xref: /xsrc/external/mit/libXt/dist/specs/CH13.xml

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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN" "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd">

<chapter id='Evolution_of_the_Intrinsics'>
<title>Evolution of the Intrinsics</title>

<para>
The interfaces described by this specification have undergone several
sets of revisions in the course of adoption as an X Consortium
standard specification.  Having now been adopted by the Consortium as
a standard part of the X Window System, it is expected that this and
future revisions will retain
backward compatibility in the sense that fully conforming
implementations of these specifications may be produced that provide
source compatibility with widgets and applications written to
previous Consortium standard revisions.
</para>

<para>
The Intrinsics do not place any special requirement on widget
programmers to retain source or binary compatibility for their widgets
as they evolve, but several conventions have been established to
assist those developers who want to provide such compatibility.
</para>

<para>
In particular, widget programmers may wish to conform to the convention
described in <xref linkend='Class_Extension_Records' /> when defining class extension records.
</para>
<sect1 id="Determining_Specification_Revision_Level">
<title>Determining Specification Revision Level</title>
<para>
Widget and application developers who wish to maintain a common source
pool that will build properly with implementations of the Intrinsics
at different revision levels of these specifications but that take
advantage of newer features added in later revisions may use the
symbolic macro
<function>XtSpecificationRelease</function>.
</para>
<programlisting>
#define XtSpecificationRelease 7
</programlisting>
<para>
As the symbol
<function>XtSpecificationRelease</function>
was new to Release 4, widgets and
applications desiring to build against earlier implementations should
test for the presence of this symbol and assume only Release 3
interfaces if the definition is not present.
</para>
</sect1>

<sect1 id="Release_to_Release_Compatibility">
<title>Release 3 to Release 4 Compatibility</title>
<para>
At the data structure level, Release 4 retains binary compatibility
with Release 3 (the first X Consortium standard release) for all data
structures except
<function>WMShellPart,</function>
<function>TopLevelShellPart</function>,
and
<function>TransientShellPart</function>.
Release 4 changed the argument type to most procedures that now take
arguments of type
<function>XtPointer</function>
and structure members that are now of type
<function>XtPointer</function>
in order to avoid potential ANSI C conformance problems.  It is
expected that most implementations will be binary compatible with the
previous definition.
</para>

<para>
Two fields in
<function>CoreClassPart</function>
were changed from
<function>Boolean</function>
to
<function>XtEnum</function>
to allow implementations additional freedom in specifying the
representations of each.  This change should require no source
modification.
</para>
<sect2 id="Additional_Arguments">
<title>Additional Arguments</title>
<para>
Arguments were added to the procedure definitions for
<xref linkend='XtInitProc' xrefstyle='select: title'/>,
<xref linkend='XtSetValuesFunc' xrefstyle='select: title'/>,
and
<xref linkend='XtEventHandler' xrefstyle='select: title'/>
to provide more information and to
allow event handlers to abort further dispatching of the current event
(caution is advised!).  The added arguments to
<xref linkend='XtInitProc' xrefstyle='select: title'/>
and
<xref linkend='XtSetValuesFunc' xrefstyle='select: title'/>
make the initialize_hook and set_values_hook methods
obsolete, but the hooks have been retained for those widgets that used
them in Release 3.
</para>
</sect2>

<sect2 id="set_values_almost_Procedures">
<title>set_values_almost Procedures</title>
<para>
The use of the arguments by a set_values_almost procedure was poorly
described in Release 3 and was inconsistent with other conventions.
</para>

<para>
The current specification for the manner in which a set_values_almost
procedure returns information to the Intrinsics is not compatible with
the Release 3 specification, and all widget implementations should
verify that any set_values_almost procedures conform to the current
interface.
</para>

<para>
No known implementation of the Intrinsics correctly implemented the
Release 3 interface, so it is expected that the impact of this
specification change is small.
</para>
</sect2>

<sect2 id="Query_Geometry">
<title>Query Geometry</title>
<para>
A composite widget layout routine that calls
<xref linkend='XtQueryGeometry' xrefstyle='select: title'/>
is now expected to store the complete new geometry in the intended structure;
previously the specification said &ldquo;store the changes it intends to
make&rdquo;.  Only by storing the complete geometry does the child have
any way to know what other parts of the geometry may still be
flexible.  Existing widgets should not be affected by this, except
to take advantage of the new information.
</para>
</sect2>

<sect2 id="unrealizeCallback_Callback_List">
<title>unrealizeCallback Callback List</title>
<para>
In order to provide a mechanism for widgets to be notified when they
become unrealized through a call to
<xref linkend='XtUnrealizeWidget' xrefstyle='select: title'/>,
the callback
list name &ldquo;unrealizeCallback&rdquo; has been defined by the Intrinsics.  A
widget class that requires notification on unrealize may declare a
callback list resource by this name.  No class is required to declare
this resource, but any class that did so in a prior revision may find
it necessary to modify the resource name if it does not wish to use the new
semantics.
</para>
</sect2>

