Enabling and Disabling AutoRun

There are many situations where AutoRun may need to be temporarily or persistently disabled. For example, AutoRun might interfere with the operation of a running application and need to be disabled for the duration. The system provides several ways to disable AutoRun.

• Suppressing AutoRun Programmatically
• Using the Registry to Disable AutoRun
• AutoRun for Other Types of Storage Media

Suppressing AutoRun Programmatically

There are a variety of situations where AutoRun may need to be suppressed programmatically. Two examples are:

• Your application has a setup program that requires the user to insert another disc that may contain an Autorun.inf file.
• During the operation of your application, the user may need to insert another disc that may contain an Autorun.inf file.

In either case, you will normally not want to launch another application while the original is in progress.

Users can manually suppress AutoRun by holding down the SHIFT key when they insert the CD-ROM. However, it is usually preferable to handle this operation programmatically rather than depending on the user.

With systems that have Shell version 4.70 and later, Microsoft Windows sends a "QueryCancelAutoPlay" message to the foreground window. Your application can respond to this message to suppress AutoRun. This approach is used by system utilities such as the Open common dialog box to disable AutoRun. You will not get a "QueryCancelAutoPlay" message with versions of Windows 95 that do not have the Microsoft Internet Explorer 4.0 integrated Shell installed.

The following code fragments illustrate how to set up and handle this message. Your application must be running in the foreground window. First, register "QueryCancelAutoPlay" as a Windows message:

uMessage = RegisterWindowMessage(TEXT("QueryCancelAutoPlay")); 
            

Your application's window must be in the foreground to receive this message. The message handler should return TRUE to cancel AutoRun and FALSE to enable it. The following code fragment illustrates how to use this message to disable AutoRun.

UINT g_uQueryCancelAutoPlay = 0;

LRESULT WndProc(HWND hwnd, UINT uMsg,  WPARAM wParam, LPARAM lParam) 
{ 
    switch (uMsg) 
    { 
    ... 
    default: 
        if (!g_uQueryCancelAutoPlay)
        { 
            g_uQueryCancelAutoPlay = RegisterWindowMessage(TEXT("QueryCancelAutoPlay"));
        } 
        if (uMsg && uMsg == g_uQueryCancelAutoPlay)
        { 
            return TRUE;       // cancel auto-play
        }
    }
}
            

If your application is using a dialog box and needs to respond to a "QueryCancelAutoPlay" message, it cannot simply return TRUE or FALSE. Instead, call SetWindowLong with nIndex set to DWL_MSGRESULT. Set the dwNewLong parameter to TRUE to cancel AutoRun, and FALSE to enable it. For example, the following sample dialog box procedure cancels AutoRun when it receives a "QueryCancelAutoPlay" message.

UINT g_uQueryCancelAutoPlay = 0;

BOOL DialogProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) 
{ 
    switch (uMsg) 
    { 
    ...
    default: 
        if (!g_uQueryCancelAutoPlay)
        {
            g_uQueryCancelAutoPlay = RegisterWindowMessage(TEXT("QueryCancelAutoPlay"));
        } 
        if (uMsg == g_uQueryCancelAutoPlay) 
        {
            SetWindowLong(hDlg, DWL_MSGRESULT, TRUE);          
            return 1;               
        }
    } 
}
            

Using the Registry to Disable AutoRun

There are two registry values that can be used to persistently disable AutoRun: NoDriveAutoRun and NoDriveTypeAutoRun. The first value disables AutoRun for specified drive letters and the second disables AutoRun for a class of drives. If either of these values is set to disable AutoRun for a particular device, it will be disabled.

Note   The NoDriveAutoRun and NoDriveTypeAutoRun values should only be modified by system administrators to change the value for the entire system for testing or administrative purposes. Applications should not modify these values, as there is no way to reliably restore them to their original values.

The NoDriveAutoRun value disables AutoRun for specified drive letters. It is a REG_DWORD data value, found under the following key:

HKEY_CURRENT_USER

Software

Microsoft

Windows

CurrentVersion

Policies

• Explorer

The first bit of the value corresponds to drive A:, the second to B:, and so on. To disable AutoRun for one or more drive letters, set the corresponding bits. For example, to disable the A: and C: drives, set NoDriveAutoRun to 0x00000005.

The NoDriveTypeAutoRun value disables AutoRun for a class of drives. It is a REG_DWORD or 4-byte REG_BINARY data value, found under the same key.

HKEY_CURRENT_USER

Software

Microsoft

Windows

CurrentVersion

Policies

• Explorer

By setting the bits of this value's first byte, different drives can be excluded from working with AutoRun.

The following table gives the bits and bitmask constants, that can be set in the first byte of NoDriveTypeAutoRun to disable AutoRun for a particular drive type. For Microsoft Windows NT and later systems, you must restart Windows Explorer before the changes take effect.

AutoRun for Other Types of Storage Media

AutoRun is primarily intended for public distribution of applications on CD-ROM and DVD-ROM. However, it is often useful to enable AutoRun on other types of removable storage media. This feature is typically used simplify the debugging of AutoRun.inf files. AutoRun only works on removable storage devices when the following criteria are met:

• The device must have AutoRun-compatible drivers. To be AutoRun-compatible, a driver must notify the system that a disk has been inserted by sending a WM_DEVICECHANGE message.
• The root directory of the inserted media must contain an Autorun.inf file.
• The device must not have AutoRun disabled through the registry.
• The foreground application has not suppressed AutoRun.

Note   This feature should not be used to distribute applications on floppy disks. Because implementing AutoRun on a floppy disk provides an easy way to spread computer viruses, users should be suspicious of any publicly distributed floppy disk that contains an Autorun.inf file.

Normally, AutoRun starts automatically, but it can also be started manually. If the device meets the criteria listed above, the drive letter's shortcut menu will include an AutoPlay command. To run AutoRun manually, either right-click the drive icon and select AutoPlay from the shortcut menu or double-click the drive icon. If the drivers are not AutoRun-compatible, the shortcut menu will not have an AutoPlay item and AutoRun cannot be started.

AutoRun-compatible drivers are provided with some floppy disk drives, as well as some other types of removable media such as CompactFlash cards. AutoRun also works with network drives that are mapped to a drive letter with Windows Explorer or mounted with the Microsoft Management Console (MMC). As with mounted hardware, a mounted network drive must have an Autorun.inf file in its root directory, and must not be disabled through the registry.