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Overview of VMD flow of execution


The VMD startup process is somewhat system dependent. Generally speaking, VMD performs various initialization tasks that depend on command line options, environment variables, and Windows Registry keys, depending on the platform.

On Unix systems, VMD is started via a shell script which is created when the program is initially installed. The startup script sets a few key environment variables. The most important environment variable is the VMDDIR variable, which tells VMD where to find its installation directory, so that it can source various initialization scripts and find helper programs such as Surf, MSMS, and Tachyon. The VMDGetOptions() routine processes the environment variables and command line options. On Windows, a special win32vmdstart() routine does some initial registry queries and environment variable manipulations to get the program bootstrapped.

Command line options affect the choice of display device which can be a text-only command interpreter, text + full GUI and OpenGL, or special CAVE and FreeVR modes which can be run with or without the regular GUI forms. The command line display device options and the environment variables are all documented in the User's Guide. After the main routine gets the appropriate VMDApp state initialized, the DisplayDevice is initialized and the title screen is displayed.

Scripts can be executed at startup time through the use of the -e command line option. These scripts are then "played" after VMD is initialized but before the user's .vmdrc file is read. This is done inside of the VMDreadStartup() routine.

Main execution loop

Once the initialization stages are complete, VMD enters an event loop that continues until the user quits or the program terminates itself due to errors. The main event loop simply calls the VMDApp::VMDupdate() method to check for events and act on them.

Roughly speaking the event loop looks like this:

  1. The CommandQueue is checked for events.

  2. The VMDApp::VMDupdate() routine requests each UIObject to check for any events such as mouse button presses or GUI button/slider/browser/whatever presses. These events are entered into the CommandQueue as Command object instances.

  3. FLTK gets a Fl::wait() call

  4. If the user has pressed the Quit button in the GUI the event loop returns immediately.

  5. All commands in the CommandQueue are processed until the queue is empty. Typically, the processing of an event will result in one or more new Command object being created and placed at the end of the CommandQueue. The CommandQueue will just continue processing each command until the queue is complete empty, which will deal with all the events found in the previous step, as well as any other commands generated while processing the events.

  6. Each UIObject is told to do any update that needs to be done each cycle through the execution loop. This accomplishes such things as updating the current frame in the form used for molecule trajectory animation.

  7. The current Scene is rendered to the current DisplayDevice after some prepare calls and checks to see if the update is actually necessary.

  8. If there is no graphics display or VMD is idle not requiring a graphics update, or a special environment variable is set, VMD will call vmd_msleep() to prevent from eating excessive amounts of CPU time polling for events while idle.


When the user quits VMD via the GUI or by text commands, or when VMD encounters an unrecoverable error or some kind, the program frees all dynamically allocated data, destroys the GUI forms and the DisplayDevice in use, and calls any special termination routines before returning from main.

pg_execution.dox,v 1.5 2008/03/04 20:15:49 johns Exp

Generated on Wed Nov 11 02:45:08 2020 for VMD (current) by doxygen1.2.14 written by Dimitri van Heesch, © 1997-2002