<sect2 id="Subclasses_of_WMShell">
<title>Subclasses of WMShell</title>
<para>
The formal adoption of the <emphasis remap='I'>Inter-Client Communication Conventions Manual</emphasis> as
an X Consortium standard has meant the addition of four fields to
<function>WMShellPart</function>
and one field to
<function>TopLevelShellPart</function>.
In deference to some
widget libraries that had developed their own additional conventions
to provide binary compatibility, these five new fields were added at
the end of the respective data structures.
</para>

<para>
To provide more convenience for TransientShells, a field was added
to the previously empty
<function>TransientShellPart</function>.
On some architectures the size of the part structure will not
have changed as a result of this.
</para>

<para>
Any widget implementation whose class is a subclass of
TopLevelShell
or
TransientShell
must at minimum be
recompiled with the new data structure declarations.  Because
<function>WMShellPart</function>
no longer contains a contiguous
<function>XSizeHints</function>
data structure,
a subclass that expected to do a single structure assignment of an
<function>XSizeHints</function>
structure to the <emphasis remap='I'>size_hints</emphasis> field of
<function>WMShellPart</function>
must be revised, though the old fields remain at the same positions within
<function>WMShellPart</function>.
</para>
</sect2>

<sect2 id="Resource_Type_Converters">
<title>Resource Type Converters</title>
<para>
A new interface declaration for resource type converters was defined
to provide more information to converters, to support conversion
cache cleanup with resource reference counting, and to allow
additional procedures to be declared to free resources.  The old
interfaces remain (in the compatibility section), and a new set of
procedures was defined that work only with the new type converter
interface.
</para>

<para>
In the now obsolete old type converter interface, converters are
reminded that they must return the size of the converted value as well
as its address.  The example indicated this, but the description of
<xref linkend='XtConverter' xrefstyle='select: title'/>
was incomplete.
</para>
</sect2>

<sect2 id="KeySym_Case_Conversion_Procedure">
<title>KeySym Case Conversion Procedure</title>
<para>
The specification for the
<xref linkend='XtCaseProc' xrefstyle='select: title'/>
function type has been changed
to match the Release 3 implementation, which included necessary
additional information required by the function (a pointer to the
display connection), and corrects the argument type of the source
KeySym parameter.  No known implementation of the Intrinsics
implemented the previously documented interface.
</para>
</sect2>

<sect2 id="Nonwidget_Objects_2">
<title>Nonwidget Objects</title>
<para>
Formal support for nonwidget objects is new to Release 4.  A
prototype implementation was latent in at least one Release 3
implementation of the Intrinsics, but the specification has changed
somewhat.  The most significant change is the requirement for a
composite widget to declare the
<function>CompositeClassExtension</function>
record with the <emphasis remap='I'>accepts_objects</emphasis> field set to
<function>True</function>
in order to permit a client to create a nonwidget child.
</para>

<para>
The addition of this extension field ensures that composite widgets
written under Release 3 will not encounter unexpected errors if an
application attempts to create a nonwidget child.  In Release 4 there
is no requirement that all composite widgets implement the extra
functionality required to manage windowless children, so the
<emphasis remap='I'>accepts_objects</emphasis> field allows a composite widget to declare that it
is not prepared to do so.
</para>
</sect2>
</sect1>

<sect1 id="Release_to_Release_Compatibility_2">
<title>Release 4 to Release 5 Compatibility</title>
<para>
At the data structure level, Release 5 retains complete binary
compatibility with Release 4.  The specification of the
<function>ObjectPart</function>,
<function>RectObjPart</function>,
<function>CorePart</function>,
<function>CompositePart</function>,
<function>ShellPart</function>,
<function>WMShellPart</function>,
<function>TopLevelShellPart</function>,
and
<function>ApplicationShellPart</function>
instance records was made less strict to permit implementations to
add internal fields to these structures.  Any implementation that
chooses to do so would, of course, force a recompilation.
The Xlib specification for
<function>XrmValue</function>
and
<function>XrmOptionDescRec</function>
was updated to use a new type,
<function>XPointer</function>,
for the <emphasis remap='I'>addr</emphasis> and <emphasis remap='I'>value</emphasis> fields, respectively, to avoid
ANSI C conformance problems.  The definition of
<function>XPointer</function>
is binary compatible with the previous implementation.
</para>
<sect2 id="baseTranslations_Resource">
<title>baseTranslations Resource</title>
<para>
A new pseudo-resource, XtNbaseTranslations, was defined to permit
application developers to specify translation tables in application
defaults files while still giving end users the ability to augment
or override individual event sequences.  This change will affect
only those applications that wish to take advantage of the new
functionality or those widgets that may have previously defined
a resource named &ldquo;baseTranslations&rdquo;.
</para>

<para>
Applications wishing to take advantage of the new functionality
would change their application defaults file, e.g., from
<programlisting>
        app.widget.translations:         <emphasis remap='I'>value</emphasis>
to
        app.widget.baseTranslations:     <emphasis remap='I'>value</emphasis>
</programlisting>
If it is important to the application to preserve complete
compatibility of the defaults file between different versions
of the application running under Release 4 and Release 5,
the full translations can be replicated in both the &ldquo;translations&rdquo;
and the &ldquo;baseTranslations&rdquo; resource.
</para>
</sect2>

