August 2015 | The Old New Thing

How do I enumerate remembered connections that are not currently connected?

Raymond ChenAugust 31, 2015Aug 31, 201508/31/15

Harry Johnston wanted to know how to get a list of remembered (but not currently connected) drive mappings. The idea here is to make a tweak to the Little Program. Start with what we had and make these changes: This changes the program from enumerating connected resources to enumerating remembered resources. The last step is to skip the ...

Debugging walkthrough: Access violation on nonsense instruction, episode 3

Raymond ChenAugust 28, 2015Aug 28, 201508/28/15

A colleague of mine asked for help debugging a strange failure. Execution halted on what appeared to be a nonsense instruction. This is clearly an invalid instruction. But observe that the offset is +2, which is normally the start of the function, because the first two bytes of Windows operating system functions are a mov edi, edi instruction...

The Windows 95 I/O system assumed that if it wrote a byte, then it could read it back

Raymond ChenAugust 27, 2015Aug 27, 201508/27/15

In Windows 95, compressed data was read off the disk in three steps. The raw compressed data was read into a temporary buffer. The compressed data was uncompressed into a second temporary buffer. The uncompressed data was copied to the application-provided I/O buffer. But you could save a step if the I/O buffer was a full cluster: ...

Rules can exist not because there’s a problem, but in order to prevent future problems

Raymond ChenAugust 26, 2015Aug 26, 201508/26/15

I lost the link, but one commenter noted that the ReadFile function documentation says Applications must not read from, write to, reallocate, or free the input buffer that a read operation is using until the read operation completes. The commenter noted, "What is the point of the rule that disallows reading from or writing to the input ...

Microspeak: DRI, the designated response individual

Raymond ChenAugust 25, 2015Aug 25, 201508/25/15

Someone sent a message to a peer-to-peer discussion group and remarked, "This is critical. I'm a DRI at the moment and have some issues to fix." The term DRI was new to many people on the mailing list (including me), and while others on the mailing list helped to solve the person's problem, I also learned that DRI stands for designated ...

Insightful graph: The ship date predictor

Raymond ChenAugust 24, 2015Aug 24, 201508/24/15

The best graphs are the ones that require no explanation. You are just told what the x- and y-axes represent, and the answer just jumps out at you. One of the greatest graphs I've seen at Microsoft is this one that a colleague of mine put together as Windows 95 was nearing completion. He took each email message from management that ...

Handy delegate shortcut hides important details: The hidden delegate

Raymond ChenAugust 21, 2015Aug 21, 201508/21/15

One of my colleagues was having trouble with a little tool he wrote. I installed a low-level keyboard hook following the code in this article, but it crashes randomly. Here's what I know so far: I spawn a new STA thread to register the hook, so that it can run a message pump, which is a requirement for low-level hooks. After setting ...

I saw a pinvoke signature that passed a UInt64 instead of a FILETIME, what’s up with that?

Raymond ChenAugust 20, 2015Aug 20, 201508/20/15

A customer had a question about a pinvoke signature that used a UInt64 to hold a FILETIME structure. Is this legal? The documentation for FILETIME says Do not cast a pointer to a FILETIME structure to either a ULARGE_INTEGER* or __int64* value because it can cause alignment faults on 64-bit Windows. Are we guilty of this cast in the above ...

If you are going to call Marshal.GetLastWin32Error, the function whose error you’re retrieving had better be the one called most recently

Raymond ChenAugust 19, 2015Aug 19, 201508/19/15

Even if you remember to set SetLastError=true in your p/invoke signature, you still have to be careful with Marshal.GetLastWin32Error because there is only one last-error code, and it gets overwritten each time. So let's try this program: The expectation is that the call to OpenIcon will fail, and the error code...

If you are going to call Marshal.GetLastWin32Error, the function whose error you’re retrieving had better have SetLastError=true

Raymond ChenAugust 18, 2015Aug 18, 201508/18/15

A customer reported that their p/invoke to a custom DLL was failing, and the error code made no sense. The customer reported that their Fribble function was returning −1, indicating a failure to open the event, but the error code returned by Marshal.GetLastWin32Error is 87, "The parameter is incorrect." But all of the ...

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