Article: Q202179
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:5.0,6.0
Operating System(s):
Keyword(s): kbAPI kbSDKWin32 kbVBp500 kbVBp600 kbGrpDSVB kbDSupport
Last Modified: 11-JAN-2001
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The information in this article applies to:
- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0
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SUMMARY
=======
Some Microsoft Windows API functions require an extra workaround when they are
called from Visual Basic because Visual Basic cannot handle pointer type
variables. This behavior is very common when you call Microsoft Windows NT
security and printer related APIs. This article discusses the following cases:
- Case 1 - A function that returns a variable-length structure.
- Case 2 - A function that returns a structure by using a second level of
indirection (requiring a pointer to a pointer as a parameter).
- Case 3 - Using a structure returned with a second level of indirection.
MORE INFORMATION
================
This article provides some sample code that uses Windows NT security APIs to
illustrate the preceding three cases. A Windows NT computer is required to test
the sample.
Step-by-Step Example
--------------------
1. Start a new Standard EXE project in Visual Basic. Form1 is created by
default.
2. Add a CommandButton control (Command1) to Form1.
3. In the Command1_Click event, add the following line:
MySample
4. From the Project menu, add a new .bas module.
5. Paste the following code into the module:
Option Explicit
' structures
Type ACL_SIZE_INFORMATION
AceCount As Long
AclBytesInUse As Long
AclBytesFree As Long
End Type
Type ACE_HEADER
AceType As Byte
AceFlags As Byte
AceSize As Integer
End Type
' constants
Public Const ERROR_SUCCESS = 0&
Public Const ERROR_INSUFFICIENT_BUFFER = 122 ' dderror
Public Const HKEY_CLASSES_ROOT = &H80000000
Public Const FORMAT_MESSAGE_FROM_SYSTEM = &H1000
Public Const DACL_SECURITY_INFORMATION = &H4&
Public Const AclSizeInformation = 2 ' from the ACL_INFORMATION_CLASS enum
' API function declarations
Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" _
(lpDest As Any, lpSrc As Any, ByVal Length As Long)
Declare Function RegGetKeySecurity Lib "advapi32.dll" _
(ByVal hKey As Long, ByVal SecurityInformation As Long, _
pSecurityDescriptor As Any, lpcbSecurityDescriptor As Long) As Long
Declare Function FormatMessage Lib "kernel32" Alias "FormatMessageA" _
(ByVal dwFlags As Long, lpSource As Any, ByVal dwMessageId As Long, _
ByVal dwLanguageId As Long, ByVal lpBuffer As String, _
ByVal nSize As Long, Arguments As Long) As Long
Declare Function GetSecurityDescriptorDacl Lib "advapi32.dll" _
(pSecurityDescriptor As Any, lpbDaclPresent As Long, pDacl As Long, _
lpbDaclDefaulted As Long) As Long
Declare Function GetAclInformation Lib "advapi32.dll" (pDacl As Any, _
pAclInformation As Any, ByVal nAclInformationLength As Long, _
ByVal dwAclInformationClass As Integer) As Long
Declare Function GetAce Lib "advapi32.dll" (pDacl As Any, _
ByVal dwAceIndex As Long, pAce As Any) As Long
Sub MySample()
Dim lErrorCode As Long
Dim lSdSize As Long
Dim bDaclExist As Long, bDaclPresent As Long ' booleans returned in API's
Dim pDacl As Long ' to store the address of a DACL
Dim pAce As Long ' to store the address of a ACE
Dim i As Long
Dim SecurityDescriptor() As Byte
Dim aclSizeInfo As ACL_SIZE_INFORMATION
Dim AceHeader As ACE_HEADER
'
' CASE 1
'
' initializing the buffer with a very low size
lSdSize = 0
ReDim SecurityDescriptor(lSdSize)
' first call is basically only to find out the required buffer size
lErrorCode = RegGetKeySecurity(HKEY_CLASSES_ROOT, _
DACL_SECURITY_INFORMATION, SecurityDescriptor(0), lSdSize)
If lErrorCode = ERROR_INSUFFICIENT_BUFFER Then
' redimensioning the buffer and calling the function again
' the lSdSize returned the required size from the previous call
ReDim SecurityDescriptor(lSdSize)
lErrorCode = RegGetKeySecurity(HKEY_CLASSES_ROOT, _
DACL_SECURITY_INFORMATION, SecurityDescriptor(0), lSdSize)