<sect2 id="Resource_File_Search_Path">
<title>Resource File Search Path</title>
<para>
The current specification allows implementations greater flexibility
in defining the directory structure used to hold the application class
and per-user application defaults files.  Previous specifications
required the substitution strings to appear in the default path in a
certain order, preventing sites from collecting all the files for
a specific application together in one directory.  The Release 5
specification allows the default path to specify the substitution
strings in any order within a single path entry.  Users will need to
pay close attention to the documentation for the specific
implementation to know where to find these files and how to specify
their own
<emphasis role='strong'>XFILESEARCHPATH</emphasis>
and
<emphasis role='strong'>XUSERFILESEARCHPATH</emphasis>
values when overriding the system defaults.
</para>
</sect2>

<sect2 id="Customization_Resource">
<title>Customization Resource</title>
<para>
<xref linkend='XtResolvePathname' xrefstyle='select: title'/>
supports a new substitution string, %C, for specifying separate
application class resource files according to arbitrary user-specified
categories.  The primary motivation for this addition was separate
monochrome and color application class defaults files.  The
substitution value is obtained by querying the current resource
database for the application resource name &ldquo;customization&rdquo;, class
&ldquo;Customization&rdquo;.  Any application that previously used this
resource name and class will need to be aware of the possibly
conflicting semantics.
</para>
</sect2>

<sect2 id="Per_Screen_Resource_Database">
<title>Per-Screen Resource Database</title>
<para>
To allow a user to specify separate preferences for each screen of a
display, a per-screen resource specification string has been added and
multiple resource databases are created; one for each screen.  This
will affect any application that modified the (formerly unique)
resource database associated with the display subsequent to the Intrinsics
database initialization.  Such applications will need to be aware
of the particular screen on which each shell widget is to be created.
</para>

<para>
Although the wording of the specification changed substantially in the
description of the process by which the resource database(s) is
initialized, the net effect is the same as in prior releases with the
exception of the added per-screen resource specification and the new
customization substitution string in
<xref linkend='XtResolvePathname' xrefstyle='select: title'/>.
</para>
</sect2>

<sect2 id="Internationalization_of_Applications">
<title>Internationalization of Applications</title>
<para>
Internationalization as defined by ANSI is a technology that
allows support of an application in a single locale.  In
adding support for internationalization to the Intrinsics
the restrictions of this model have been followed.
In particular, the new Intrinsics interfaces are designed not to
preclude an application from using other alternatives.
For this reason, no Intrinsics routine makes a call to establish the
locale.   However, a convenience routine to establish the
locale at initialize time has been provided, in the form
of a default procedure that must be explicitly installed
if the application desires ANSI C locale behavior.
</para>

<para>
As many objects in X, particularly resource databases, now inherit
the global locale when they are created, applications wishing to use
the ANSI C locale model should use the new function
<function>XtSetLanguageProc</function>
to do so.
</para>

<para>
The internationalization additions also define event filters
as a part of the Xlib Input Method specifications.  The
Intrinsics enable the use of event filters through additions to
<xref linkend='XtDispatchEvent' xrefstyle='select: title'/>.
Applications that may not be dispatching all events through
<xref linkend='XtDispatchEvent' xrefstyle='select: title'/>
should be reviewed in the context of this new input method mechanism.
</para>

<para>
In order to permit internationalization of error messages, the name
and path of the error database file are now allowed to be
implementation-dependent.
No adequate standard mechanism has yet been suggested to
allow the Intrinsics to locate the database from localization information
supplied by the client.
</para>

<para>
The previous specification for the syntax of the language string
specified by
<function>xnlLanguage</function>
has been dropped to avoid potential conflicts with other standards.
The language string syntax is now implementation-defined.
The example syntax cited is consistent with the previous
specification.
</para>
</sect2>

<sect2 id="Permanently_Allocated_Strings">
<title>Permanently Allocated Strings</title>
<para>
In order to permit additional memory savings, an Xlib interface was
added to allow the resource manager to avoid copying certain string
constants.  The Intrinsics specification was updated to explicitly require
the Object <emphasis remap='I'>class_name</emphasis>, <emphasis remap='I'>resource_name</emphasis>, <emphasis remap='I'>resource_class</emphasis>,
<emphasis remap='I'>resource_type</emphasis>, <emphasis remap='I'>default_type</emphasis> in resource tables, Core <emphasis remap='I'>actions</emphasis>
<emphasis remap='I'>string</emphasis> field, and Constraint <emphasis remap='I'>resource_name</emphasis>, <emphasis remap='I'>resource_class</emphasis>,
<emphasis remap='I'>resource_type</emphasis>, and <emphasis remap='I'>default_type</emphasis> resource fields to be
permanently allocated.  This explicit requirement is expected to
affect only applications that may create and destroy classes
on the fly.
</para>
</sect2>

