248 lines
9.1 KiB
XML
248 lines
9.1 KiB
XML
<chapter id='Data_Structures'>
|
||
<title>Data Structures</title>
|
||
|
||
<para>
|
||
An Xkb keyboard description consists of a variety of data structures, each of
|
||
which describes some aspect of the keyboard. Although each data structure has
|
||
its own peculiarities, there are a number of features common to nearly all Xkb
|
||
structures. This chapter describes these common features and techniques for
|
||
manipulating them.
|
||
</para>
|
||
|
||
|
||
<para>
|
||
Many Xkb data structures are interdependent; changing a field in one might
|
||
require changes to others. As an additional complication, some Xkb library
|
||
functions allocate related components as a group to reduce fragmentation and
|
||
allocator overhead. In these cases, simply allocating and freeing fields of Xkb
|
||
structures might corrupt program memory. Creating and destroying such
|
||
structures or keeping them properly synchronized during editing is complicated
|
||
and error prone.
|
||
</para>
|
||
|
||
|
||
<para>
|
||
Xkb provides functions and macros to allocate and free all major data
|
||
structures. You should use them instead of allocating and freeing the
|
||
structures yourself.
|
||
</para>
|
||
|
||
<sect1 id='Allocating_Xkb_Data_Structures'>
|
||
<title>Allocating Xkb Data Structures</title>
|
||
|
||
<para>
|
||
Xkb provides functions, known as allocators, to create and initialize Xkb data
|
||
structures. In most situations, the Xkb functions that read a keyboard
|
||
description from the server call these allocators automatically. As a result,
|
||
you will seldom have to directly allocate or initialize Xkb data structures.
|
||
</para>
|
||
|
||
|
||
<para>
|
||
However, if you need to enlarge an existing structure or construct a keyboard
|
||
definition from scratch, you may need to allocate and initialize Xkb data
|
||
structures directly. Each major Xkb data structure has its own unique
|
||
allocator. The allocator functions share common features: allocator functions
|
||
for structures with optional components take as an input argument a mask of
|
||
subcomponents to be allocated. Allocators for data structures containing
|
||
variable-length data take an argument specifying the initial length of the data.
|
||
</para>
|
||
|
||
|
||
<para>
|
||
You may call an allocator to change the size of the space allocated for
|
||
variable-length data. When you call an allocator with an existing data
|
||
structure as a parameter, the allocator does not change the data in any of the
|
||
fields, with one exception: variable-length data might be moved. The allocator
|
||
resizes the allocated memory if the current size is too small. This normally
|
||
involves allocating new memory, copying existing data to the newly allocated
|
||
memory, and freeing the original memory. This possible reallocation is
|
||
important to note because local variables pointing into Xkb data structures
|
||
might be invalidated by calls to allocator functions.
|
||
</para>
|
||
|
||
</sect1>
|
||
<sect1 id='Adding_Data_and_Editing_Data_Structures'>
|
||
<title>Adding Data and Editing Data Structures</title>
|
||
|
||
<para>
|
||
You should edit most data structures via the Xkb-supplied helper functions and
|
||
macros, although a few data structures can be edited directly. The helper
|
||
functions and macros make sure everything is initialized and interdependent
|
||
values are properly updated for those Xkb structures that have
|
||
interdependencies. As a general rule, if there is a helper function or macro to
|
||
edit the data structure, use it. For example, increasing the width of a type
|
||
requires you to resize every key that uses that type. This is complicated and
|
||
ugly, which is why there’s an <emphasis>
|
||
XkbResizeKeyType</emphasis>
|
||
function.
|
||
</para>
|
||
|
||
|
||
<para>
|
||
Many Xkb data structures have arrays whose size is reported by two fields. The
|
||
first field, whose name is usually prefixed by <emphasis>
|
||
sz_</emphasis>
|
||
, represents the total number of elements that can be stored in the array. The
|
||
second field, whose name is usually prefixed by <emphasis>
|
||
num_</emphasis>
|
||
, specifies the number of elements currently stored there. These arrays
|
||
typically represent data whose total size cannot always be determined when the
|
||
array is created. In these instances, the usual way to allocate space and add
|
||
data is as follows:
|
||
</para>
|
||
|
||
<itemizedlist>
|
||
<listitem>
|
||
<para>
|
||
Call the allocator function with some arbitrary size, as a hint.
|
||
</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>
|
||
For those arrays that have an <emphasis>
|
||
Xkb...Add...</emphasis>
|
||
function, call it each time you want to add new data to the array. The
|
||
function expands the array if necessary.
|
||
</para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
|
||
<para>
|
||
For example, call:
|
||
</para>
|
||
|
||
<para>
|
||
XkbAllocGeomShapes(geom,4)
|
||
</para>
|
||
|
||
<para>
|
||
to say "I’ll need space for four new shapes in this geometry." This makes
|
||
sure that <emphasis>
|
||
sz_shapes</emphasis>
|
||
- <emphasis>
|
||
num_shapes</emphasis>
|
||
>= 4, and resizes the shapes array if it isn’t. If this function
|
||
succeeds, you are guaranteed to have space for the number of shapes you need.
