Microsoft KB Archive/202179

= How To Call Windows API Functions with Special Requirements from Visual Basic =

Article ID: 202179

Article Last Modified on 7/15/2004

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APPLIES TO


 * Microsoft Visual Basic 5.0 Learning Edition
 * Microsoft Visual Basic 6.0 Learning Edition
 * Microsoft Visual Basic 5.0 Professional Edition
 * Microsoft Visual Basic 6.0 Professional Edition
 * Microsoft Visual Basic 5.0 Enterprise Edition
 * Microsoft Visual Basic 6.0 Enterprise Edition

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This article was previously published under Q202179



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
 Start a new Standard EXE project in Visual Basic. Form1 is created by default. Add a CommandButton control (Command1) to Form1.  In the Command1_Click event, add the following line: MySample  From the Project menu, add a new .bas module.  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  Save the project. Press the F5 key to run the project.</li> Click Command1 to test the code.</li></ol>

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.

Keywords: kbhowto kbapi KB202179

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