End If
' display message error if not successful
If lErrorCode <> ERROR_SUCCESS Then
DisplayError lErrorCode, "RegGetKeySecurity"
Exit Sub
End If
'
' CASE 2
'
' get a pointer (pDacl) to the discretionary access-control list (ACL)
' pDacl was declared as a variable of type Long and will store the
' address of the DACL list
lErrorCode = GetSecurityDescriptorDacl(SecurityDescriptor(0), _
bDaclPresent, pDacl, bDaclExist)
If lErrorCode = 0 Then
lErrorCode = Err.LastDllError
DisplayError lErrorCode, "GetSecurityDescriptorDacl"
Exit Sub
End If
If pDacl = 0 Then
MsgBox "Key has a NULL DACL"
Exit Sub
End If
' retrieving DACL's information; information is returned in the
' aclSizeInfo structure
lErrorCode = GetAclInformation(ByVal pDacl, aclSizeInfo, _
Len(aclSizeInfo), AclSizeInformation)
If lErrorCode = 0 Then
lErrorCode = Err.LastDllError
DisplayError lErrorCode, "GetAclInformation"
Exit Sub
End If
'
' if Dacl is present, get ACE's information
' for each ACE in the DACL list we are going to display the ACE's size
'
If bDaclPresent Then
MsgBox "DACL contains " & aclSizeInfo.AceCount & " ACEs"
If aclSizeInfo.AceCount > 0 Then
For i = 0 To aclSizeInfo.AceCount - 1
' The GetAce function obtains a pointer to an ACE in an ACL
' GetAce expects a reference to DACL in the first
' parameter, thus we pass it ByVal
' GetAce returns the address of an ACE in the second
' parameter, thus we pass pAce ByRef
' pAce was declared as a variable of type Long and will
' store the address of an ACE
lErrorCode = GetAce(ByVal pDacl, i, pAce)
If lErrorCode = 0 Then
lErrorCode = Err.LastDllError
DisplayError lErrorCode, "GetAce"
Else
' copying the memory block pointed by pAce to the
' ACE_HEADER structure
' pAce stores the address of an ACE; we want this
' address to be passed
' to the CopyMemory function, thus we pass this
' parameter ByVal.
CopyMemory AceHeader, ByVal pAce, Len(AceHeader)
' use the AceHeader variable to access structure members
MsgBox "Size of ACE(" & i + 1 & ") is: " _
& AceHeader.AceSize
End If
Next i
End If
End If
End Sub
Sub DisplayError(ByVal dwError As Long, RelatedApi As String)
Dim ErrorMsg As String, SysMsg As String
Dim MsgSize As Long
' get the error's description
If dwError <> 0 Then
MsgSize = 1000
SysMsg = Space(MsgSize)
MsgSize = FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM, ByVal 0&, _
dwError, 0, SysMsg, MsgSize, ByVal 0&)
' function returns number of characters in string; 0=function failed
If MsgSize = 0 Then
SysMsg = "System error code: " & Str$(dwError)
Else
' resizing the string for output
SysMsg = Left$(SysMsg, MsgSize)
End If
Else
SysMsg = ""
End If
' including additional information in the string
ErrorMsg = "ErrorCode: " & Str$(dwError) & vbCrLf & "API: " _
& RelatedApi & vbCrLf & "System error: " & SysMsg
MsgBox ErrorMsg
End Sub
6. Save the project.
7. Press the F5 key to run the project.
8. Click Command1 to test the code.
Case 1 - A Function Returns a Variable-Length Structure:
For a function to return a structure with variable length, you need to pass to
the function a pointer to a memory buffer large enough for the structure to be
stored. Usually, functions that return a structure with variable length provide
a way of finding out the size of the memory buffer to allocate. This is what the
RegGetKeySecurity function illustrates in this case. The RegGetKeySecurity
function retrieves a copy of the security descriptor of a registry key, which is
a variable size structure. The third parameter is a memory buffer that is used
to store the structure itself, and the fourth parameter is used as an in-and-out
parameter. When you call the function, the fourth parameter contains the size of
the allocated buffer. If this size is not large enough to store the structure,
then the function fails, returns the following error message
ERROR_INSUFFICIENT_BUFFER
and uses the fourth parameter to return the size required for the structure. This
gives you a chance to allocate a larger buffer and call the function again.
