winapiwritefile

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

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.

Solution

  • 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.