Wrong use of ctypes.GetLastError() in windows_console.py

@eryksun

Bug report

Bug description:

Everywhere ctypes.GetLastError() is used, even though use_last_error is specified

cpython/Lib/_pyrepl/windows_console.py

Line 617 in e1c09ff

_KERNEL32 = WinDLL("kernel32", use_last_error=True)

This will always return the "swapped" error, as can be seen here

cpython/Modules/_ctypes/callbacks.c

Lines 201 to 222 in e1c09ff

    
           #ifdef MS_WIN32 
        
                   if (flags & FUNCFLAG_USE_LASTERROR) { 
        
                       int temp = space[1]; 
        
                       space[1] = GetLastError(); 
        
                       SetLastError(temp); 
        
                   } 
        
           #endif 
        
               } 
        
               result = PyObject_Vectorcall(callable, args, nargs, NULL); 
        
               if (result == NULL) { 
        
                   PyErr_FormatUnraisable("Exception ignored while " 
        
                                          "calling ctypes callback function %R", 
        
                                          callable); 
        
               } 
        
           #ifdef MS_WIN32 
        
               if (flags & FUNCFLAG_USE_LASTERROR) { 
        
                   int temp = space[1]; 
        
                   space[1] = GetLastError(); 
        
                   SetLastError(temp); 
        
               }

or carefully reading the docs (the code was easier to read for me :)

The rule is easy: use_last_error=True implies using ctypes.get_last_error(), which @eryksun has for example explained here: https://discuss.python.org/t/a-ctypes-function-to-list-all-loaded-shared-libraries/36370/10

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

@ambv

The easiest fix would be to search/replace, but I'd like to take the opportunity to also introduce errcheck callables like

from functools import partial

def errcheck(result, func, args, error_val):
    if result == error_val:
        raise WinError(get_last_error())
    return result

SetConsoleMode.errcheck = partial(errcheck, error_val=False)
WaitForSingleObject.errcheck = partial(errcheck, error_val=WAIT_FAILED)

This will make the code much cleaner and "automatically check everywhere". E.g., currently none of the SetConsoleMode or GetConsoleMode calls is checked - and thus a search/replace would not fix them.

This makes the diff slightly larger but IMHO still qualifies for backport, since 3.13 is affected, too.
And the missing checks are bugs IMHO, so we'd have to touch those code parts, anyway, and will most probably end up with not really less diff lines ...

I will do a PR if the repl maintainer agree with the above (@ambv, @lysnikolaou, @pablogsal).

My preference is still that we ought to be actively migrating from ctypes to a proper native module for these. Performance and reliability will thank us.

Beyond that, I don't have a real preference, though I'd vote for the simpler fix just because it's not worth investing heavily in code that I think should be completely rewritten 😉

@chris-eibl

Hi @chris-eibl are you working on this?

No, because there hasn't been any feedback from the pyrepl maintainers.

I'd vote for the simpler fix

But feel free to just implement the simple "mechanical" fixes, to get the ball rolling?

But feel free to just implement the simple "mechanical" fixes, to get the ball rolling?

Thank you for your reply. I will work on a simpler fix first, so that the bug is at least resolved before any larger changes or refactoring.

gh-132888: Fix Windows API error checking in pyrepl.windows_console (#…

pythongh-132888: Fix Windows API error checking in pyrepl.windows_con…

On Windows an exited process reads as alive, so a crashed process's l…

@aisk

A recent report showed that we needed a backport of this but I don't know if it's possible.

cc @aisk @chris-eibl

@picnixz

@picnixz cc @chris-eibl I don't see any concerns with backporting this. I've added the needs backport to 3.14 label, and the backport is in gh-161633.

[3.14] gh-132888: Fix Windows API error checking in pyrepl.windows_co…

@zooba

I think this can be closed now. The incorrect use of GetLastError() was fixed with a simpler approach, and the fix was backported. As @zooba suggested, the plan is to migrate from ctypes to a native extension. That work is tracked in #150582.

cc @chris-eibl

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Wrong use of `ctypes.GetLastError()` in windows_console.py #132888

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CPython versions tested on:

Operating systems tested on:

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	#ifdef MS_WIN32
	if (flags & FUNCFLAG_USE_LASTERROR) {
	int temp = space[1];
	space[1] = GetLastError();
	SetLastError(temp);
	}
	#endif
	}

	result = PyObject_Vectorcall(callable, args, nargs, NULL);
	if (result == NULL) {
	PyErr_FormatUnraisable("Exception ignored while "
	"calling ctypes callback function %R",
	callable);
	}

	#ifdef MS_WIN32
	if (flags & FUNCFLAG_USE_LASTERROR) {
	int temp = space[1];
	space[1] = GetLastError();
	SetLastError(temp);
	}

Wrong use of ctypes.GetLastError() in windows_console.py #132888

Description

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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chris-eibl commented on Apr 24, 2025

zooba commented on Apr 29, 2025

aisk commented on Jan 26, 2026

chris-eibl commented on Jan 26, 2026

aisk commented on Jan 26, 2026

picnixz commented on Nov 8, 2026

aisk commented on Nov 8, 2026

aisk commented on Nov 9, 2026

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Wrong use of `ctypes.GetLastError()` in windows_console.py #132888