Chapter 4. Guest Additions

To overcome the limitations for mouse support that were described in Section 1.8.2, “Capturing and releasing keyboard and mouse”, this provides you with seamless mouse support. You will only have one mouse pointer and pressing the Host key is no longer required to "free" the mouse from being captured by the guest OS. To make this work, a special mouse driver is installed in the guest that communicates with the "real" mouse driver on your host and moves the guest mouse pointer accordingly.

These provide an easy way to exchange files between the host and the guest. Much like ordinary Windows network shares, you can tell VirtualBox to treat a certain host directory as a shared folder, and VirtualBox will make it available to the guest operating system as a network share, irrespective of whether guest actually has a network. For details, please refer to Section 4.3, “Shared folders”.

While the virtual graphics card which VirtualBox emulates for any guest operating system provides all the basic features, the custom video drivers that are installed with the Guest Additions provide you with extra high and non-standard video modes as well as accelerated video performance.

In addition, with Windows, Linux and Solaris guests, you can resize the virtual machine's window if the Guest Additions are installed. The video resolution in the guest will be automatically adjusted (as if you had manually entered an arbitrary resolution in the guest's display settings). Please see Section 1.8.5, “Resizing the machine's window” also.

Finally, if the Guest Additions are installed, 3D graphics and 2D video for guest applications can be accelerated; see Section 4.5, “Hardware-accelerated graphics”.

With this feature, the individual windows that are displayed on the desktop of the virtual machine can be mapped on the host's desktop, as if the underlying application was actually running on the host. See Section 4.6, “Seamless windows” for details.

The Guest Additions enable you to control and monitor guest execution in ways other than those mentioned above. The so-called "guest properties" provide a generic string-based mechanism to exchange data bits between a guest and a host, some of which have special meanings for controlling and monitoring the guest; see Section 4.7, “Guest properties” for details.

Additionally, applications can be started in a guest from the host; see Section 4.8, “Guest control”.

With the Guest Additions installed, VirtualBox can ensure that the guest's system time is better synchronized with that of the host.

For various reasons, the time in the guest might run at a slightly different rate than the time on the host. The host could be receiving updates via NTP and its own time might not run linearly. A VM could also be paused, which stops the flow of time in the guest for a shorter or longer period of time. When the wall clock time between the guest and host only differs slightly, the time synchronization service attempts to gradually and smoothly adjust the guest time in small increments to either "catch up" or "lose" time. When the difference is too great (e.g., a VM paused for hours or restored from saved state), the guest time is changed immediately, without a gradual adjustment.

The Guest Additions will re-synchronize the time regularly. See Section 9.14.3, “Tuning the Guest Additions time synchronization parameters” for how to configure the parameters of the time synchronization mechanism.

With the Guest Additions installed, the clipboard of the guest operating system can optionally be shared with your host operating system; see Section 3.4, “General settings”.

For details, please see Section 9.2, “Automated guest logons”.