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"No enumerators available" - on Windows 10 #30

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feerrenrut opened this issue Apr 11, 2020 · 4 comments · May be fixed by #32
Open

"No enumerators available" - on Windows 10 #30

feerrenrut opened this issue Apr 11, 2020 · 4 comments · May be fixed by #32
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help wanted windows

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@feerrenrut
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Getting a No enumerators available error.

  • Windows 10
  • Python 3.7.3
  • screeninfo version - 0.6.3

The exception being raised in get_monitors is

Procedure probably called with not enough arguments (4 bytes missing)

This is coming from the EnumDisplayMonitors call in enumerators\windows

Originally posted by @feerrenrut in https://github.com/rr-/screeninfo/issues/27#issuecomment-611994044
@hhannine
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Do you get this error on older versions of screeninfo? Would you mind sharing what values the variable dc_full gets?

@hhannine
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I'm not reproducing this issue on Win10 with Python 3.8.1.
@feerrenrut , would you either describe your setup in more detail or try python 3.8.1?

@feerrenrut
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This is the only version of screeninfo I have tried and on Python 3.7.3. I've worked around the issue in the application that was failing for me.

However, looking at this again I can see a few problems. Rectifying them fixes the issue for me.

  • For me, dc_full had a value of -16704030. The docs for GetDC specify that if it fails NULL will be returned. Rather than checking the value is greater than 0, it should be checked against None or 0 (I'd have to verify what ctypes does in this case)
  • The call to releaseDC is incorrect the docs for ReleaseDC suggest it should take two params int ReleaseDC( HWND hWnd, HDC hDC). Passing in ReleaseDC(None, dc_full) fixes the error for me.

@hhannine
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Thanks for looking into it. Would you mind posting your changes so I could test them? Or submit a PR? Since I can't reproduce the problem I can't do more right now.

feerrenrut added a commit to feerrenrut/screeninfo that referenced this issue Apr 13, 2020
@feerrenrut
fix 'No enumerators available' - on Windows 10
119c26a 
Fixes rr-#30

dc_full is a pointer, if interpretted as a signed number may be negative. Null -> 0 indicates failure.
Set return types for foriegn function so return value can be compared to
None.

ReleaseDC was not being called with enough arguments, it should also
take the same Window Handle (HWND) as GetDC.
@feerrenrut feerrenrut linked a pull request Apr 13, 2020 that will close this issue
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fix 'No enumerators available' - on Windows 10 feerrenrut/screeninfo
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@rr- @feerrenrut @hhannine