<sect2 id="Arguments_to_Existing_Functions">
<title>Arguments to Existing Functions</title>
<para>
The <emphasis remap='I'>args</emphasis> argument to
<xref linkend='XtAppInitialize' xrefstyle='select: title'/>,
<xref linkend='XtVaAppInitialize' xrefstyle='select: title'/>,
<xref linkend='XtOpenDisplay' xrefstyle='select: title'/>,
<xref linkend='XtDisplayInitialize' xrefstyle='select: title'/>,
and
<xref linkend='XtInitialize' xrefstyle='select: title'/>
were changed from
<function>Cardinal</function>*
to int* to conform to pre-existing convention and avoid otherwise
annoying typecasting in ANSI C environments.
</para>
</sect2>
</sect1>

<sect1 id="Release_to_Release_Compatibility_3">
<title>Release 5 to Release 6 Compatibility</title>
<para>
At the data structure level, Release 6 retains binary compatibility
with Release 5 for all data structures except
<function>WMShellPart</function>.
Three resources were added to the specification.
The known implementations had unused space in the data structure,
therefore on some architectures and implementations,
the size of the part structure will not have changed as a result of this.
</para>
<sect2 id="Widget_Internals">
<title>Widget Internals</title>
<para>
Two new widget methods for instance allocation and deallocation were added
to the Object class.  These new methods
allow widgets to be treated as C++ objects in the C++ environment
when an appropriate allocation method is specified or inherited
by the widget class.
</para>

<para>
The textual descriptions of the processes of widget creation and
widget destruction have been edited to provide clarification to widget
writers.  Widgets writers may have reason to rely on the specific order of
the stages of widget creation and destruction; with that motivation,
the specification now more exactly describes the process.
</para>

<para>
As a convenience, an interface to locate a widget class extension
record on a linked list,
<xref linkend='XtGetClassExtension' xrefstyle='select: title'/>,
has been added.
</para>

<para>
A new option to allow bundled changes to the managed set of a Composite
widget is introduced in the Composite class extension record.
Widgets that define a change_managed procedure that can accommodate
additions and deletions to the managed set of children in a single
invocation should set allows_change_managed_set to <function>True</function> in the
extension record.
</para>

<para>
The wording of the process followed by
<xref linkend='XtUnmanageChildren' xrefstyle='select: title'/>
has changed slightly to better handle changes to the managed set
during phase 2 destroy processing.
</para>

<para>
A new exposure event compression flag,
<function>XtExposeNoRegion</function>,
was added.  Many widgets specify exposure compression, but either
ignore the actual damage region passed to the core expose procedure or
use only the cumulative bounding box data available in the event.
Widgets with expose procedures that do not make use of exact
exposure region information can indicate that the Intrinsics need not
compute the region.
</para>
</sect2>

<sect2 id="General_Application_Development">
<title>General Application Development</title>
<para>
<xref linkend='XtOpenApplication' xrefstyle='select: title'/>
is a new convenience procedure to initialize the toolkit, create an
application context, open an X display connection, and create the
root of the widget instance tree.  It is identical to the interface
it replaces,
<xref linkend='XtAppInitialize' xrefstyle='select: title'/>,
in all respects except that it takes an additional argument specifying
the widget class of the root shell to create.
This interface is now the recommended one so that clients may easily
become session participants.
The old convenience procedures appear in the compatibility section.
</para>

<para>
The toolkit initialization function
<xref linkend='XtToolkitInitialize' xrefstyle='select: title'/>
may be called multiple times without penalty.
</para>

<para>
In order to optimize changes in geometry to a set of geometry-managed
children, a new interface,
<xref linkend='XtChangeManagedSet' xrefstyle='select: title'/>,
has been added.
</para>
</sect2>

<sect2 id="Communication_with_Window_and_Session_Managers">
<title>Communication with Window and Session Managers</title>
<para>
The revision of the <emphasis remap='I'>Inter-Client Communication Conventions Manual</emphasis> as an X Consortium standard has resulted
in the addition of three fields to the specification of
<function>WMShellPart</function>.
These are <emphasis remap='I'>urgency</emphasis>, <emphasis remap='I'>client_leader</emphasis>, and <emphasis remap='I'>window_role</emphasis>.
</para>

<para>
The adoption of the <emphasis remap='I'>X Session Management Protocol</emphasis> as an X Consortium
standard has resulted in the addition of a new shell widget,
<function>SessionShell</function>,
and an accompanying subclass verification interface,
<function>XtIsSessionShell</function>.
This widget provides support for communication between an application
and a session manager, as well as a window manager.
In order to preserve compatibility with existing subclasses of
<function>ApplicationShell</function>,
the
<function>ApplicationShell</function>
was subclassed to create the new widget class.
The session protocol requires a modal response to certain checkpointing
operations by participating applications.  The
<function>SessionShell</function>
structures
the application's notification of and responses to messages from the session
manager by use of various callback lists and by use of the new interfaces
<xref linkend='XtSessionGetToken' xrefstyle='select: title'/>
and
<xref linkend='XtSessionReturnToken' xrefstyle='select: title'/>.
There is also a new command line argument, -xtsessionID, which facilitates
the session manager in restarting applications based on the Intrinsics.
</para>

