September 2015 | The Old New Thing

Why can’t I create a file equal to the available disk space?

Raymond ChenSeptember 30, 2015Sep 30, 201509/30/15

A customer was trying to exercise some boundary cases in their application, one of which entailed "out of disk space" scenarios. The way they did this was very simple: Call GetDiskFreeSpaceEx to see how much disk space is available, and then create a file of that size. This worked on some volumes, but on other volumes, it ...

The trust relationship between this workstation and the primary domain failed, what does this mean?

Raymond ChenSeptember 29, 2015Sep 29, 201509/29/15

Just like users, machines have domain accounts. And just like user accounts, machine accounts have passwords. And just like user account passwords, machine account passwords expire (by default, every 30 days). Now, you don't normally notice any of this because machines automatically change their machine account passwords before they expire. ...

What happens to lost timer messages if I don’t process them fast enough?

Raymond ChenSeptember 28, 2015Sep 28, 201509/28/15

Some time ago, I noted that if your WM_TIMER handler takes longer than the timer period, your queue will not fill up with WM_TIMER messages. The WM_TIMER message is generated on demand, and if your handler doesn't check for messages, then there is no demand for timer messages. But what happens when your thread finally starts processing ...

Why doesn’t my keyboard hook get called for keyboard messages I manually posted?

Raymond ChenSeptember 25, 2015Sep 25, 201509/25/15

A customer was generating synthesized keyboard input by posting WM_KEYDOWN and related messages to a window. They found that a WH_KEYBOARD keyboard hook was not triggering for these keyboard messages and they wanted to know why. You already know enough to answer this question, though the pieces are scattered about over several years for ...

How do I call SetTimer with a timer ID that is guaranteed not to conflict with any other timer ID?

Raymond ChenSeptember 24, 2015Sep 24, 201509/24/15

Suppose you want to create a timer for a window, but you don't know what timers that window already has running. You don't want your timer to overwrite any existing timer, so you need to find a unique ID for your timer. If this were for a window message, you could use RegisterWindowMessage, but there is no corresponding Register...

It rather involved being on the other side of this airtight hatchway: Elevation from Administrator to SYSTEM

Raymond ChenSeptember 23, 2015Sep 23, 201509/23/15

A security vulnerability report arrived that took the following form: I have discovered a critical security vulnerability in Windows which I intend to present at the XYZ conference. It allows any user with administrator privileges to perform operation Q, something that should be available only to SYSTEM. I think you know how this story ends...

Thanks for informing us that you know about the compatibility issue in your application, so we don’t have to fix it

Raymond ChenSeptember 22, 2015Sep 22, 201509/22/15

During the public prerelease period of a version of Windows some time ago, I remember a bug came in from an application developer. It went something like this: Hi, we just tried our program on the most recent public prerelease version of Windows, and we noticed that you changed the undocumented XYZ function. Our program uses that undocumented...

How can I tell if Windows Update is waiting for the system to reboot?

Raymond ChenSeptember 21, 2015Sep 21, 201509/21/15

Today's Little Program shows how to ask whether Windows Update is waiting for the system to reboot. You might want to check this in your installer, so that you don't try to install your program while the system is in a mixed state where an update is partially-installed. Testing this from script is easy. MSDN contains sample code to do that. ...

Why are there all these processes lingering near death, and what is keeping them alive?

Raymond ChenSeptember 18, 2015Sep 18, 201509/18/15

A customer (via their customer liaison) asked for assistance debugging a failure that occurs when their system remains up for several weeks. The customer seems to have a complicated system where they create and kill processes, and I am seeing hundreds of processes in the following state. This got me curious: Why are there so many of these ...

How can I start my service as soon as possible, before any other service?

Raymond ChenSeptember 17, 2015Sep 17, 201509/17/15

A customer wanted to know how they could make their service start before any other service. FIRST! Of course, a simple application of the What if two programs did this? principle shows that this is not possible. Upon closer questioning, the real problem was that they had a service which configured some piece of hardware, and they wanted to ...

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