Table of Contents DOS

4.9 X Window System

The X Window System (hereafter referred to as X) is one of the most widely used Graphical User Interface (GUI), or bitmapped-window display systems. It is supported by all major workstation vendors, and is used by a large and growing number of users worldwide. The X Window System offers more than just a raw environment. It also offers a platform for uniquely incorporated commercial packages. In addition to writing application software, some industry groups have created proprietary software packages and standards for interfaces which leverage the display capabilities of the X Window System. These packages are then integrated into applications to improve the look and feel of them. The two most significant commercial packages in this area are the Open Software Foundation's MOTIF and UNIX International's Open Look. X was the brainchild of Robert Scheifler, Jim Gettys, and others at MIT, as part of Project Athena, a research project devoted to the examination of very large networks of personal computers and workstations. (For an overview of the Project Athena, please refer to Project Athena: Supporting Distributed Computing at MIT). As part of this study, a unifying window system environment extending over all systems was deemed necessary. X was envisioned as this window system, one that could be used among the varied heterogeneous computers and networks.

As Project Athena progressed, X evolved into a portable network-based window system. Much of the early work on X was derived from an extant Stanford window system called W. In fact the name X was simply a play on the previous name W. The MIT X Consortium, founded in 1988, is dedicated to the advancement of the X Window System and to the promotion of cooperation within the computer industry in standardizing the X Window System interfaces.

Current X releases contain two numbers: the version number indicating major protocol or standards revisions, and a release number indicating minor changes. At the time of writing, the latest version is X11 Release 6, also known as X11R6. The latest release of OSF/MOTIF is V1.2 (based on X11R5). Major revisions of X are incompatible, but there is backward compatibility with minor releases within major revision categories.

The aim of X was to allow the user to control all sessions from one screen, with applications either running in a window, or in separate virtual terminals but with an icon on the primary screen reminding him of the existence of that application (the same function as OS/2 Presentation Manager).

The X Window System provides the capability of managing both local and remote windows. Remote windows are established through TCP/IP, and local windows through the use of BSD sockets.

4.9.1 Functional Concept

Basically there are two parts communicating with each other:

1. The application, which gets input from the user, executes code and sends output back to the user. Instead of reading and writing directly to a display, the application uses the Xlib programming interface to send and receive data to/from the user's terminal. The application part is also called the X client.
2. The user's terminal, running a display-managing software which receives/sends data from/to the application and is called the X server.

Terminology:

• X Server: This is a dedicated program that provides display services on a graphic terminal, on behalf of a user, at the request of the user's X client program. It controls the screen and handles the keyboard and the mouse (or other input devices) for one or more X clients. Equally, it is responsible for output to the display, the mapping of colors, the loading of fonts and the keyboard mapping. Typically X server programs run on high performance graphics PCs and workstations, as well as "X terminals", which are designed to run only the X server program.

  The X11R5 X server has provided some speed improvements and new font interfaces. An X Font Service Protocol is available to allow the X servers to delegate the task of managing fonts to a font server.

• X Client: This is the actual application and is designed to employ a graphical user interface to display its output. Typically, many X clients compete for the service of one X server per display per user. Conflict for services are resolved by the X Window Manager, a separate entity altogether. Xterm and Xclock are two examples of X clients.

  X11R5 added some more clients, new demos and a completely new implementation of bitmap and xmag.

• X Window Manager: This is an X client program located on the workstation where the X server runs. While windows can be created without a window manager in place, a window manager permits windows to be resized, moved, and otherwise modified on demand.
• X Protocol: This runs within the network connection, and allows requests-responses between client and server. It uses a reliable byte stream connection (that is TCP) and describes the format of messages exchanged between client and server over this connection.
• Xlib: The rudimentary application programming interface is contained in the Xlib. It is a collection of C primitive subroutines embedded in all X clients, which gives the lowest level access to the X protocol. The procedures in Xlib translate client requests to X protocol requests, parse incoming messages (events, replies, and errors) from the X server, and provide several additional utilities, such as storage management and operating system independent operations. It is possible to write application programs entirely with Xlib. In fact, most existing X clients are or were developed in this fashion.

  X11R5 added two new major pieces of functionality to Xlib:

  • Device-independent color
  • Internationalization (i18n): this means that X client application programs can adapt to the requirements of different native languages, local customs, and character string encodings.
• X Toolkits: The complexity of the low-level Xlib interface and of the underlying X Protocol is handled by an increasing variety of available X Toolkits. The X Toolkits are software libraries that provide high-level facilities for implementing common user-interface objects such as buttons, menus, and scrollbars, as well as layout tools for organizing these objects on the display. The basis for a family of toolkits is provided with the standard X releases from MIT. The library, called the X Intrinsics or Xt, forms the building blocks for sets of user interface objects called widgets.
• Widgets: For toolkits based on the X Intrinsics, a common interface mechanism called a widget is used. A widget is essentially an X window plus some additional data and a set of procedures for operating on that data. Widgets are a client-side notion only. Neither the X server nor the X protocol understand widgets. A sample widget set, Xaw, more commonly referred to as the Athena Widget Set, is distributed by MIT with the X11 source.