<para>
The resource name and class strings defined by the Intrinsics shell
widgets in
<filename class="headerfile">&lt;X11/Shell.h&gt;</filename>
are now listed in Appendix E.  The addition of a new symbol
for the
<function>WMShell</function>
<emphasis remap='I'>wait_for_wm</emphasis> resource was made to bring the external symbol
and the string it represents into agreement.  The actual resource name
string in
<function>WMShell</function>
has not changed.
The resource representation type of the XtNwinGravity resource of the
<function>WMShell</function>
was changed to XtRGravity in order to register a type
converter so that window gravity resource values could be specified by name.
</para>
</sect2>

<sect2 id="Geometry_Management_2">
<title>Geometry Management</title>
<para>
A clarification to the specification was made to indicate that geometry
requests may include current values along with the requested changes.
</para>
</sect2>

<sect2 id="Event_Management_2">
<title>Event Management</title>
<para>
In Release 6, support is provided for registering selectors
and event handlers for events generated by X protocol extensions
and for dispatching those events to the appropriate widget.
The new event handler registration interfaces are
<xref linkend='XtInsertEventTypeHandler' xrefstyle='select: title'/>
and
<xref linkend='XtRemoveEventTypeHandler' xrefstyle='select: title'/>.
Since the mechanism to indicate selection of extension events is specific
to the extension being used, the Intrinsics introduces
<xref linkend='XtRegisterExtensionSelector' xrefstyle='select: title'/>,
which allows the application to select for the events of interest.
In order to change the dispatching algorithm to accommodate extension events
as well as core X protocol events,
the Intrinsics event dispatcher may now be replaced or enveloped
by the application with
<xref linkend='XtSetEventDispatcher' xrefstyle='select: title'/>.
The dispatcher may wish to call
<xref linkend='XtGetKeyboardFocusWidget' xrefstyle='select: title'/>
to determine the widget with the current Intrinsics keyboard focus.
A dispatcher, after determining the destination widget, may use
<xref linkend='XtDispatchEventToWidget' xrefstyle='select: title'/>
to cause the event to be dispatched to the event handlers registered
by a specific widget.
</para>

<para>
To permit the dispatching of events
for nonwidget drawables, such as pixmaps that are not associated
with a widget's window,
<xref linkend='XtRegisterDrawable' xrefstyle='select: title'/>
and
<xref linkend='XtUnregisterDrawable' xrefstyle='select: title'/>
have been added to the library.  A related update was made to
the description of
<xref linkend='XtWindowToWidget' xrefstyle='select: title'/>.
</para>

<para>
The library is now thread-safe, allowing one thread at a time to
enter the library and protecting global data as necessary from concurrent use.
Threaded toolkit applications are supported by the
new interfaces
<xref linkend='XtToolkitThreadInitialize' xrefstyle='select: title'/>,
<xref linkend='XtAppLock' xrefstyle='select: title'/>,
<xref linkend='XtAppUnlock' xrefstyle='select: title'/>,
<xref linkend='XtAppSetExitFlag' xrefstyle='select: title'/>,
and
<xref linkend='XtAppGetExitFlag' xrefstyle='select: title'/>.
Widget writers may also use
<xref linkend='XtProcessLock' xrefstyle='select: title'/>
and
<xref linkend='XtProcessUnlock' xrefstyle='select: title'/>.
</para>

<para>
Safe handling of POSIX signals and other asynchronous notifications
is now provided by use of
<xref linkend='XtAppAddSignal' xrefstyle='select: title'/>,
<xref linkend='XtNoticeSignal' xrefstyle='select: title'/>,
and
<xref linkend='XtRemoveSignal' xrefstyle='select: title'/>.
</para>

<para>
The application can receive notification of an impending block
in the Intrinsics event manager by registering interest through
<xref linkend='XtAppAddBlockHook' xrefstyle='select: title'/>
and
<xref linkend='XtRemoveBlockHook' xrefstyle='select: title'/>.
</para>

<para>
<xref linkend='XtLastEventProcessed' xrefstyle='select: title'/>
returns the most recent event passed to
<xref linkend='XtDispatchEvent' xrefstyle='select: title'/>
for a specified display.
</para>
</sect2>

<sect2 id="Resource_Management_2">
<title>Resource Management</title>
<para>
Resource converters are registered by the Intrinsics for window gravity
and for three new resource types associated with session participation:
RestartStyle, CommandArgArray and DirectoryString.
</para>

<para>
The file search path syntax has been extended to make it easier to
include the default search path, which controls resource
database construction, by using the new substitution string, %D.
</para>
</sect2>

