ID Number: Q85108
1.00
WINDOWS
Summary:
When you call a dynamic link library (DLL) function from Visual Basic,
the "Bad DLL Calling Convention" error is often caused by incorrectly
omitting or including the ByVal keyword from the Declare statement or
Call statement. The ByVal keyword affects the size of data placed on
the stack. Visual Basic checks the change in the position of the stack
pointer to detect this error.
When Visual Basic generates the run time error "Bad DLL Calling
Convention," the most common cause when calling API functions is
omitting the ByVal keyword from the Declaration of the external
function or from the call itself. It can also occur due to including
the ByVal keyword when the function is expecting a 4 byte pointer to
the parameter instead of the value itself. This changes the size
(number of bytes) of the values placed on the stack, and upon return
from the DLL, Visual Basic detects the change in the position of the
stack frame and generates the error.
More Information:
There are two calling conventions, or inter-language protocols: the
Pascal/Basic/FORTRAN calling convention, and the C calling convention.
Visual Basic uses the Pascal calling convention, as do the Window API
functions and other Microsoft Basic language products. Under the
Pascal convention, it is the responsibility of the called procedure to
adjust or clean the stack. (In addition, parameters are pushed onto
the stack in order from the leftmost parameter to the rightmost.)
Because the DLL function is responsible for adjusting the stack based
on the type and number of parameters it expects, Visual Basic checks
the position of the stack pointer upon return from the function. If
the called routine has adjusted the stack to an unexpected position,
then Visual Basic generates a "Bad DLL Calling Convention" error.
Visual Basic assumes a stack position discrepancy because the DLL
function uses the C calling convention. With the C calling convention,
the calling program is responsible for adjusting the stack immediately
after the called routine returns control.
Steps to Reproduce Behavior
---------------------------
Create a simple DLL using Quick C for Windows or any compiler capable
of creating Windows DLLs. The following example is in C and written
for Quick C for Windows:
Stacking.C
----------
#include <windows.h>
long far pascal typecheck (long a, float b, short far *c, char far *buff)
{
short retcode;
a = a * 3;
retcode = MessageBox(NULL, "I am in the DLL", "BOX", MB_OK);
return (a);
}
Stacking.DEF
------------
LIBRARY STACKING
EXETYPE WINDOWS 3.0
STUB 'winstub.exe'
STACKSIZE 5120
HEAPSIZE 1024
DATA PRELOAD MOVEABLE SINGLE ;ADD THESE TWO LINES
CODE PRELOAD MOVEABLE DISCARDABLE ;TO AVOID WARNINGS
EXPORTS
typecheck @1
WEP @2
In a Visual Basic form, in the general Declarations module:
Declare Function typecheck Lib "d\stacking.dll" (ByVal a As Long,
ByVal b As Single, c As Integer, ByVal s As String) As Long
Note: The above declaration must be placed on one line.
In the Form_Click event:
Sub Form_Click ()
Dim a As Long 'Explicitly type the variables
Dim b As Single
Dim c As Integer
Dim s As String
a = 3 'Initialize the variables
b = 4.5
c = 6
s = "Hello there! We've been waiting for you!"
Print typecheck(a, b, c, s)
End Sub
Running the program as written above will not generate the error. Now
add the ByVal keyword before the variable named c in the Visual Basic
Declaration. Run the program. Note that the MessageBox function pops a
box first, and then the error box pops up indicating that Visual Basic
checks the stack upon return to see if it has been correctly adjusted.
Because the DLL expected a 4-byte pointer and received a 2-byte value,
the stack has not adjusted back to the initial frame.
As another test, first remove the ByVal keyword before the variable c
that you added in the previous test. Declare the parameter a As Any
instead of As Long. Change the type of the variable a in the
Form_Click to Integer. Run the program again. Using As Any turns off
type checking by Visual Basic. Because the program passed an integer
ByVal instead of the long that the DLL expected, the stack frame is
off and the error is generated.
Reference(s):
"Microsoft BASIC 7.0: Programmer's Guide" for versions 7.0 and 7.1,
pages 423-426
"Microsoft Visual Basic Programmer's Guide," version 1.0, pages
379-386
Additional reference words: 1.00