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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.

  
Rundll Command Line
The command line for Rundll is as follows:

    RUNDLL.EXE <dllname>,<entrypoint> <optional arguments>

An example is as follows:

    RUNDLL.EXE SETUPX.DLL,InstallHinfSection 132 C:\WINDOWS\INF\SHELL.INF

There are 3 issues to consider carefully in the above command line:

1. Rundll or Rundll32 search for the given DLL filename in the standard places (see the documentation for the LoadLibrary() function for details). It is recommended that you provide a full path to the DLL to ensure that the correct one is found. For best results, use the short file name instead of the long file name to ensure that no illegal characters will appear. Note in particular that this means a DLL in the "C:\Program Files" folder should be converted to its short name.
2. The may not contain any spaces or commas or quotation marks. This is a limitation in the Rundll command line parser.
3. In the above command line, the comma (,) between the <dllname> and the <entrypoint> function name is extremely important. If the comma separator is missing, Rundll or Rundll32 will fail without indicating any errors. In addition, there cannot be any white spaces in between the <dllname>, the comma, and the <entrypoint> function.

  
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.

  
How to Write Your DLL
In your DLL, write the function with the following prototype:

16-bit DLL:

    void FAR PASCAL __loadds
        EntryPoint(HWND hwnd, HINSTANCE hinst, LPSTR lpszCmdLine, int nCmdShow);

32-bit DLL:

    void CALLBACK
        EntryPoint(HWND hwnd, HINSTANCE hinst, LPSTR lpszCmdLine, int nCmdShow);

Again, there are 3 issues to consider with the EntryPoint function:

1. Obviously, the name "EntryPoint" should be replaced with the actual name of your entry point function. Note that the Rundll32's entry point is completely unrelated to the DllEntryPoint function in a 32-bit DLL which handles process and thread attach/detach notifications.
2. The entry point function for Rundll32 must be defined with the _stdcall calling convention (CALLBACK defaults to using the _stdcall attribute). If the _stdcall attribute is missing, then the function defaults to _cdecl calling convention and then Rundll32 will terminate abnormally after calling the function.
3. Since you must declare the function with _stdcall calling convention as described above, it follows that the Visual C++ compiler will actually export it as _EntryPoint@16 if the DLL is written in C or will use further name decoration if the DLL is written in C++. So, be careful to use the correctly exported name in the command line for Rundll or Rundll32. If you want to avoid using decorated names, use a .def file and export the entry point function by name. Please refer to the product documentation and the following article for further information on name decoration when using Visual C++ compilers:

          ARTICLE ID: Q140485
          TITLE : Exporting PASCAL-Like Symbols in 32-bit DLLs

The parameters to the Rundll entry point are as follows:

  hwnd - window handle that should be used as the owner window for any windows your DLL creates
  hinst - your DLL's instance handle
  lpszCmdLine - ASCIIZ command line your DLL should parse
  nCmdShow - describes how your DLL's windows should be displayed.

In the following example:

  RUNDLL.EXE SETUPX.DLL,InstallHinfSection 132 C:\WINDOWS\INF\SHELL.INF

Rundll would call the InstallHinfSection() entrypoint function in Setupx.dll and pass it the following parameters:

  hwnd = (parent window handle)
  hinst = HINSTANCE of SETUPX.DLL
  lpszCmdLine = "132 C:\WINDOWS\INF\SHELL.INF"
  nCmdShow = (whatever the nCmdShow was passed to CreateProcess)

Note that it is the function (or InstallHinfSection() in the above example) that has to parse its own command line (the lpszCmdLine parameter above) and use the individual parameters as necessary. Rundll.exe parses only up to the optional arguments passed to its command line. The rest of the parsing is up to the function.

  
Special Notes On Differences Between Windows 95 And Windows NT

On Windows NT, the behavior of Rundll32.exe is slightly different, in order to accommodate UNICODE command lines.

Windows NT first attempts to GetProcAddress for <EntryPoint>W. If this entry point is found, then the prototype is assumed to be:

    void CALLBACK
        EntryPointW(HWND hwnd, HINSTANCE hinst, LPWSTR lpszCmdLine, int nCmdShow);

This is the same as the ANSI EntryPoint, except that the lpszCmdLine parameter is now a UNICODE string.

If the <EntryPoint>W entry point is not found, then Windows NT will GetProcAddress for <entrypoint>A and for <entrypoint>. If either is found, then it is considered an ANSI entry point and is treated the same way as Windows 95. Therefore, if you want your DLL to run on Windows 95 with ANSI support and on Windows NT with UNICODE support, you should export two functions: EntryPointW and EntryPoint. On Windows NT, the EntryPointW function will be called with a UNICODE command line; on Windows 95, the EntryPoint function will be called with an ANSI Command line.

   
References

For an example on the usage of Rundll, refer to the following article on how to launch a Control Panel Applet in Windows 95 using the Rundll command line utility:

  ARTICLE ID: Q135068
  TITLE : How to Start a Control Panel Applet in Windows 95

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