3.11. Shared folders
Shared folders allow you to easily exchange data between a virtual machine and your host. This feature requires that the VirtualBox Guest Additions be installed in a virtual machine and is described in detail in Section 4.3, “Shared folders”.
The VirtualBox Guest Additions for all supported guest operating systems are provided as a single CD-ROM image file which is called VBoxGuestAdditions.iso. This image file is located in the installation directory of VirtualBox. To install the Guest Additions for a particular VM, you mount this ISO file in your VM as a virtual CD-ROM and install from there.（使用光盘加载方式加载VBoxGuestAdditions.iso）
The Guest Additions offer the following features:
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”.
`4.3.2. Automatic mounting
Starting with version 4.0, VirtualBox can mount shared folders automatically, at your option. If automatic mounting is enabled for a specific shared folder, the Guest Additions will automatically mount that folder as soon as a user logs into the guest OS. The details depend on the guest OS type:
With Windows guests, any auto-mounted shared folder will receive its own drive letter (e.g. E:) depending on the free drive letters remaining in the guest.
If there no free drive letters left, auto-mounting will fail; as a result, the number of auto-mounted shared folders is typically limited to 22 or less with Windows guests.
With Linux guests, auto-mounted shared folders are mounted into the /media directory, along with the prefix sf_. For example, the shared folder myfiles would be mounted to /media/sf_myfiles on Linux （linux系统中自动挂载在/media目录中，在共享目录前加上sf_前缀，如共享目录为myfiles，则在linux中挂载的目录为/media/sf_myfiles）and /mnt/sf_myfiles on Solaris.
The guest property /VirtualBox/GuestAdd/SharedFolders/MountPrefix determines the prefix that is used. Change that guest property to a value other than “sf” to change that prefix; see Section 4.7, “Guest properties” for details.
Access to auto-mounted shared folders is only granted to the user group vboxsf, which is created by the VirtualBox Guest Additions installer. Hence guest users have to be member of that group to have read/write access or to have read-only access in case the folder is not mapped writable.
To change the mount directory to something other than /media, you can set the guest property /VirtualBox/GuestAdd/SharedFolders/MountDir.
Solaris guests behave like Linux guests except that /mnt is used as the default mount directory instead of /media.`