Question

Loading and working with DLLs

Asked by: mzimmer74

I'm attempting to write a c++ program to add a currently installed printer to the machine such that it is installed for all users.  I can accomplish that in dos by using this command:
RUNDLL32 PRINTUI.DLL,PrintUIEntry /ga /c\\machine /n\\machine\printer /j"LanMan Print Services"

I've gotten a lot of the program done (I'm using visual c++ 2008 Express Edition) but am having some problems figuring out the DLL portions (my background is in Java so I'm struggling with c++).  After browsing around and finding some solutions that were similar, I have the code I attached to this question.

Currently, I can load the dll into hDLL just fine.  However, the call to GetProcAddress is returning null.  Any idea what I'm doing wrong here?  Also, once the GetProcAddress call actually works, what do I need to do to make the dwParam1 compile?  I get error C2664 with a conversion problem.  I'm assuming that's the correct way to call the params, but if not what would be correct for the desired DLL call?

I'm not limited to using DLL loading, so if there is another method to do this I'd be glad to hear it.  However, as mentioned, I'm still learning C++ so simpler is better. :)

Thanks,
Matthew

typedef UINT (CALLBACK* LPFNDLLFUNC1)(DWORD,UINT); 
HINSTANCE hDLL;               // Handle to DLL
LPFNDLLFUNC1 lpfnDllFunc1;    // Function pointer
DWORD dwParam1;
UINT  uParam2, uReturnVal;
 
hDLL = LoadLibraryA("C:\\Windows\\system32\\printui.dll"); //this works
if (hDLL != NULL)
{
   lpfnDllFunc1 = (LPFNDLLFUNC1)GetProcAddress(hDLL, "PrintUIEntry"); //this fails
   if (!lpfnDllFunc1)
   {
      FreeLibrary(hDLL);       
      return -1;
   }
   else
   {
      dwParam1="/ga /c\\machine /n\\machine\printer /j\"LanMan Print Services\""
      uReturnVal = lpfnDllFunc1(dwParam1, uParam2);
   }
   return 0;
}

                                  
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Asked On
2008-07-29 at 07:58:34ID23604187
Tags

c++

Topic

C++ Programming Language

Participating Experts
3
Points
250
Comments
11

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Answers

 

by: jkrPosted on 2008-07-29 at 08:04:37ID: 22112185

The problem is that printui.dll does not export a function named 'PrintUIEntry()' - that one is actually called 'PrintUIEntryW()' as 'dumpbin /exports' shows:

File Type: DLL
 
  Section contains the following exports for PRINTUI.dll
 
           0 characteristics
    41107EB7 time date stamp Wed Aug 04 08:14:15 2004
        0.00 version
           1 ordinal base
         200 number of functions
          24 number of names
 
    ordinal hint RVA      name
 
          6    0 00038AC7 ConnectToPrinterDlg
          7    1 00038BF7 ConnectToPrinterPropertyPage
          1    2 00027CB3 ConstructPrinterFriendlyName
          8    3 00031EDB DllCanUnloadNow
          9    4 000327F2 DllGetClassObject
         10    5 0000790F DllMain
          2    6 00028A94 DocumentPropertiesWrap
          3    7 00025E5B PnPInterface
         11    8 0003E9FD PrintNotifyTray_Exit
         12    9 00005000 PrintNotifyTray_Init
          4    A 0002E533 PrintUIEntryW
          5    B 00034B7A PrinterPropPageProvider
         13    C 00008FDD RegisterPrintNotify
         14    D 00027838 ShowErrorMessageHR
         15    E 0002780A ShowErrorMessageSC
         16    F 0000671D UnregisterPrintNotify
         17   10 00008B46 bFolderEnumPrinters
         18   11 00003D8F bFolderGetPrinter
         19   12 00009171 bFolderRefresh
         20   13 0002625E bPrinterSetup
         21   14 00018D6D vDocumentDefaults
         22   15 0001CEEE vPrinterPropPages
         23   16 0002695F vQueueCreate
         24   17 00013490 vServerPropPages
        200      0003803E [NONAME]

                                              
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by: GurudenisPosted on 2008-07-29 at 08:09:03ID: 22112232

There are a couple of things you might want to check:
1) Is the function name actually "PrintUIEntry"? Note that this is case-sensitive.
2) Is the function really exported without name mangling (this can be checked with Dependency Walker, depends.exe)? See: http://en.wikipedia.org/wiki/Name_mangling
3) What does GetLastError() return if called just after GetProcAddress fails? You can look up the corresponding textual error message in the Error Lookup tool.

 

by: GurudenisPosted on 2008-07-29 at 08:10:08ID: 22112241

I'm sorry, jkr, your post wasn't there when I started typing :)
Your comment explains it all.

 

by: mzimmer74Posted on 2008-07-29 at 08:11:37ID: 22112261

Ah...that explains the failure.  I didn't know about dumpbin.  That's a nice feature to get info like this.  The function load now works, but any ideas on the parameter?  Am I correct about what needs to be entered in?  If so, how do I actually get it to compile?

Thanks,
Matthew

 

by: jkrPosted on 2008-07-29 at 08:16:48ID: 22112334

The 'rundll' interface is standardized by MS, see http://support.microsoft.com/kb/164787 ("INFO: Windows Rundll and Rundll32 Interface"). The signature of the functions is as shown below. Be sure to use an UNICODE string as the command line, the trailing 'W' in the function name indicates that.

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

                                              
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by: GurudenisPosted on 2008-07-29 at 08:19:54ID: 22112361

 

by: JohnGabyPosted on 2008-07-29 at 08:20:17ID: 22112367

I am not familiar with the function 'PrintUIEntryW' that you are calling, but I might also point out that the 'W' at the end of the name usually means that it is the wide character version of the function.  So if the first parameter is a string (and it seems from your code that it is), then it needs to be a wide character (unicode) string, not a byte character string.

 

by: mzimmer74Posted on 2008-07-29 at 08:47:04ID: 22112673

Once I changed the DWORD to String^ I was able to compile and run to the point where the function is actually called.  This is outside the scope of the original question, but I do get an System.AccessViolationException (Attempted to read or write protected memory) and am curious if there is some easy way to set permissions on a call such as this.  I'll be doing some research on it but hoped somebody with more knowledge (i.e. you guys :)) could point me in the right direction.  I am a domain administrator on this computer/domain so it shouldn't be a permissions problem and I can run the command in dos.

Thanks,
Matthew

 

by: jkrPosted on 2008-07-29 at 08:56:29ID: 22112785

Err, "String^" is not valid C++ anyway...

 

by: mzimmer74Posted on 2008-07-29 at 09:07:38ID: 22112896

Seriously?  How is it working then?  Is there something special to Visual C++ that would make it work?  It's things like this that make me love java all the more! :)  However, for the sake of proper programming what should I use instead of String^?

By the way, I'll be accepting your first answer as the solution as it did answer my original question.

Thanks,
Matthew

 

by: GurudenisPosted on 2008-07-29 at 11:45:16ID: 22114250

String^ is C++/CLI (managed C++). In unmanaged C++, you'd use std::string (or rather std::wstring) instead.

20120131-EE-VQP-002

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