<sect2 id="Translation_Management_2">
<title>Translation Management</title>
<para>
The default key translator now recognizes the NumLock modifier.
If NumLock is on and the second keysym is a keypad keysym
(a standard keysym named with a &ldquo;KP&rdquo; prefix or a
vendor-specific keysym in the hexadecimal range 0x11000000 to 0x1100FFFF),
then the default key translator will
use the first keysym if Shift and/or ShiftLock is on and will
use the second keysym if neither is on.  Otherwise, it will
ignore NumLock and apply the normal protocol semantics.
</para>
</sect2>

<sect2 id="Selections">
<title>Selections</title>
<para>
The targets of selection requests may be parameterized, as described
by the revised <emphasis remap='I'>Inter-Client Communication Conventions Manual</emphasis>.
When such requests are made,
<xref linkend='XtSetSelectionParameters' xrefstyle='select: title'/>
is used by the requestor to specify the target parameters and
<xref linkend='XtGetSelectionParameters' xrefstyle='select: title'/>
is used by the selection owner to retrieve the parameters.
When a parameterized target is specified in the context of a bundled
request for multiple targets,
<xref linkend='XtCreateSelectionRequest' xrefstyle='select: title'/>,
<xref linkend='XtCancelSelectionRequest' xrefstyle='select: title'/>,
and
<xref linkend='XtSendSelectionRequest' xrefstyle='select: title'/>
are used to envelop the assembly of the request.
When the parameters themselves are the names of properties,
the Intrinsics provides support for the economical use of property atom names;
see
<xref linkend='XtReservePropertyAtom' xrefstyle='select: title'/>
and
<xref linkend='XtReleasePropertyAtom' xrefstyle='select: title'/>.
</para>
</sect2>

<sect2 id="External_Agent_Hooks">
<title>External Agent Hooks</title>
<para>
External agent hooks were added for the benefit of applications
that instrument other applications for purposes of accessibility,
testing, and customization.  The external agent and the application
communicate by a shared protocol which is transparent to the application.
The hook callbacks permit the external agent to register interest
in groups or classes of toolkit activity and to be notified of the
type and details of the activity as it occurs.  The new interfaces
related to this effort are
<xref linkend='XtHooksOfDisplay' xrefstyle='select: title'/>,
which returns the hook registration widget, and
<xref linkend='XtGetDisplays' xrefstyle='select: title'/>,
which returns a list of the X displays associated with an application context.
</para>
</sect2>
</sect1>

<sect1 id="Release_to_Release_Compatibility_4">
<title>Release 6 to Release 7 Compatibility</title>

<sect2 id="Changes_During_X11R6">
<title>Changes During X11R6</title>
<para>
The Toolkit was proposed in X10R4, released at the end of 1986.
The X11R6 documentation was completed in mid&ndash;1994.
Over most of the eleven years through X11R6.9,
only minor changes were made to the specification.
Some changes are documented only in the release notes:
</para>