One important point to consider is the declaration of the function. You should
always check the function declaration defined in the API Text Viewer and change
it according to the way you are calling the function. In this case, the
declaration is changed to:
Declare Function RegGetKeySecurity Lib "advapi32.dll" _
(ByVal hKey As Long, ByVal SecurityInformation As Long, _
pSecurityDescriptor As Any, lpcbSecurityDescriptor As Long) As Long
Note that the third parameter is declared as pSecurityDescriptor As Any instead
of being declared as a structure. This occurs because you should pass a
reference to a memory buffer instead of a reference to the structure itself. In
this case, the memory buffer is the SecurityDescriptor array that is an array of
Bytes. You need to pass the memory buffer to the function by passing the first
element of the array (SecurityDescriptor(0)) by reference.
Case 2 - A Function Returns a Structure by Using a Second Level of Indirection (Requiring a Pointer to a Pointer as a Parameter):
When a function requires a pointer to a pointer as an output parameter, this is
the same as returning the address of a variable. Just as an address can be
stored in a 4-bytes integer (a type Long in Visual Basic), you can replace this
parameter with a Long parameter passed by reference.
The most common situation when a function returns an address of a structure is
when the function allocates the memory for the structure or when the function
just retrieves the position of a given structure from an already allocated
buffer. In the preceding sample, the GetSecurityDescriptorDacl function is used
to retrieve a pointer to the discretionary access control list (DACL) from the
security descriptor. Retrieving a pointer means retrieving an address, and to
store an address you need a variable of type Long in Visual Basic. Again, it's
very important to use the right function declaration. Note that the declaration
used in the preceding sample declares the third parameter, the one that
retrieves the pointer as a Long by reference.
Declare Function GetSecurityDescriptorDacl Lib "advapi32.dll" _
(pSecurityDescriptor As Any, lpbDaclPresent As Long, pDacl As Long, _
lpbDaclDefaulted As Long) As Long
Case 3 - Using a Structure Returned with a Second Level of Indirection:
Once you retrieve the pointer of a structure from a function, as in Case 2, you
can face two types of problems. The first problem is how to pass this structure
to another function which is expecting to receive the structure by reference.
The second problem is the need to access elements of this structure directly in
Visual Basic. Both problems can be solved by using the GetAce function. The
GetAce function is used to retrieve the pointer of an ACE from a DACL list. The
first parameter expects a reference to a DACL (in other words, the address of a
DACL), and the second parameter returns the address of an ACE. The address of
the DACL has been retrieved by the GetSecurityDescriptorDacl function and is
stored in the pDacl variable. Because the GetAce function is expecting a
reference to a DACL, you can just pass the pDacl parameter by value. Note the
difference here; when you retrieve pDacl in the GetSecurityDescriptorDacl , you
passed pDacl by reference, and now you are passing it by value. This solves the
first problem, which is to pass a structure retrieved by a second level of
indirection to another function.
Take a look at the second parameter of GetAce, which returns the address of an
ACE structure. As in Case 2, you are using a variable of type Long (pAce) to
retrieve the structure's address. However, you cannot do anything with this
variable in Visual Basic if you need to access members of this structure. Unlike
C, Visual Basic doesn't provide you with any features to manipulate pointers.
The workaround here is to allocate another variable of type UDT (user defined
type) with the structure type you need, and copy into it the content of the
structure you are trying to access. In the preceding sample, the type was
declared as ACE_HEADER, and the variable was defined as AceHeader. You can copy
the content of the structure pointed by pAce to AceHeader by using the
CopyMemory function. Now you can access the members of the structure using the
AceHeader variable. Note that pAce is passed ByVal to CopyMemory.
General Comments:
1. The RtlMoveMemory function (usually declared as CopyMemory) is a Win32 API.
It is not documented in the API Text Viewer, but is documented in MSDN.
2. Always check the declarations for these types of functions carefully. A wrong
declaration may generate an exception.
3. Always check the declaration of structures used in these types of functions
carefully.
Additional query words:
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Keywords : kbAPI kbSDKWin32 kbVBp500 kbVBp600 kbGrpDSVB kbDSupport
Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVBA600 kbVB500 kbVB600
Version : WINDOWS:5.0,6.0
Issue type : kbhowto
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