Functionality:

• X client and X server can be on different hosts. Then they use the TCP/IP protocol to communicate over the network. They can also be on the same machine, using IPC (inter-process communication) to communicate (through sockets).
• There is only one X server per terminal. Multiple X client applications can communicate with this one X server. The duty of the X server is to display the application windows and to send the user input to the appropriate X client application.
• It is up to the X client to maintain the windows that it created. It is notified by events from the X server whenever something is changed on the display by other clients. However, they don't have to care about which part of their windows are visible or not when they are drawing or redrawing their windows.
• The X server keeps track of the visibility of the windows, by maintaining stacks. A stack contains all "first generation" children of a parent window. A child window can also be a parent window by having one or more child windows itself, which are again held in a substack. The primary stack is the stack which holds all the windows located directly below the root. See Figure - X Window System Window Structure for an illustration. Subwindows can only be fully visible when their parent is on the top of its respective stack and mapped to the display.
• X server itself has no management functions; it only performs window clipping according to its stacks. Every client is responsible for its own windows. There is a Window Manager which manipulates the top level windows of all the clients. The Window Manager is not part of the X server but is itself a client. As soon as the Window Manager changes something on the screen (for instance resizing a window), it makes the X server send out an exposure event to all the other clients.
• The client applications send request messages to the X server, which replies with a reply message or an error message. The X server can also send event messages to the applications. Event messages indicate changes to the windows (and their visibility), and user input (mouse and keyboard).

Applying this client/server concept gives the following advantages:

• Applications don't have to know the hardware characteristics of the terminal.
• Applications don't have to be on the same computer as the terminal.
• Programs written to Xlib are portable.
• New terminal types can be added by providing an appropriate X server.
• The programmers do not have to deal with the communications. They just write graphic applications to Xlib, regardless of whether the users will be remote or local.

4.9.2 Protocol

An X Window System protocol can be implemented on top of any reliable byte stream transport mechanism. It uses a simple block protocol on top of the stream layer. Four kinds of messages are used:

• Request format. Requests flow from the client application to the X server.
  
  
  
  
  

  Where:

  Major and minor are opcodes, each 1-byte long.
  Length is 2-bytes long.
  Data may be zero or more bytes, depending on the request.
• Reply format: 32-byte block.
• Error format: 32-byte block.
• Event format: 32-byte block.

Reply, error and event messages are sent by the X server to the X client applications.

Displays are always numbered from zero. For TCP connections, display number N is associated with port 5800+N (hex 5800) and port 5900+N. The X server treats connections on the 58xx ports as connections with hosts which use the "low-order byte first" format, and the 59xx ports as "high-order byte first".

There are more than a hundred different possible requests, each corresponding to an Xlib application call. As this document is not a programmer's guide, we will not deal with the Xlib functions. RFC 1013 - X Window System Protocol, Version 11 contains the 1987 alpha update of the X11 protocol. For documentation on the current release X11R6, please contact either MIT or a commercial computer books publisher. For documentation on IBM X Windows implementations, please see the appropriate section in Bibliography.

4.9.3 Implementations

4.9.3.1 VM

The TCP/IP for VM API is from the X Window System Version 11, Release 4 and includes the following components:

• X11LIB TXTLIB (Xlib, Xmu, Xext, and Xau routines).
• OLDXLIB TXTLIB (X Release 10 compatibilty routines).
• XTLIB TXTLIB (Xt Intrinsics).
• XAWLIB TXTLIB (Athena Widget set).
• Header files (H files) needed for compiling X clients.
• Standard MIT X clients.
• Sample X clients (Xlib sample program, Athena Widget set sample program, OSF/Motif-based widget sample program).

In addition, it also includes an API based on Version 1.1.2 of the OSF/Motif-based widget set which has the following components:

• XMLIB TXTLIB (OSF/Motif-based widget set)
• Header files needed for compiling clients using the OSF/Motif-based widget set.

GDDMXD is an interface to the Graphical Data Display Manager/VM (GDDM/VM or GDDM/VMXA). This interface permits graphics display output from the IBM GDDM/VM to be displayed on workstations that support the X Window System. The interface translates the data stream, created by GDDM, to the X protocol, and transmits it by TCP/IP to the X server. If GDDMXD is installed on your system, but is not active, GDDM transmits data as if the interface were not present. The X GDDM interface should be installed by the person responsible for installing GDDM, because the shared segment of GDDM needs to be reinstalled during X GDDM interface installation. Please refer to IBM TCP/IP Version 2 Release 3 for VM: Planning and Customization for more details about the installation of the X GDDM interface, and to IBM TCP/IP Version 2 Release 3 for VM: Programmer's Reference for more programming considerations.

No X server function is provided for VM users.

