How do I Call a function form a dll file

Posted on 2007-12-02
Last Modified: 2008-02-01
I made a simple dll file from but i get the error Bad DLL calling convention when I try to use it. Can someone show me the correct way to use this dll from VB.

Thanks for your help
Question by:rcherne1
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LVL 92

Expert Comment

by:Patrick Matthews
ID: 20391791
Hello rich,

How are you trying to call it now?



Author Comment

ID: 20391804

Private Declare Function RetrieveTempFilename Lib "c:\work\MakeTempFilenameDLL.dll" (szDirectory As String, szPrefix As String) As String

Private Sub Command1_Click()
Dim my_szDirectory As String
Dim my_szPrefix As String
my_szDirectory = "c:\work\"
my_szPrefix = "tst"
MsgBox RetrieveTempFilename(my_szDirectory, my_szPrefix)
End Sub
LVL 143

Expert Comment

by:Guy Hengel [angelIII / a3]
ID: 20391982
can you show how exactly you defined the function in your dll code?
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Author Comment

ID: 20391997

char szReturn[MAX_PATH];
__declspec(dllexport) LPCTSTR RetrieveTempFilename(LPCTSTR szDirectory, LPCTSTR szPrefix)
     char szBuffer[MAX_PATH];
     if(GetTempFileName(szDirectory,szPrefix,0,szBuffer) == 0)
          // It failed - return the string "ERROR"
     } else {
          // Success!
     return szReturn;

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LVL 143

Accepted Solution

Guy Hengel [angelIII / a3] earned 250 total points
ID: 20392030
please try to change the declaration:

Private Declare Function RetrieveTempFilename Lib "c:\work\MakeTempFilenameDLL.dll" (byval szDirectory As String, byval szPrefix As String) As String

LVL 22

Assisted Solution

danaseaman earned 250 total points
ID: 20392042
VB6 can only call __stdcall functions, not __cdecl functions so you will need to change this to __stdcall.

Error is "Bad DLL calling convention"

   __declspec(dllexport) LPCTSTR RetrieveTempFilename(LPCTSTR szDirectory, LPCTSTR szPrefix)

Try changing to this and recompile the DLL:
   __stdcall(dllexport) LPCTSTR RetrieveTempFilename(LPCTSTR szDirectory, LPCTSTR szPrefix)


Author Comment

ID: 20392282
Hum? That wont compile but I changed it to
"LPCTSTR __stdcall  RetrieveTempFilename(LPCTSTR szDirectory,LPCTSTR szPrefix)"
Now I dont get an error but i think its returning a pointer to the string not a string.

Sound like I should posted this in the vc++ area to

LVL 22

Expert Comment

ID: 20392307
I probably have the wrong syntax (I don't use C++):

Perhaps it should be:
__declspec(dllexport) __stdcall LPCTSTR RetrieveTempFilename(LPCTSTR szDirectory, LPCTSTR szPrefix)


Author Comment

ID: 20392968
angelIII and danaseaman you both had part of my answer.
I had to pass my string byval and use __stdcall
I also found that it returns a pointer.

Thanks for your help.

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