The X Window System is a network transparent protocol that supports windowing and graphics. The protocol is communicated between a client or application and an X server over a reliable bidirectional byte stream. This byte stream is provided by the TCP/IP communication protocol. In the MVS™ environment, X Window System support consists of a set of application calls that create the X protocol, as requested by the application. This application programming interface allows an application to be created, which uses the X Window System protocol to be displayed on an X server.

In an X Window System environment, the X server distributes user input to and accepts requests from various client programs located either on the same system or elsewhere on a network. The X client code uses sockets to communicate with the X server.

Figure 1 shows a high-level abstraction of how the X Window System works in an MVS environment. As an application writer, you need to be concerned only with the client API in writing your application.

The communication path from the MVS X Window System application to the server involves the client code and TCP/IP. The application program that you create is the client part of a client-server relationship. The X server provides access to the resources that are shared among many X applications, such as the screen, keyboard, mouse, fonts, and graphics contexts. A single X server can control more than one physical screen.

Each client can interact with multiple servers, and each server can interact with multiple clients.

If your application is written to the Xlib interface, it calls XOpenDisplay() to start communication with an X server on a workstation. The Xlib code opens a communication path called a socket to the X server, and sends the appropriate X protocol to initiate client-server communication.

The X protocol generated by the Window System client code uses an ISO Latin-1 encoding for character strings, while the MVS encoding for character strings is EBCDIC. The X Window System client code in the MVS environment automatically transforms character strings from EBCDIC to ISO Latin-1 or from ISO Latin-1 to EBCDIC, as needed using internal translate tables.

In the MVS environment, external names must be eight characters or less. Many of the X Window System application programming interface names exceed this limit. To support the X API in MVS, all X names longer than eight characters are remapped to unique names using the C compiler preprocessor. This name remapping is found in a file called X11GLUE.H, which is automatically included in your program when you include the standard X header file called XLIB.H. When debugging your application, you can see the X11GLUE.H file to find the remapped names of the X API routines.