Re: Passing UDT from VB 6.0 to C++ DLL

[WizBang]

here is the DLL info.

extern "C" __declspec(dllexport) int GetHash(HASHINFO*);

typedef struct {
        unsigned char* pHash;      // Address of buffer for saving hash
        unsigned char* szPassword; // Password
        int nPasswordLen;          // Password length
        unsigned char* szSalt;     // Salt
        int nSaltLen;              // Salt length
        unsigned char* szName;     // User name
        int nNameLen;              // User name length
        DWORD dwFlags;             // Flags
} HASHINFO;

Here is the VB

Private Type HASHINFO
   hash As String
   szPassword As String * 10
   nPasswordLen As Long
   szSalt As String * 10
   nSaltLen As Long
   szName As String * 10
   nNameLen As Long
   dwFlags As Long
End Type

Private Declare Sub GetHash Lib "c:\md5\md5.dll" (ByRef pudt As HASHINFO)

Private Sub Command1_Click()
Dim udt As HASHINFO
   udt.hash = vbNullString
   udt.szPassword = "test" & Chr(0)
   udt.nNameLen = 4
   udt.szSalt = "test" & Chr(0)
   udt.nSaltLen = 4
   udt.szName = "test" & Chr(0)
   udt.nNameLen = 4
   udt.dwFlags = 0
Call GetHash(udt)
End Sub

I just get a crash.

Anyone out there to help me?

What jumps out at me first is that the DLL UDT structure given above says (for the hash string) "Address of buffer for saving hash". That to me means it's a reference to the memory where the string is stored, not the actual string itself. If true, then might the VB UDT structure use a Long, not a string? Whichever it is, the other string members would be the same, no?

So basically, if the hash member and the string members are of the same type, you can't make them different in VB. They either have to both be fixed length strings, or variable length strings.

Incidentally, VB pads fixed length strings with zeros (Chr$(0) or vbNullChar) if you don't assign all the available characters. The value returned by Len(TheUDT) will always be the length of the structure in memory. When fixed length strings are defined in a UDT, the actual string is stored in the space allocated for the structure, thus will be included in the value returned by Len. On the other hand, variable length strings are not stored within the space in memory for the structure, because the string member is a reference to the location of the string, not the actual string.