Call C function in DLL that returns char * from MS-Access VBA - ms-access

Question

I have a DLL with a function declared like this:
char * __declspec(dllexport) WINAPI test(int x);
I want to call it in Access VBA like this:
Private Declare Function "test" Lib "MyDLL.dll"(ByVal x As Long) As String
Sub MySub
Dim s As String
s = test(1)
End Sub
Based on articles found in Google, I wrote a function in the DLL:
BSTR CStrToVBStr(char *str)
{
int wslen = MultiByteToWideChar(CP_ACP, 0, str, lstrlen(str), 0, 0);
BSTR bstr = SysAllocStringLen(0, wslen);
MultiByteToWideChar(CP_ACP, 0, str, strlen(str), bstr, wslen);
return bstr;
}
After the call to test(), s apparently contains Unicode characters. That is, a char * "test" is returned to VBA so that Mid(s, 1, 1) = "t", Mid(s, 2, 1) = Chr(0), Mid(s, 3, 1) = "e", etc. I decide to skip the conversion to Unicode and wrote CstrToVBStr as
BSTR CStrToVBStr(char *str)
{
return SysAllocString(str);
}
and ignored the warning that SysAllocString takes an OLECHAR * and not a char * parameter.
The returned string looks good now, but the null character terminator is included, so if the char * is "test", then in VBA len(s) = 5 and Right(s,1) = Chr(0).
What is the right way to do what I am doing? All examples that I have seen are about char * as parameters, not return values. I can change test() to be
void __declspec(dllexport) WINAPI test(int x, char *result);
but I would like to know if what I am trying to do is possible.
I am using Access 2007 on Windows 7 (64-bit).

Answer 1

A char is the same as an integer. In VBA that means a long (as VBA uses 16 bit terminology). There are no strings involved. To convert an integer or char to a string use chr(x).