<itemizedlist>
<listitem>
<para>
The X11R6.3 release notes (1997) mention one new feature (section 3.15)
<emphasis remap='I'>Xt Geometry Management Debugger</emphasis>, saying
</para>
<blockquote>
<para>
Daniel Dardailler's &ldquo;GeoTattler&rdquo; code has been merged into the Xt
Intrinsics library implementation.
This is not a standard.
If libXt is compiled with the <code>XT_GEO_TATTLER</code> symbol defined
(currently there is no build configuration support to do this)
then a &ldquo;geoTattler&rdquo; resource
may be specified for any widget in an application.
If the <code>geoTattler</code>
resource for a widget instance is <code>True</code>
then libXt will generate debugging information to
<emphasis remap='I'>stdout</emphasis> when the widget makes geometry change requests.
</para>
<para>
For example, if the resources specify:
</para>
<programlisting>
myapp*draw.XmScale.geoTattler: ON
*XmScrollBar.geoTattler:ON
*XmRowColumn.exit_button.geoTattler:ON
</programlisting>
<para>
then geometry management debugging information will be generated for all
the <code>XmScale</code> children
of the widget named <emphasis remap='I'>draw</emphasis>,
all the XmScrollBars,
and the widget named <emphasis remap='I'>exit_button</emphasis>
in any <code>XmRowColumn</code>.
</para>
</blockquote>
</listitem>
<listitem>
<para>
X11R6.4 (1998) added
<xref linkend='Resource_Configuration_Management' />.
The release notes explain that by saying
<blockquote>
<para>
The X Toolkit Intrinsics library (libXt) now has IBM's Easy Resource
Configuration support included.
</para>
</blockquote>
</para>
<para>
but goes on to say (section 14) that
<blockquote>
<para>
Easy Resource Configuration
is not a standard part of the X Toolkit Intrinsics (libXt).
It is neither an X Consortium standard nor an X Project Team specification.
</para>
</blockquote>
</para>
</listitem>
<listitem>
<para>
X11R6.5 (2000) documented a bug-fix for XtAppPeekEvent in the release notes,
stating that it now worked as described in the specification.
It also modified the description of XtAppPeekEvent in the specification.
Previously the specification stated that no known implementations behaved
as specified.
</para>
</listitem>
<listitem>
<para>
Subsequent releases X11R6.6 (2001) through X11R6.9 (2005)
did not document any new or improved features.
</para>
</listitem>
</itemizedlist>
<para>
Throughout this interval,
there were undocumented fixes and improvements made to the X Toolkit Intrinsics library.
The documentation was modified to fix minor errors,
improve the formatting, and
update version numbers.
</para>
</sect2>
<sect2 id="Changes_During_X11R7">
<title>Changes During X11R7</title>
<para>
X11R7 releases starting in 2005 continued this trend,
converting the documentation from nroff to sgml.
X11R7.7 (2012) provides the same Intrinsics specification
(aside from details of formatting and version numbers) as X11R6 (1995).
</para>
<para>
The updates for this specification are a continuation of X11R7.7,
because (as of April 2019) there are no plans for an X11R7.8 release.
</para>
</sect2>
<sect2 id="Converting_to_Standard_C">
<title>Converting to Standard C</title>
<para>
The Intrinsics specification was first released with X11R3 in 1989.
That was too early to take Standard C (i.e., ANSI C) into account.
Because vendors generally did not provide a no-cost Standard C compiler,
the X Toolkit Intrinsics library initially supported both K&amp;R and ANSI C.
The X11R5 release notes mention using gcc, with some caveats.
As a result, the specification and implementation gave equal attention
to both K&amp;R and ANSI C.
</para>
<para>
This example shows how a function prototype was used in the C header files:
</para>
<programlisting>
extern Display *XtOpenDisplay(
#if NeedFunctionPrototypes
    XtAppContext        /* app_context */,
    _Xconst _XtString   /* display_string */,
    _Xconst _XtString   /* application_name */,
    _Xconst _XtString   /* application_class */,
    XrmOptionDescRec*   /* options */,
    Cardinal            /* num_options */,
    int*                /* argc */,
    char**              /* argv */
#endif
);
</programlisting>
<para>
The parameters for the ANSI C prototype were conditionally compiled.
Used with a K&amp;R compiler, those parameters were ignored.
</para>
<itemizedlist>
<listitem>
<para>
The X Toolkit Intrinsics library used <type>const</type> in just a few cases.
The specification did not mention it at all.
</para>
<para>
Over time, that was seen as a problem,
partly because of gcc's warning options
such as <emphasis remap='I'>write-strings</emphasis>,
introduced in early 1988 (released with gcc 1.27 in late 1988).
Quoting from gcc 2.58's documentation (late 1993):
<programlisting>
`-Wwrite-strings'
     Give string constants the type `const char[LENGTH]' so that
     copying the address of one into a non-`const' `char *' pointer
     will get a warning.  These warnings will help you find at compile
     time code that can try to write into a string constant, but only
     if you have been very careful about using `const' in declarations
     and prototypes.  <emphasis remap='I'>Otherwise, it will just be a nuisance; this is
     why we did not make `-Wall' request these warnings.</emphasis>
</programlisting>
</para>
<para>
Others did not agree that it was a nuisance. Besides the obvious advantage
of improving program correctness, making a symbol &ldquo;const&rdquo;
gave the compiler and linker a hint that the symbol could be put into
the text (read-only) section, eliminating some steps needed by the linker
to adjust addresses and thereby reducing the time it took to load a
program into memory.
</para>
<para>
Other gcc warning options (such as
such as <emphasis remap='I'>cast-qual</emphasis>)
are useful for improving programs.
They give similar information, because unless told otherwise,
gcc would treat string values as nonwritable.
Quoting from gcc 1.27:
<programlisting>
   * GNU CC normally makes string constants read-only.  If several
     identical-looking string constants are used, GNU CC stores only
     one copy of the string.
     ...
     The best solution to these problems is to change the program to
     use `char'-array variables with initialization strings for these
     purposes instead of string constants.  But if this is not
     possible, you can use the `-fwritable-strings' flag, which
     directs GNU CC to handle string constants the same way most C
     compilers do.
</programlisting>
and
<programlisting>
    `-fwritable-strings'
          Store string constants in the writable data segment and
          don't uniquize them.  This is for compatibility with old
          programs which assume they can write into string constants.
          Writing into string constants is a very bad idea;
          ``constants'' should be constant.
</programlisting>
</para>
</listitem>
<listitem>
<para>
Several prototypes in the implementation
use the private type <type>_XtString</type>.
The specification and implementation also used a <type>String</type>
type without explaining when it is appropriate.
<programlisting>
typedef char *String;

