RUNDLL32.ORG

Microsoft Windows 95, Windows 98, and Windows Millennium Edition (Me) contains two command-line utility programs named Rundll.exe and Rundll32.exe that allow you to invoke a function exported from a DLL, either 16-bit or 32-bit. However, Rundll and Rundll32 programs do not allow you to call any exported function from any DLL. For example, you can not use these utility programs to call the Win32 API (Application Programming Interface) calls exported from the system DLLs. The programs only allow you to call functions from a DLL that are explicitly written to be called by them. This article provides more details on the use of Rundll and Rundll32 programs under the Windows operating systems listed above.

MIcrosoft Windows NT 4.0, Windows 2000, and Windows XP ship with only Rundll32. There is no support for Rundll (the Win16 utility) on either platform.

The Rundll and Rundll32 utility programs were originally designed only for internal use at Microsoft. But the functionality provided by them is sufficiently generic that they are now available for general use. Note that Windows NT 4.0 ships only with the Rundll32 utility program and supports only Rundll32.

Rundll vs. Rundll32

Rundll loads and runs 16-bit DLLs, whereas Rundll32 loads and runs 32-bit DLLs. If you pass the wrong type of DLL to Rundll or Rundll32, it may fail to run without indicating any error messages.

How Rundll Works

Rundll performs the following steps:

1.	It parses the command line.
2.	It loads the specified DLL via LoadLibrary().
3.	It obtains the address of the <entrypoint> function via GetProcAddress().
4.	It calls the <entrypoint> function, passing the command line tail which is the <optional arguments>.
5.	When the <entrypoint> function returns, Rundll.exe unloads the DLL and exits.