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

**user65536** · July 10th, 2012, 02:19 AM

I have the same problem as this topic.

here is the DLL info.

Code:

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

Code:

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

Code:

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)

Code:

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?

**GremlinSA** · July 10th, 2012, 03:57 AM

[Split Topic]
Please Start your own thread, and you can add reference to older topic...

**GremlinSA** · July 10th, 2012, 04:16 AM

Code:

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

Code:

Private Type HASHINFO
   hash As String  '<--- Variable Length
   szPassword As String * 10 '<--- Fixed Length
   nPasswordLen As Long
   szSalt As String * 10  '<--- Fixed Length
   nSaltLen As Long
   szName As String * 10  '<--- Fixed Length
   nNameLen As Long
   dwFlags As Long
End Type

Made a few pointers here ... Notice what i'm pointing out...

Another one is ..
C++ Int = 4 bytes..
VB6 Long = 8 Bytes..

Your two data types need to match up exactly, so that the External C DLL can correctly parse out the info .

Code:

Private Sub Command1_Click()
Dim udt As HASHINFO
   udt.hash = vbNullString
   udt.szPassword = "test" & Chr(0)  ' <-- using 5 of the 10 allocated..
   udt.nNameLen = 4                ' <--- Wrong Variable used in here......
   udt.szSalt = "test" & Chr(0)  ' <-- using 5 of the 10 allocated..
   udt.nSaltLen = 4                ' <-- Saying Prev Char is only 4 long when its defined as 10 long..
   udt.szName = "test" & Chr(0) ' same here
   udt.nNameLen = 4                ' Same Here...
   udt.dwFlags = 0
Call GetHash(udt)
End Sub

this should help you fix it...

**user65536** · July 10th, 2012, 04:33 AM

Thanks for checking thigns out.

I'm pretty sure VB Long is 4 bytes, Double is 8 bytes.

I have tried this

Code:

Public Type HASHINFO
   pHash As String
   szPassword As String
   nPasswordLen As Long
   szSalt As String
   nSaltLen As Long
   szName As String
   nNameLen As Long
   dwFlags As Long
End Type

Public Declare Function GetHash Lib "C:\md5\md5.dll" (ByRef xx As HASHINFO) As Long

Call code:

Code:

Private Sub Command1_Click()
Dim test as long

Dim udt As HASHINFO

    udt.szPassword = "test"
    udt.nPasswordLen = 4
    udt.szSalt = "test"
    udt.nSaltLen = 4
    udt.szName = "test"
    udt.nNameLen = 4
    udt.dwFlags = &H0
    
test = GetHash(udt)

End Sub

I'm not sure if I need to specify something for udt.pHash, have tried various things and it still crashes.

**GremlinSA** · July 10th, 2012, 05:15 AM

Originally Posted by user65536

Thanks for checking thigns out.

I'm pretty sure VB Long is 4 bytes, Double is 8 bytes.

you actually are right.. VB.NET Long = 8 bytes in VB6 = 4 bytes.. Havn't done VB6 in a long time..

Originally Posted by user65536

I have tried this

Code:

Public Type HASHINFO
   pHash As String
   szPassword As String
   nPasswordLen As Long
   szSalt As String
   nSaltLen As Long
   szName As String
   nNameLen As Long
   dwFlags As Long
End Type

    Dim udt As HASHINFO

    udt.szPassword = "test"
    udt.nPasswordLen = 4
    udt.szSalt = "test"
    udt.nSaltLen = 4
    udt.szName = "test"
    udt.nNameLen = 4
    udt.dwFlags = &H0
    
test = GetHash(udt)

End Sub

I'm not sure if I need to specify something for udt.pHash, have tried various things and it still crashes.

You will need to assign the right amount of space for the Hash to be put in (32 Byte hash ?) as the C DLL cannot assign new/ more memory to the referenced Datatype.

You were on the right track with the Null (&H0) termination..

But something else i'm thinking is that VB stores stings in Unicode (2 bytes per char) and the char array in C is expecting one byte per char.. (this may need clarification from a C Expert) ...

in this case an array of byte may be better to use, and you load the ASCII val of each char in the string to the byte array..

You will have to play with it a little, if your Defs and Variable lengths are out by one, it will crash ...

try this for debugging

Code:

 Debug.print len(udt)

After you've assigned all the values and then see if it matches what you should have...

**WoF** · July 11th, 2012, 09:42 AM

In my opinion the strings in the UDT should either be of fixed length or, if dynamic, they must be initialized.
If you expect a string to be filled in by the dll yo must provide enough buffer space for the resulting string.
If udt.pHash is the string where you expect the result to be returned, you have to preallocate space like

Code:

Private Sub Command1_Click()
Dim test as long

Dim udt As HASHINFO
    udt.pHash = Space$(256) '<---- preallocate a buffer of 256 characters
    udt.szPassword = "test"
    udt.nPasswordLen = 4
    udt.szSalt = "test"
    udt.nSaltLen = 4
    udt.szName = "test"
    udt.nNameLen = 4
    udt.dwFlags = &H0
    
test = GetHash(udt)

End Sub

If this fails you try to declare pHash as a constant length string, like in your first version you did with the other strings

Code:

Public Type HASHINFO
   pHash As String * 256
   ....

