How SCO Visual Tcl works

The following is a high-level look at what happens when a user runs a SCO Visual Tcl application:

1. A user starts an application (also referred to as a client) written using SCO Visual Tcl, either from the command line (by entering vtcl progname) or by clicking on the application's icon. In either case, the vtcl interpreter processes the script.

2. vtcl attempts to contact the server process over a guaranteed connection (named pipe). If the SCO Visual Tcl server process for the type of display is not yet running, one starts automatically. For example, if a user at a character terminal starts an application and cm_vtcld is not running, the process starts.

3. The SCO Visual Tcl application (client) makes a connection to the server, and the server displays the appropriate information on the user's screen. This information consists of ``widgets'', items such as menus, buttons, and text-entry fields.

4. The user performs ``events'' on these widgets. An event is an action such as clicking on a button, choosing a menu item, entering text in a text field, and so on.

5. For each widget, the programmer defined the possible events in the application's script. When the user performs the event (such as pressing a button), a specified ``callback'' executes. This callback calls a procedure in the application's script, which can then call additional operating system or Tcl commands.

6. Additional events and callbacks are performed until the user exits the program. At that time, the communication channel between the client and server is dropped, but the server process remains active until the system shuts down, listening for additional requests.