Windows API - Win32
A core set of Windows application programming interfaces (APIs) for desktop and server applications. Previously known as Win32 API.
2,651 questions
BackupWrite sets lpContext to 0xFFFF.... when called with nNumberOfBytesToWrite=0 in versions after 2022
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Anatol Piotrowski
0
Reputation points
Seems like behavior of BackupWrite (https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-backupwrite) changed after Server 2022.
In versions windows 10.0.20348.2461 and before, BackupWrite when called with nNumberOfBytesToWrite=0 does not change lpContext, it returns 1 and GetLastError() returns 0
However in newer windows versions, when BackupWrite is called with nNumberOfBytesToWrite=0
- After BackupWrite call, lpContext is changed to 0xFFFFFFF...
- BackupWrite still returns 1 and GetLastError() returns 0
- This affects WinPE (versions 10.0.22621.1, 10.0.26100.1) as well as Server 2025 preview (version 10.0.26100.560)
This is definitely a change in behavior. I do not see it documented.
Is this intended behavior?
Since file has multiple streams, when code tries to write next stream with BackupWrite, it fails due to invalid lpContext. No error is returned, GetLastError() is 0, but lpNumberOfBytesWritten is 0.
I can reproduce it every time when using BackupWrite to restore data of c:\ProgramData\Microsoft\Network\Downloader\qmgr.db
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David Lowndes 2,545 Reputation points MVP2024-06-11T15:48:51.84+00:00 Given that the documentation doesn't say specifically that lpContext shouldn't change (as far as I can see), I have to wonder why you're calling it with a 0, and how does this change in behaviour affect your code?
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Jeanine Zhang-MSFT 9,771 Reputation points Microsoft Vendor2024-06-12T02:11:59.0366667+00:00 Due to the lack of instructions in the documentation, it seems to be a documentation issue.
I suggest you could feedback the issue base on the document link.
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Anatol Piotrowski 0 Reputation points
2024-06-12T13:11:11.3933333+00:00 Answering "How does this change in behavior affect your code?
Since file has multiple streams, when code tries to write next stream with BackupWrite, it fails due to invalid lpContext. No error is returned, GetLastError() is 0, but lpNumberOfBytesWritten is 0.
If I skip calling BackupWrite when nNumberOfBytesToWrite=0, there is no problem.
Other streams get written BackupWrite without errors. -
David Lowndes 2,545 Reputation points MVP
2024-06-12T13:16:29.62+00:00 I guess you have your work-around then.
As Jeanine suggested, perhaps add feedback to the documentation to note this change in behaviour?
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