is it legal to pass STD_OUTPUT_HANDLE directly as input HANDLE to WriteFile?

Question

The WinApi WriteFile() function seems to accept the STD_xxx_HANDLE constants directly as a first argument. I executed the following:

#include <windows.h>
main() {
    DWORD bw;

    WriteFile( (void *)STD_OUTPUT_HANDLE,       "output", 6, &bw, NULL);
    WriteFile( GetStdHandle(STD_OUTPUT_HANDLE), "output", 6, &bw, NULL);

    WriteFile( (void *)STD_ERROR_HANDLE,        "error", 5, &bw, NULL);
    WriteFile( GetStdHandle(STD_ERROR_HANDLE),  "error", 6, &bw, NULL);
}

The above example writes "output" twice to stdout and "error" twice to stderr. I have tested on Win XP and Win 7 (I have no access to WINE or Win 10).

I can see GetStdHandle() does convert the value (from -11 to 7 in the case of STD_OUTPUT_HANDLE), but either value works the same as a WriteFile() input HANDLE.

Is WriteFile implicitly doing GetStdHandle() when it recognizes a STD_xxx_HANDLE constant as its HANDLE input? I have looked but can't find this documented.

Yes I know if it's not documented, don't do it... I'm simply wondering if I missed something.

yes, `WriteFile` check `hFile` for `STD_ERROR_HANDLE, STD_OUTPUT_HANDLE, STD_INPUT_HANDLE` and convert it by using `GetStdHandle` — RbMm, Jul 22 '19 at 00:12
My guess is that this was added to support some automatic sourcecode-rewriting tool... and now kept forever in order to maintain backward-compatibility — Ben Voigt, Jul 22 '19 at 00:35
Actually, It doesn't make sense to tangle with this. It's considered an invalid handle in other APIs :) — Strive Sun, Jul 23 '19 at 10:38

score 1 · Accepted Answer · answered Jul 23 '19 at 15:51

1

Surprisingly, this works even on Windows 95 RTM and NT 4 (and XP and 8) so it seems like it is on purpose. I can't tell you why nor how far back in NT 3.x this goes.

But it is undocumented and not really part of the ABI so I would suggest that you continue calling GetStdHandle to avoid a nasty surprise in the future.

answered Jul 23 '19 at 15:51

Anders

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Appreciate the additional research. I have come to my senses and shall continue to religiously perform `GetStdHandle()` mapping, unnecessary though it may be. – Paul Houle Jul 23 '19 at 19:14

score 0 · Answer 2 · answered Jul 22 '19 at 09:22

0

WriteFile expects a handle. The STD_xxx_HANDLE values are not handles. It is not legal to pass those values to WriteFile. You have to convert them to handles by calling GetStdHandle.

answered Jul 22 '19 at 09:22

IInspectable

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Ex-Kyuto · Answer 3 · 2023-06-04T04:07:09.603

Nope, it's treated as pseudohandles, same as GetCurrentProcess() handle.

Try to peek inside KernelBase.dll, It checks the hFile argument if it's equal to STD_X_HANDLE macros.

If yes, the hFile handles turn into NtCurrentPeb()->ProcessParameter->StandardXxx handle.

And there are other functions that recognized STD_XXX_HANDLE macros. GetFiletype , FlushFileBuffers, CloseHandle, and WaitForSingleObject(Ex)also recognize STD_XXX_HANDLE.

is it legal to pass STD_OUTPUT_HANDLE directly as input HANDLE to WriteFile?

3 Answers3