4.9.3.2 MVS

The TCP/IP for MVS API is from the X Window System Version 11, Release 4 and includes the following components:

• tcpip.SEZAX11L (Xlib, Xmu, Xext, and Xau routines).
• tcpip.SEZAOLDX (X Release 10 compatibilty routines).
• tcpip.SEZAXTLB (Xt Intrinsics).
• tcpip.SEZAXAWL (Athena Widget set).
• Header files (H files) needed for compiling X clients.
• Standard MIT X clients.
• Sample X clients (Xlib sample program, Athena Widget set sample program, OSF/Motif-based widget sample program).

In addition, it also includes an API based on Version 1.1.2 of the OSF/Motif-based widget set which has the following components:

• tcpip.SEZAXMLB (OSF/Motif-based widget set)
• Header files needed for compiling clients using the OSF/Motif-based widget set.

GDDMXD is an interface that allows graphics from the IBM Graphical Data Display Manager/MVS to be displayed on workstations that support the X Window System.

The interface translates the data stream, created by GDDM, to the X protocol, and transmits it by TCP/IP to the X server. GDDMXD was implemented such that existing GDDM applications can function as X Window clients without modifications or relink.

If the GDDMXD is installed on your system and not activated, or has been made inactive, GDDM transmits data as if the interface were not present. Please refer to IBM TCP/IP Version 3 Release 1 for MVS: Customization and Administration Guide for more details about the installation of the GDDMXD interface, and to IBM TCP/IP Version 3 Release 1 for MVS: Programmer's Reference for more programming considerations.

IBM TCP/IP for MVS supports the OpenEdition/MVS with the following set of object libraries with the X Windows routines compiled with the C/370 RENT compiler option (Reentrant):

• tcpip.v3r1.SEZAROE1
• tcpip.v3r1.SEZAROE2
• tcpip.v3r1.SEZAROE3

These libraries can be used as input for linking reentrant OpenEdition/MVS X Windows applications.

No X server function is provided for MVS users.

4.9.3.3 AIX/6000 V3.2.5

Both a client and server X Window System is implemented by AIXwindows Environment/6000 Version 1.2.5.

AIXwindows Environment/6000 Version 1.2.5 provides a graphical interface to AIX/6000 V3.2.5. It is based on and compatible with the industry-accepted X Window System and the OSF/Motif 1.2.2 graphical user interface. It can also interact with other AIX and other equipment manufacturer systems implementing the X Window System and OSF/Motif interfaces. AIXwindows Environment/6000 provides a sophisticated graphical desktop (AIXwindows Desktop) that can be tailored for integrating and launching applications. AIXwindows Environment/6000 provides the facilities to execute and develop X applications, OSF/Motif applications or applications requiring Display PostScript support.

If you require 3D capability, the AIXwindows/3D feature provides the facilities for the development and execution of 3D applications using a variety of industry standard APIs. This includes hardware accelerated support for PEXlib, graPHIGS and GL 3.2 as well as a pure software implementation of OpenGL, PEXlib and graPHIGS, referred to as Softgraphics. Softgraphics allows all 3D functions to be performed by software where the graphics adapter is used simply as a frame buffer to display the image. This implementation makes it possible to run 3D applications on any 2D graphics adapter.

4.9.3.4 AIX/6000 Version 4.1.1

AIX Version 4.1.1 contains the AIXwindows 2D environment, and this function is loaded automatically at installation time upon detection of a supported graphics card. The AIXwindows 2D environment is based upon X11R5 and Motif 1.2.3.

A new graphical user interface based on the Common Desktop Environment (CDE) was added.

4.9.3.5 AIX/ESA

AIXwindows Environment/ESA Version 1 Release 2 implements the X Window System Version X11R5 and OSF/Motif Version 1.1.4 with single byte, double byte, and multibyte data streams in which characters can be encoded up to 4 bytes each. The code set support in AIX/ESA Version 2 Release 2 includes the support of ISO 8859-1, ISO 8859-7 (Greek), ISO 8859-9 (Turkish), IBM PC Code Page 850, IBM PC Code Page 932 (Shift-JIS), and IBM Japanese EUC (Extended UNIX Code).

AIXwindows Environment/ESA provides the X Window System client functions.

4.9.3.6 OS/2

The X server function is available in the X Window Server Kit (PMX) of TCP/IP V2.0 for OS/2 (X11R5).

The X server function uses OS/2 PM as the X Window manager and supports all the keyboard, display, and pointer functions that are supported by OS/2 PM. Using PM as the X Window manager enables OS/2 PM windowed applications and X client applications to share the same screen. As a result, X client applications cannot act as the X Window manager for the OS/2 X server.

The X client function is available in the X Window Client Kit of TCP/IP V2.0 for OS/2 (X11R5).

Motif 1.2 function is available in the OSF/Motif Kit of TCP/IP V2.0 for OS/2. The X Window Client Kit mentioned above is a prerequisite for the OSF/Motif Kit.

Table of Contents Remote Procedure Call (RPC)