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Export DLL functions- Link error

Posted on 2000-02-23
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Last Modified: 2008-03-06
I cant get the C++ V6 syntax correct to get a clean link of this DLL.  The intent is to create a function library to be used in VB6.

My confusion stems from a lack of understanding of decorated and undecorated symbnols.

I documented below the 1) the link error, 2) C++ code and 3) .def file
/////////////////////////////////
Link error:
Linking...
.\notes2clarifydll.def : warning LNK4022: cannot find unique match for symbol "Connect"
.\notes2clarifydll.def : warning LNK4002: "long __stdcall Connect(char *,char *,char *,char *)" (?Connect@@YGJPAD000@Z) defined in .\Debug\notes2clarifydll.obj
.\notes2clarifydll.def : warning LNK4002: "public: int __thiscall CAsyncSocket::Connect(char const *,unsigned int)" (?Connect@CAsyncSocket@@QAEHPBDI@Z) defined in D:\Program Files\Microsoft Visual Studio\VC98\MFC\LIB\nafxcwd.lib
.\notes2clarifydll.def : warning LNK4002: "public: int __thiscall CAsyncSocket::Connect(struct sockaddr const *,int)" (?Connect@CAsyncSocket@@QAEHPBUsockaddr@@H@Z) defined in D:\Program Files\Microsoft Visual Studio\VC98\MFC\LIB\nafxcwd.lib
.\notes2clarifydll.def : warning LNK4002: "protected: int __thiscall CDatabase::Connect(unsigned long)" (?Connect@CDatabase@@IAEHK@Z) defined in D:\Program Files\Microsoft Visual Studio\VC98\MFC\LIB\nafxcwd.lib
LINK : fatal error LNK1152: cannot resolve one or more undecorated symbols
LINK : fatal error LNK1141: failure during build of exports file
Error executing link.exe.
Creating browse info file...

notes2clarifydll.dll - 2 error(s), 5 warning(s)

//////////////////////////////////
Extracted code of the C function:

//Prototype functions called externally
long __stdcall Connect(char *,char *,char *,char *);
...

//Establish a connection to the database
long  __stdcall Connect(char *strServer,char *strDatabase,char *strUsername,char *strPassword)
{
      char *MOD = "ServerLogin";
      long iConnect_rc;

      //Login to the database.
      thedb.dbo_status = cpi_sess_login(strServer,  /* server name */
            strDatabase,  /* database name */
            strUsername,  /* login name */
            strPassword,  /* password */
            CPI_OPTIONS_DEFAULT,
            &dbid);
    iConnect_rc = check_status(thedb.dbo_status, MOD, "Error logging into the database. \n",1L);
      return iConnect_rc;
}


///////////////////////////////////
; notes2clarifydll.def : Declares the module parameters for the DLL.

LIBRARY      "notes2clarifydll"
DESCRIPTION  'notes2clarifydll Windows Dynamic Link Library'

EXPORTS
    ; Explicit exports can go here
      Connect
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Question by:GregoryGr
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Accepted Solution

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jhance earned 100 total points
ID: 2551592
Declare your C functions as:

extern "C" __declspec(dllexport) long __stdcall Connect(char *,char *,char *,char *);

In your VB program, declare it as:

Private Declare Function Connect Lib "MyDll.Dll" Alias _Connect@16 (ByVal p1 As String, ByVal p2 As String, ByVal p3 As String, ByVal p4 As String) As Long
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LVL 32

Expert Comment

by:jhance
ID: 2551594
Also, forget the DEF file, you don't need it...
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Author Comment

by:GregoryGr
ID: 2551639
That did it...thanks!
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LVL 32

Expert Comment

by:jhance
ID: 2551684
BTW, the only really trick part is:

_Connect@16

The @16 varies depending on the number and type of arguments to the function.  The 16 in this case means 16 bytes are passed in, that's 4 bytes for each char *.  Most things in Windows are 32 bit values so this number is usually just 4 x number of args but there are 8, 16, and even 64 bit types possible.  If in doubt, check the actual exported name with:

DUMPBIN /EXPORTS mydll.dll

DUMPBIN is in the VC++ bin directory.
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