one only
needs to write a server that listens on messages
passed between servers.

@item
If a server crashes it does not crash the entire
session. Crashes can most often be repaired.

@item
Because servers can easily be omitted and replaced
when starting the display server, it becomes much
easier to mount the display server on top of an
already running display server. For example, if you
want the performance of @code{weston} but then
flexibility and functionallity of @code{mds}, you
could start @code{mds} inside @code{weston} and
replace a small set of the servers with variants
written to running on top of Wayland; of course
with some functionallity of @code{mds} missing.

@item
It is trivial to only have setuid on for the
part of the display server where it is required.
@end itemize



@node The Monolithic Server Architecture
@subsection The Monolithic Server Architecture

Description: The display server is implemented
as one binary.

@noindent
Implementations: X11, Mir, Wayland, Surface Flinger,
Quartz Compositor, Desktop Window Manager.

@noindent
Advantages:

@itemize @bullet{}
@item
The monolithic architecture makes it trivial
to isolate information for clients to achieve
confidentiality. Prioritising confidentiality
however leads to problems implementing features
such as screenshooting and global hotkeys.

@item
Monolithic server does not need to pass messages
between modules, but can rather perform normal
function calls and achieve greater performance.

@item
Monolithic display servers can have a smaller memory
footprint than its full-fledged counterparts.
@end itemize



@node The Hybrid Server Architecture
@subsection The Hybrid Server Architecture

Description: The display server is implmeneted
with the microserver architecture except some
components are built into the master server for
performance or security reasons.

@noindent
Hybrid display server could arguably be called
milli-display servers to emphasis that they are
small, but not as small as micro-display servers,
are much more closely related to micro-display
servers than monolithic display servers, and, in
constrast with OS kernels, have a proper distinction
from monolithic systems and microsystems.
@footnote{I don't know about calling them
macro-display servers, that implies that they are
the total opposite of micro-display servers.}

@noindent
Implementations: none?

@noindent
Advantages:

@itemize @bullet{}
@item
Can achieve most of the microserver architecture's
advantages, but not always to the same extent.

@item
By integrating some servers into the master server,
the hybrid architecture can isolate information for
clients to achieve confidentiality. Prioritising
confidentiality however leads to problems implementing
features such as screenshooting and global hotkeys.

@item
Large and high frequency messages does not need to
be passed around to other servers if they are integrated
into the master server. This lets hybrid display server
achieve the same perfomance performance as monolithic
display servers for tasks where it is desirable.
@end itemize

@noindent
The mds protocol and its reference implemention can
easily be made into a hybrid display server protocol
and an implementation thereof.



@node The Megalithic Server Architecture
@subsection The Megalithic Server Architecture

Description: A monolithic display server where
applications are loaded or compiled into the display
server itself.

@noindent
These are also known as mega-display servers.

@noindent
Implementations: none?

@noindent
Advantages:

@itemize @bullet{}
@item
No interprocess communication is required, apart from
letting the display server know to load modules if it
does not compile in its programs. This lets megalithic
display server achieve even greater performance than
monolithic display servers.
@end itemize

@noindent
Disadvantages:

@itemize @bullet{}
@item
Imposes restrictions on which languages applications can use.

@item
Imposes restrictions on how applications can behave.

@item
Cannot be networked without exposing an alternative
display server protocol.

@item
The display becomes more crash prune; if an application
crashes it is likely to crash the entire display.

@item
Applications will run with the same privileges as the display
server, which is root on most operating systems.
@end itemize

@noindent
Megalithic display servers could be interesting for high
performing gaming consoles.



@node The Modular Server Architecture
@subsection The Modular Server Architecture

Description: A monolithic display server where server-like
programs can be loaded as modules into the display server
but applicates are connected with interprocess communication.

@noindent
Implementations: none?@footnote{Desktop Window Manager
is partially modular, but as of yet, this cannot be
utilised by end-users.}

@noindent
Advantages:

@itemize @bullet{}
@item
Can achieve that flexibility of micro-display servers,
but not when networked, with the same memory footprint
as monolithic display servers.

@item
Has the same advantages as monolithic kernels.

@item
Applications that require absolute performance can
be loaded as modules and achieve the same performance
as megalithic kernels, however with the same caveats.
@end itemize

@noindent
With a little work the mds protocol could be transformed
into a modular server display protocol, and with some work
the reference implementation could be made into a modular
server display.



@node The Modular Microserver Architecture
@subsection The Modular Microserver Architecture

Description: A modular display server with a module
that enables clients to act as modules that communicates
via interprocess communication rather than being loaded
into the display server.

@noindent
Implementations: none?

@noindent
Advantages:

@itemize @bullet{}
@item
The modular microserver architecture seem to provide
all of the advantages of the other architecture but
none of the disadvantages. However, modules can still
crash and bring down the display server, but the idea
is to not load unstable modules but let the be servers.
Therefore exo-diplay server are slightly more robust.
@end itemize

@noindent
With a little work the mds protocol could be transformed
into a modular server display protocol, and with some work
the reference implementation could be made into a modular
server display. Then the untransformed version of
@command{mds-server} cound be made into a module for the
transformed version.