/* We do this in order to get "const" declarations to work right.  We
 * use _XtString instead of String so that C++ applications can
 * #define String to something else if they choose, to avoid conflicts
 * with other C++ libraries.
 */
#define _XtString char*
</programlisting>
That is, the developers were providing for some workaround to allow
C++ applications to use the stricter compiler checking
associated with <type>const</type>.
</para>
</listitem>
<listitem>
<para>
The <type>String</type> type is not the only type used in the
prototypes for the X Toolkit Intrinsics library.
Its developers were also concerned with porting the library
to platforms with different size-constraints.
They defined different types (used in the function prototypes)
depending on whether a &ldquo;wide&rdquo; parameter type was appropriate: 
<programlisting>
/* _Xt names are private to Xt implementation, do not use in client code */
#if NeedWidePrototypes
#define _XtBoolean	int
#define _XtDimension	unsigned int
#define _XtKeyCode	unsigned int
#define _XtPosition	int
#define _XtXtEnum	unsigned int
#else
#define _XtBoolean	Boolean
#define _XtDimension	Dimension
#define _XtKeyCode	KeyCode
#define _XtPosition	Position
#define _XtXtEnum	XtEnum
#endif /* NeedWidePrototypes */
</programlisting>
and
<programlisting>
#ifdef CRAY
typedef long		Boolean;
typedef char*		XtArgVal;
typedef long		XtEnum;
#else
typedef char		Boolean;
typedef long		XtArgVal;
typedef unsigned char	XtEnum;
#endif
</programlisting>
In practice, wide-prototypes are rarely used, not well supported.
The specification did not clarify the distinction
between <type>Bool</type> (mentioned as a resource type)
and <type>Boolean</type> (used in all of the data structures).
The implementation used both, predominantly the latter.
</para>
</listitem>
</itemizedlist>
<para>
Other features of Standard C were neglected in the specification because
it was accommodating K&amp;R C:
</para>
<itemizedlist>
<listitem>
<para>
K&amp;R C has no <type>void</type> keyword.
The specification used it for return-types,
but not to indicate an empty parameter list.
The specification also stated that
<type>void*</type> would be used for the <type>XtPointer</type> type.
</para>
<para>
The conversion to sgml lost the <type>void</type> return-type.
</para>
</listitem>
<listitem>
<para>
Standard C uses an ellipsis for variable-length argument lists, e.g., for
<xref linkend='XtVaAppCreateShell' />.
Again, there was a conditional-compilation symbol
(<code>NeedVarargsPrototypes</code>)
to handle the different forms used.
Here is an example:
<programlisting>
#if NeedVarargsPrototypes
void
XtVaGetApplicationResources(Widget widget, XtPointer base, XtResourceList resources, Cardinal num_resources, ...)
#else
/*VARARGS4*/
void XtVaGetApplicationResources(widget, base, resources, num_resources, va_alist)
    Widget widget;
    XtPointer base;
    XtResourceList resources;
    Cardinal num_resources;
    va_dcl
#endif
</programlisting>
</para>
<para>
One problem with the conditional-compilation was
that it was easy to make a mistake with the order
of parameters between the two forms.
Developers would frequently group together parameters
which used the same type, whether or not they were adjacent in
the Standard C prototype.
</para>
<para>
A comment in the X11R4 header file said that this was temporary,
until function prototypes worked everywhere.
That was finally removed in X11R6.7 (fourteen years later).
However, the subsequent conversion to sgml
lost the ellipsis from the prototypes shown in the specification.
</para>
</listitem>
</itemizedlist>
<para>
Support for K&amp;R C was removed from the header files in 2003
(released in X11R6.7),
and from the library source in 2004
(released in X11R6.9).
The wide-prototype feature is still present in 2019, but generally unused.
</para>
<para>
Removing support for K&amp;R C did not address the issues of <type>const</type>.
That was done in 2019:
</para>
<itemizedlist>
<listitem>
<para>
The <type>String</type> is conditionally defined,
to provide compatibility with existing applications.
</para>
</listitem>
<listitem>
<para>
If the symbol <symbol>_CONST_X_STRING</symbol> is defined,
<type>String</type> is read-only as shown here.
<programlisting>
/*
 * As used in its function interface, the String type of libXt can be readonly.
 * But compiling libXt with this feature may require some internal changes,
 * e.g., casts and occasionally using a plain "char*".
 */
#ifdef _CONST_X_STRING
typedef const char *String;
#else
typedef char *String;
#endif
</programlisting>
</para>
</listitem>
<listitem>
<para>
Applications which use the newer <type>const</type> feature must define
<symbol>_CONST_X_STRING</symbol> to enable this feature.
</para>
</listitem>
<listitem>
<para>
By default, the X Toolkit Intrinsics library
uses the <type>const</type> feature.
It has been updated to make use of the <type>const</type> feature
for improved type-checking.
</para>
</listitem>
<listitem>
<para>
Because the X Toolkit Intrinsics library uses <type>const</type>,
some prototypes have been modified.
For example:
<itemizedlist>
<listitem>
<para>
Most of the parameters which used <type>String</type> are unmodified; 
a few (such as the <emphasis remap='I'>argv</emphasis>&ndash;parameters)
are actually read/write.
They are now <type>char*</type> parameters.
</para>
<para>
Many of the strings passed to the library are stored in widgets
without reallocating a copy.
Those are treated as read-only, and use the <type>String</type> type.
</para>
</listitem>
<listitem>
<para>
Each change to the documentation was verified using scripts that
extracted the function prototypes and used the C compiler to check
for compatibility.
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
</itemizedlist>
</sect2>
</sect1>
</chapter>