The length of the string depends on how many characters you expect to be returned.

**user65536** · July 11th, 2012, 07:44 PM

in order to determine how many bytes pHash is suppose to be buffered the dll call was attempted in c.

It appears that only

szPassword
nPasswordLen

are the necessary inputs

while pHash returns a Byte Array of (16 bytes), this byte array needs to be then converted to hex to give the full 32byte hexstring of the hash.

The VB Code was then modified to reflect this.

Code:

Private Sub Command1_Click()
Dim test as long

Dim udt As HASHINFO
    udt.pHash = Space$(16) 
    udt.szPassword = "test"
    udt.nPasswordLen = 4

test = GetHash(udt)

End sub

Code:

Public Type HASHINFO
   pHash As String * 16
   szPassword As String * 4
   nPasswordLen As Long
End Type

Public Declare Function GetHash Lib "C:\md5\md5.dll" (ByRef xx As HASHINFO) As Long

Too bad it still crashes.

**WizBang** · July 13th, 2012, 08:40 AM

Originally Posted by user65536

here is the DLL info.

Code:

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

Code:

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

Code:

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)

Code:

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.

**user65536** · July 13th, 2012, 09:14 AM

Ok since i can use StrPrt and VarPtr to pass references I may be able to pass PHash as long. So this is what is tried.

According to: http://www.xtremevbtalk.com/showthread.php?t=230453 I must pass with VtrPtr rather than StrPtr to return pointer of the BSTR so this is what I tried.

Call Code:

Code:

Private Sub Command1_Click()

testString = Space$(16)

Dim udt As HASHINFO
    
    udt.pHash = VarPtr(testString)
    udt.szPassword = "test"
    udt.nPasswordLen = 4
    
test = GetHash(udt)

MsgBox udt.pHash
End Sub

Code:

Public Type HASHINFO
   pHash As Long
   szPassword As String
   nPasswordLen As Long
End Type

Public Declare Function GetHash Lib "C:\md5\md5.dll" (ByRef udt As HASHINFO) As Long

Public testString As String * 16

Crash.

Looks like i'm following everything correctly.

-Buffered string to 16 bytes which I know is the return size
-Passing the references of the string as long from VB
-Only passing requires inputs for DLL others were omitted

I guess this dll probably just isn't callable from VB6

**dglienna** · July 13th, 2012, 10:55 AM

As I've said. Pointers can rarely be used from C++ (unless it was specifically written to pass a pointer AND a string)

**GremlinSA** · July 13th, 2012, 11:30 AM

Here are some hints to match up a UDT, from C to VB6.....

Use Array of Byte instead of Strings, to match array of char ( VB uses Unicode to store a string, 2 Bytes per Char + 2 bytes for length)

Match the Length Variable Exactly to the length of the matching Byte array.

use something along the lines of...

Code:

    udt.nPasswordLen = length(udt.szPassword)

Do not forget to add a Null terminator to the last position in the variable been used for strings...

**WizBang** · July 14th, 2012, 05:01 AM

If you didn't change the declaration of the UDT so that pHash is a Long instead of a String, then it'll still have the same result. And again, it follows that the others would need to be the same.

I may just get curious enough to try some stuff with it myself.

**user65536** · July 14th, 2012, 05:09 AM

MD.zip

Well for anyone who wants to give this a shot.

Here are the files. You will need to move the dll into the correct area.

Thanks

**GremlinSA** · July 14th, 2012, 01:40 PM

Played with it a little..

and got a little news for ya..

The Dll is not properly compiled for VB calling... (see this thread on VB forums for what i mean)

Right so this is the story...
The proper UDT you need is like this..

Code:

Public Type HASHINFO
   pHash() As Byte
   szPassword() As Byte
   nPasswordLen As Long
   szSalt() As Byte
   nSaltLen As Long
   szName() As Byte
   nNameLen As Long
   dwFlags As Long
End Type

and to use it your code is like this...

Code:

Dim udt As HASHINFO
    ReDim udt.pHash(16)
    ReDim udt.szName(1)
    ReDim udt.szPassword(5)
    ReDim udt.szSalt(1)
    
    udt.nNameLen = 1
    udt.szName(0) = 0
    udt.nSaltLen = 1
    udt.szSalt(0) = 0
    udt.nPasswordLen = 5
    udt.szPassword(0) = Asc("t")
    udt.szPassword(1) = Asc("e")
    udt.szPassword(2) = Asc("s")
    udt.szPassword(3) = Asc("t")
    udt.szPassword(4) = 0

Problem is now you get a Error 49 - Bad DLL calling convention..

**WoF** · July 18th, 2012, 07:14 AM

If the calling convention is bad (no STDCALL) then also the string-constant versions of the UDT might work, as long as the calling convention is adapted or changed.