|
||
</para>
|
||
|
||
|
||
<para>
|
||
When you call an editing function for a structure, you do not need to check for
|
||
space, because the function automatically checks the <emphasis>
|
||
sz_</emphasis>
|
||
and <emphasis>
|
||
num_</emphasis>
|
||
fields of the array, resizes the array if necessary, adds the entry to the
|
||
array, and then updates the <emphasis>
|
||
num_</emphasis>
|
||
field.
|
||
</para>
|
||
|
||
|
||
</sect1>
|
||
<sect1 id='Making_Changes_to_the_Servers_Keyboard_Description'>
|
||
<title>Making Changes to the Server’s Keyboard Description</title>
|
||
|
||
<para>
|
||
In Xkb, as in the core protocol, the client and server have independent copies
|
||
of the data structures that describe the keyboard. The recommended way to
|
||
change some aspect of the keyboard mapping in the X server is to edit a local
|
||
copy of the Xkb keyboard description and then send only the changes to the X
|
||
server. This method helps eliminate the need to transfer the entire keyboard
|
||
description or even an entire data structure for only minor changes.
|
||
</para>
|
||
|
||
|
||
<para>
|
||
To help you keep track of the changes you make to a local copy of the keyboard
|
||
description, Xkb provides separate special <emphasis>
|
||
changes</emphasis>
|
||
data structures for each major Xkb data structure. These data structures do
|
||
not contain the actual changed values: they only indicate the changes that have
|
||
been made to the structures that actually describe the keyboard.
|
||
</para>
|
||
|
||
|
||
<para>
|
||
When you wish to change the keyboard description in the server, you first
|
||
modify a local copy of the keyboard description and then flag the modifications
|
||
in an appropriate changes data structure. When you finish editing the local
|
||
copy of the keyboard description, you pass your modified version of the
|
||
keyboard description and the modified changes data structure to an Xkb
|
||
function. This function uses the modified keyboard description and changes
|
||
structure to pass only the changed information to the server. Note that
|
||
modifying the keyboard description but not setting the appropriate flags in the
|
||
changes data structure causes indeterminate behavior.
|
||
</para>
|
||
|
||
|
||
</sect1>
|
||
<sect1 id='Tracking_Keyboard_Changes_in_the_Server'>
|
||
<title>Tracking Keyboard Changes in the Server</title>
|
||
|
||
<para>
|
||
The server reports all changes in its keyboard description to any interested
|
||
clients via special Xkb events. Just as clients use special changes data
|
||
structures to change the keyboard description in the server, the server uses
|
||
special changes data structures to tell a client what changed in the server’s
|
||
keyboard description.
|
||
</para>
|
||
|
||
|
||
<para>
|
||
Unlike clients, however, the server does not always pass the new values when it
|
||
reports changes to its copy of the keyboard description. Instead, the server
|
||
only passes a changes data structure when it reports changes to its keyboard
|
||
description. This is done for efficiency reasons — some clients do not always
|
||
need to update their copy of the keyboard description with every report from
|
||
the server.
|
||
</para>
|
||
|
||
|
||
<para>
|
||
When your client application receives a report from the server indicating the
|
||
keyboard description has changed, you can determine the set of changes by
|
||
passing the event to an Xkb function that "notes" event information in the
|
||
corresponding changes data structure. These "note changes" functions are
|
||
defined for all major Xkb components, and their names have the form <emphasis>
|
||
XkbNote{Component}Changes</emphasis>
|
||
, where <emphasis>
|
||
Component</emphasis>
|
||
is the name of a major Xkb component such as <emphasis>
|
||
Map</emphasis>
|
||
or <emphasis>
|
||
Names</emphasis>
|
||
. When you want to copy these changes from the server into a local copy of the
|
||
keyboard description, use the corresponding <emphasis>
|
||
XkbGet{Component}Changes</emphasis>
|
||
function<emphasis>
|
||
, </emphasis>
|
||
passing it the changes structure. The function then retrieves only the changed
|
||
structures from the server and copies the modified pieces into the local
|
||
keyboard description.
|
||
</para>
|
||
|
||
</sect1>
|
||
<sect1 id='Freeing_Data_Structures'>
|
||
<title>Freeing Data Structures</title>
|
||
|
||
<para>
|
||
For the same reasons you should not directly use <emphasis>
|
||
malloc</emphasis>
|
||
to allocate Xkb data structures, you should not free Xkb data structures or
|
||
components directly using <emphasis>
|
||
free</emphasis>
|
||
or <emphasis>
|
||
Xfree</emphasis>
|
||
. Xkb provides functions to free the various data structures and their
|
||
components. Always use the free functions supplied by Xkb. There is no
|
||
guarantee that any particular field can be safely freed by <emphasis>
|
||
free</emphasis>
|
||
or <emphasis>
|
||
Xfree</emphasis>
|
||
.
|
||
</para>
|
||
</sect1>
|
||
</chapter>
|