C++ function in DLL called through Excel / VBA generates exception when passing double argument

Question

I'm trying to use a C / C++ static function in Excel / VBA through a DLL.

I'm getting an exception when debugging in VS17, and I suspect it's an issue with the way the argument is passed (it's a double)

Exception thrown at 0x00007FFA28BBA14F (kernel32.dll) in EXCEL.EXE: 0xC0000005: Access violation reading location 0xFFFFFFFFFFFFFFFF.

here's my C code:

test.h

extern "C" __declspec(dllexport) double get_sum_cpp(double x);

test.cpp

double WINAPI get_sum_cpp(double x)
{
   double res = x + x;
   return res;
}

declaration in VBA:

Declare PtrSafe Function get_sum_cpp Lib "C:\Users\bbi\source\repos\Test\x64\Debug\Test.dll" (ByVal my_var As Double) As Double

test code in VBA:

Sub testSum()

    Dim A As Double
    Dim Asum As Double

    A = 5

    Asum = get_sum_cpp(A)

end sub

I'm running 64 bits excel, and the dll is compiled in debug mode 64 bits.

I have many more issues with the overall development (for example any function with more than one argument will crash excel entirely), but this is the smallest "unit test" I could get too.

I feel it's an issue with the way the VBA double argument is passed to the DLL function, (stack misalignment ?), but I can't figure out how to set it right. When debugging in VS17, the exception occurs before I reach the line "double res = x + x", so I suspect it's happening at the function declaration, so when the double argument is passed - so an issue with casting - again maybe stack misalignment ?

my exports seem OK - checked with dumpbin / EXPORTS. The function is found and eventually returns.

Any idea ?

why a -1 ? at least motivate it so I can reformulate the question ! — Bastien, Mar 01 '22 at 05:42
Try it with a simple `int`, or `char`, If it works you can validate your suspicion that its because its a double. — Taekahn, Mar 05 '22 at 17:29
Rapid overview seems it should work. Can you post a small reproducing project somewhere on the internet so we can have a look. — Simon Mourier, Mar 05 '22 at 18:37
In your cpp file, do _not_ use WINAPI, see this SO [answer](https://stackoverflow.com/a/41910450/14973743) that explains the issue if applied. — Anand Sowmithiran, Mar 07 '22 at 15:39
try changing your cpp file to define the implementation of the function to be, `extern "C" double get_sum_cpp(double x)` — Anand Sowmithiran, Mar 07 '22 at 15:42

Afshin · Answer 1 · 2022-03-08T10:05:10.490

I have successfully ran your code exactly without any problem on my computer.

My VBA script:

Private Declare PtrSafe Function get_sum_cpp Lib "D:\Codes\VC\TestVBA\x64\Debug\Dll2.dll" (ByVal my_var As Double) As Double

Sub testSum()

    Dim A As Double
    Dim Asum As Double

    A = 5

    Asum = get_sum_cpp(A)

    MsgBox "Val is: " & Asum
End Sub

and your C++ code. I have tested both with your code and using __declspec(dllexport) and definition file. Both works. I also have tested with/without DLL runtime library. Still works.

I have used VS2019, Excel 2019. If they are not same version, make sure you are using /MT not /MTd for non-DLL runtime libraries though.

C++ function in DLL called through Excel / VBA generates exception when passing double argument

1 Answers1