Personalize

Text Size

Layout

Navigation Position

Bruce Eitman

Windows CE Musings

<< Windows CE: How to buy Windows CE or Platform Builder | Home | Platform Builder: Why can’t I call the Function in my DLL? >>

Windows CE: Reading a String from the Registry

I made a change to a string value in the registry recently. That seemed like a harmless thing to do, didn’t it? But, what I did was make the string longer than it was before, again seemed harmless. Harmless until some applications started reading the value into arrays with hard coded length, the problems began.

The problem is that RegQueryValueEx() does not have a parameter to indicate the size of the buffer that it will put data into. This means that if the data is bigger than the buffer, RegQueryValueEx() will write past the buffer causing problems for your system. The good news is that you can use RegQueryValueEx() to discover the size of the buffer that is required, and then allocate a buffer that is large enough.

So the following code reads a string from the registry by getting the string length first, then allocating a buffer, then reading the string:

TCHAR *EventName = NULL;

DWORD Result;

HKEY hKey;

DWORD NumBytes = 0;

DWORD Type;

HANDLE UserEvent = INVALID_HANDLE_VALUE;

// Open the Registry Key

Result = RegOpenKeyEx(HKEY_LOCAL_MACHINE, (LPCWSTR)GWE_REG_PATH, 0, 0, &hKey);

if( ERROR_SUCCESS == Result )

{

// This is a fake read, all it does is fill in NumBytes with the number of

// bytes in the string value plus the null character.

Result = RegQueryValueEx( hKey, ACTIVITY_VALUE, NULL, &Type, NULL, &NumBytes );

if( NumBytes > 0 )

{

// Now we know how big the string is allocate and read it

EventName = (TCHAR *)malloc( NumBytes );

if( EventName != NULL )

Result = RegQueryValueEx( hKey, ACTIVITY_VALUE, NULL, &Type,

(LPBYTE)EventName, &NumBytes );

}

RegCloseKey( hKey );

UserEvent = CreateEvent( NULL, FALSE, FALSE, EventName );

free( EventName );

}

// Do something with UserEvent

You may also want to look at Summary of Registry Posts

Tags: REG Files

Copyright © 2009 – Bruce Eitman

All Rights Reserved

Share This Post:
Short Url: http://wblo.gs/Vwf

Wednesday, June 11, 2008 12:48 PM

Your Comments.

# re: Windows CE: Reading a String from the Registry
The high risk of a leak with this code is, IMO, a bad, bad thing. I'd refactor it to have the buffer passed in as a parameter and the buffer length as well. No point in propagating bad code into the wild.

http://blog.opennetcf.com/ctacke/2006/12/19/ReturningStringsFromCFunctions.aspx

Left by Chris Tacke at 6/12/2008 4:01 PM
# re: Windows CE: Reading a String from the Registry
Of course you are right Chris. I updated it to eliminate that.

Left by Bruce Eitman at 6/12/2008 6:21 PM
# re: Windows CE: Reading a String from the Registry
Apart from the string size issue that you pointed out here, reading strings from registry can lead to some funny bugs when porting "legacy" ASCII-based code from Win32 applications to CE.
The size of the buffer is in bytes (and that' correct, since RegQueryValueEx can be used to read different kind of data). In CE (and each time you use UNICODE on the desktop) each character is stored using two bytes. On ascii systems you have 1 char=1 byte and so you'll often find code that allocate memot using the value size in byte to allocate chars. In CE (and in UNICODE) this will allocate half the memory that is needed for the string.
This may be an easy bug to find (you'll see truncated strings and, maybe, even exceptions during the call to RegQueryValue).
Thing get interesting when someone defines a "maximum lenght" for those strings and passes that defined length as number of bytes. In ASCII everything works fine. On UNICODE everything works until someone (usually not the programmer testing it's own code and inputting "fdf" o "kjk" as string value) try to load a value that is more than half the maximum length.
There you get truncated string values and any sort of "funny" and inexplicable issues....

Left by Valter Minute at 12/16/2008 5:40 AM
# re: Windows CE: Reading a String from the Registry
I am developing smard device application using .net CF 1.0 and WindowsCE 5.0
I want to read registory value in my application. I dont know which namespace is required for this.
Please help on this

thanks in advance.......

Left by Rishi at 3/19/2009 6:26 AM
# re: Windows CE: Reading a String from the Registry
Rishi:

I assume that you are not a regular reader of my blog. Regular readers know that I am not a managed code developer, and in fact not an application developer.

I bet if you read the documentation that you can find similar functions within the Compact Framework. If not, then you certainly could P/Invoke the functions that I show here.

Bruce

Left by Bruce Eitman at 3/19/2009 7:15 AM

Your Reply.

Comment Form.

Fields denoted with a "*" are required.

*Your name:

*Subject:

You may also like to leave your email address.

*Your message:

Enter the code shown above

Preview Your Comment.