@node The Exoserver Architecture
@subsection The Exoserver Architecture

Description: An exo-display server is a tiny display
server that attempts to let applications access the
underlaying system directly and implements basic
interprocess communication to let applications share
vital information and coordinate with each other.

@noindent
Implementations: none?

@noindent
Advantages:

@itemize @bullet{}
@item
Can achieve the same performance as megalithic
display servers.

@item
Can achieve the same robustness as micro-display
servers.
@end itemize

@noindent
Disadvantages:

@itemize @bullet{}
@item
Cannot be networked without exposing an alternative
display server protocol.
@end itemize

@noindent
Exo-display servers could be interesting for high
performing gaming consoles.



@node Fixing X.org Issues
@section Fixing X.org Issues

X.org is been critiqued for several shortcoming, some
of which have caused people to start on new display
servers replace X.org. This chapter will list some
issues and discuss how they can be avoided in mds.

@menu
* Automatic Cleanup::                         Cleanup up after applications.
* Input Problems::                            Problems related to human input.
* Other Issues::                              Other issues in X.org.
@end menu



@node Automatic Cleanup
@subsection Automatic Cleanup

A common critique of X.org is that the monitor resolution
is not restored if a game change the resolution and for
some reason, for instance a software crash, does not switch
back before exiting. This problem is not intrinsic to the
protocol, but rather because of a lacking protocol. You
can run a program like @command{xrandr} to change the
monitor resolution for the entirety of the session and
@command{xrandr} can exit when the resolution has changed.
This is how it should be. However, there is no way to tell
an X.org server to switch back if the connection between
the program and server is lost. This is easily fixed by
adding a lifespan parameter as found in @ref{set-gamma}.

A similar critique of X.org is that gamma ramps are not
restored when an application exits. Either the ones
complaining about this do not understand why gamma ramps
exists, namely so you can calibrate the monitor's output
in respect to the colours, and just think it is a way
to make the video in games brighter. Or they think we
should have daemons running ideally to have gamma
adjustments. Or, more likely and more validly, its is
poorly phrased and they actually want a way for
applications, like games, to inform the display server
to undo its modifications to the gamma ramps when the
program exits. This is already supported by the mds
protocol.



@node Input Problems
@subsection Input Problems

X11 allows programs to exclusively grab keyboard
and mouse input. When a program that does this
misbehaves or become unresponsive, you cannot do
anything but manage it from another computer or
restart the computer. In mds exclusively grabbing
is achieved buy setting the client priority for
the related message to the highest priority.
@footnote{If multiple clients do this, it is
arbitrary who gets the message first and can stop
the others from getting it.} This is however not
allowed (but nothing will stop you) as the idea is
that clients should either select a predefined
priority, select a priority between servers it to
be between, or select a priority of 50 percent or
150 percent of another servers priority. Thus,
unless a client breaks this rule, you can always
have your server for switching to the TTY at a
higher priority than other programs.

A similar, and probably related, problem in X.org
is that global keybindings don't work when a
popup or menu has focus. (Thankfully GTK+ will
close that item if it receives unexpected input.)
I have hard time seeing how this could become
an issue in mds.

Another issue related to the keyboard in X.org is
that hotkeys in programs do not work in a few
situtations because the program was not designed
with another keyboard layout in mind than the
keyboard layout the developer used. I suggest
that programs restrain themself from including
Alternative Graph in their hotkeys and only use
Shift for A through Z and space. However, what
I would really like to see is that toolkits lets
users modify all hotkeys. If program additionally
restrain themself to having all hotkeys contain
control or alt the keyboard layouts with non-latin
alphabets would not suffer because they do not
use the latin alphabet.



@node Other Issues
@subsection Other Issues

X11 display servers do not let you upgrade or
otherwise replace graphics drivers online. Or other
parts of it. X11 display servers could allow you to
send a signal, for instance SIGUSR1, to upgrade the
whole server, however this is not favourable, and
X.org does not do this. The reference implemention
of the mds protocol lets you safely upgrade any part
of it unline by sending SIGUSR1 to the server that
should be upgraded. On catastrophic failure in this
process 0a server would restart and lose volatile
data, but the server shoul be upgraded and it would
ask all running clients for resend information the
server lost.

Another issue with X.org is that it is not
multithreaded, which can cased intensive programs
to freeze your desktop. mds is inherently pervasively
parallel and only subsystems, rather than the whole
system, can suffer from this. It is however easy
for mds servers to implement pervasive threading,
that is, letting each request spin up a new thread
in the server.



@node GNU Free Documentation License
@appendix GNU Free Documentation License
@include fdl.texinfo

@bye