September 2017

Why does my thread handle suddenly go bad? All I did was wait on it!

Raymond Chen September 29, 2017 Sep 29, 2017 09/29/17

A customer reported that they had a very strange bug, where waiting on a thread handle causes it to become invalid. Here's a code fragment: That final call to CloseHandle results in a STATUS_INVALID_HANDLE exception when run in the debugger. How did the handle become invalid? We sucessfully waited on it just a few lines earlier. There ...

CancelIoEx can cancel synchronous I/O, which is kind of nice

Raymond Chen September 28, 2017 Sep 28, 2017 09/28/17

The CancelIoEx function can be used to cancel synchronous I/O. This is handy if you have a program that processes a file in large chunks and you want to give it a Cancel button. Without CancelIoEx, you would either have to accept that the program won't respond to the Cancel button until the large I/O is complete, or you ...

How to check if a pointer is in a range of memory

Raymond Chen September 27, 2017 Sep 27, 2017 09/27/17

Suppose you have a range of memory described by two variables, say, And suppose you want to check whether a pointers lies within that region. You might be tempted to write but is this actually guaranteed according to the standard? The relevant portion of the C standard (6.5.8 Relational Operators)¹ says If two pointers to object or ...

Who implemented the Windows NT blue screen of death?

Raymond Chen September 26, 2017 Sep 26, 2017 09/26/17

Following up on the history of the Windows 3.1 and Windows 95 blue screen messages, John Vert admits that he wrote the Windows NT blue screen of death. In particular, he explains why it was white text on a blue background. (That it happened to match the Windows 3.1 and Windows 95 color scheme was a coincidence.) As I recall, it was John Vert ...

What happens if I wake a condition variable when nobody is waiting for it? Is the wake saved for the next thread that waits?

Raymond Chen September 25, 2017 Sep 25, 2017 09/25/17

Suppose you call WakeXxxConditionVariable to wake a CONDITION_VARIABLE, but nobody is waiting on the condition variable. What happens? In particular, is the wake saved for the next thread that waits, so the next call to SleepConditionVariableXxx returns immediately instead of waiting? The answer is, "It shouldn't...

What does it mean when I get an access violation at a very low address when entering a critical section?

Raymond Chen September 22, 2017 Sep 22, 2017 09/22/17

Warning: This article talks about implementation details which can change at any time. The information provided is for debugging and diagnostic purposes only. A customer found that their server program occasionally crashes in the internal function RtlpWaitOnCriticalSection trying to dereference the address 0x00000014. The...

How accurate are the various Windows time-querying functions?

Raymond Chen September 21, 2017 Sep 21, 2017 09/21/17

Windows has a bunch of time-querying functions. One group of functions uses the system performance counter. These are as accurate as the hardware allows while still conforming to the basic requirements (such as running at a constant speed and being consistent among multiple processors). Another group uses the system timer, which usually ...

What is the correct way of using SaveDC and RestoreDC?

Raymond Chen September 20, 2017 Sep 20, 2017 09/20/17

There are these two functions SaveDC and RestoreDC. How do they work? Each device context (DC) maintains a stack of saved states. When you call SaveDC, the current state of the DC is saved and pushed onto the stack, and you get a positive integer representing that saved state. If you call SaveDC again, a new saved state is created and pushed ...

The NET HELPMSG command will decode Windows error codes, at least the simple ones

Raymond Chen September 19, 2017 Sep 19, 2017 09/19/17

A quick and dirty way to convert Windows error codes to strings is to use the NET HELPMSG command. The command is not infallible, however. If you pass a message with an insertion, such as ERROR_WRONG_DISK, then instead of getting you get¹ Still, it's better than nothing. ¹ As a bonus insult, if you type the suggested command...

The increasingly complex Kremlinology surrounding Windows

Raymond Chen September 18, 2017 Sep 18, 2017 09/18/17

(If you prefer videos, here's a video version of basically the same question.) Everybody wants to know what the Windows team is up to, and the lack of official information won't stop them. As an example of the extreme Kremlinology surrounding any future version of Windows, when Steve Ballmer in May 2011 referred to the next generation of ...

The Old New Thing

September 2017 - The Old New Thing