Tuesday, August 26, 2008

new with empty bracket

Have you ever wrote a statement like..

int *pnValue = new int[];

One of my friend asked me what will happen if the above statement is executed. Executed ??? I though the code won't even compile. Surprisingly it compile and even returned a pointer. Wow that was some thing unbelievable.

OK now the question is, what will be size of memory that pnValue points?

The two API's in windows that allocates and de allocates the memeory are HeapAlloc and HeepFree. The CRT functions malloc and free are actually wrappers the above API. The new, new[], delete, delete[] are again another wrapper around the malloc and free. So when ever you allocates some memory using new or new[], it will finally reach the HeapAlloc function. This function is defined as

__in HANDLE hHeap,
__in DWORD dwFlags,
__in SIZE_T dwBytes

From the above definition, you can see that the third parameter to this function is the number of BYTES to be allocated . So to find out what "new int[]" returns, we can put a break point in the entry point of HeapAlloc and check the value of dwBytes( in dis assembly ).

when I tried, the dwBytes turned out be 1 !!! This one byte cannot even hold one int variable. That means any further operation using such a pointer will possibly crash.

Another interesting thing is "int *pnValue = new int[0];" also returns a pointer pointing a memory of 1 byte long.

Wednesday, August 20, 2008

How to open Popup Blocker Settings window of IE programmatically

How to open Popup Blocker Settings window of IE programmatically??

Well it is with the help of "DisplayPopupWindowManagementDialog" function in the "C:\WINDOWS\system32\inetcpl.cpl".

But here rather just showing a misterious function, I would like to explain the methods by which I found out the name of function.

This question was asked by some one in the codeproject and at that time I was also unaware of any function for this. How ever I felt it interesting so just decided to have a try.

At first I took this "Popup Blocker Settings " dialog in the IE. My first intention was to find out the dll name which created the dialog. There is an application called WinID[^] with which we can find which dll created a perticular windows. In one of my previous post, I have explained How to find which dll / exe created a window.[^] using code. Below is a screenshot of what WinID showed me for the "Popup Blocker Settings" dialog .

It told me that "USER32.dll" has created the dialog. Liar... I was much sure that user32.dll isn't the real dll behind it. So this technique to find the dll didn't really work. So I had to find another way...

Every window has a thread associated with it. If you have the window handle, you can easily retirive the thread Id associated with it using the GetWindowThreadProcessId function. Any way I am not going to code for this. Tools like spy++, WinID displays the thread id of the window if we select a window using it. So the Thread ID displayed in Spy++ was 0xB94( 2964 in decimal ).

If we took the properties of a process in the process explorer, it will list all the threads in the process and each thread's call stack. So the thing next to do is to select the iexplorer process, select the thread with ID 2964 and took its call stack.

And in the call stack, there was only one un familiar dll and function the "inetcpl.cpl" and the DisplayPopupWindowManagementDialog function in it. So I found out the "inetcpl.cpl" file in the system32 folder and when double clicked on it, huiii it shows the "internet options" dialog.

The control panel files (.cpl ), are just a dll with a some special export functions( If you want to know more about control panel files I suggest you to read Paul DiLascia's Q&A[^] article ). With the dependency walker, I confirmed the presence of DisplayPopupWindowManagementDialog in it. But to call this function, I need to the find how it is declared, what all are the arguments etc. Fortunately some else has already document the function proto and with a google search, I was able to find it out. Happy ending!!!!

And ho ya here is the sample code..

typedef BOOL (WINAPI *DisplayPopupWindowP)( HANDLE hWnd, LPCTSTR lpCaption );
void CDialogBasedDlg::OnBnClickedButton1()
HMODULE hModule = LoadLibrary( _T("inetcpl.cpl") );
DisplayPopupWindowP DisplayPopupWindowManagementDialog =
( DisplayPopupWindowP)GetProcAddress(hModule,
"DisplayPopupWindowManagementDialog" );
if( !DisplayPopupWindowManagementDialog )
DisplayPopupWindowManagementDialog( m_hWnd,

Sunday, August 17, 2008

GetFinalPathNameByHandle API Hungs

GetFinalPathNameByHandle is an new API introduced in windows vista. As you can read from MSDN, the purpose of this API is to return the file name corresponding to a file handle .

Some months back, I have written a small utility OpenedFileFinder which list all the file that are opened under a certain folder. While writing this application,the main problem I came across was to get the file name from the handle. Initially I used a function called NtQueryObject(). But the problem with this API is that if we try to query the details for certain object such as pipes, directory handles etc, it will hangs. So in my application finally I have to introduce a driver to get round of this hanging problem. When I found the new GetFinalPathNameByHandle(), I was so happy and I thought at least I can avoid the driver in the vista. So I had a try on this API. But I came to find that this API also have the same hanging problem which I described earlier.

It seems the GetFinalPathNameByHandle is also using the same NtQueryObject() to get the file name and so it hangs. Any how, I have reported this bug to the microsoft connect. Let us see how they solve the problem.

Another API, that worth mentioning here is GetFileInformationByHandleEx(), which is also a new API introduced in Vista. Using this API also, I can get the file name from a handle. So I decided to check whether the same problem exists in this API also. Surprisingly it worked fine. And in MSDN also it is mentioned that it uses a driver for attaining the task. So I highly suggest every one to use GetFileInformationByHandleEx() API instead of GetFinalPathNameByHandle().