Environment > Distributing IDL Applications > Virtual Machine Applications > Preferences for Virtual Machine Applications

# Preferences for Virtual Machine Applications

IDL’s preference system allows developers, administrators, and individual users to control default values for many aspects of IDL’s environment and configuration. Creators of runtime applications can take advantage of the preference system to customize the environment in which a particular application runs.

See Preferences for Runtime Applications for a discussion of using preferences in the context of any IDL runtime application, including applications that run in the IDL Virtual Machine.

The process of specifying preferences for a Virtual Machine application is complicated by the following facts:

- Since you are relying on a standard IDL Virtual Machine distribution rather than a distribution you create, it is more difficult to install a preferences file in the application distribution.
- On Microsoft Windows and Macintosh platforms, users may launch your Virtual Machine application by clicking on the SAVE file icon, or by dragging the SAVE file icon onto the Virtual Machine icon. This prevents you from specifying preferences via a command line option.

## Options for Windows Applications

If your Virtual Machine application runs under Microsoft Windows, you have the following options:

- Have your users launch the Virtual Machine application via the Windows command line, and use the `-pref` command line option to specify a preferences file or to specify individual preferences.
- Have your users install an `idl.pref` file in the `IDL_DIR/bin/bin.platform` directory where `platform` is the platform-specific `bin` directory, and then launch the application by clicking on the SAVE file icon or by dragging it to the Virtual Machine icon.
- Instruct your users to set environment variables that correspond to the preferences you need to specify.
- If you are providing a runtime distribution for your application, you can install an `idl.pref` file in the `IDL_DIR/bin/bin.platform` directory yourself.

## Options for UNIX/Macintosh Applications

If your Virtual Machine application runs under UNIX (including Mac OS X), you have the following options:

- Have your users launch the Virtual Machine application via the shell command line, and use the `-pref` command line option to specify a preferences file or to specify individual preferences.
- Instruct your users to set environment variables that correspond to the preferences you need to specify.