[Python-Dev] Strange behavior of subprocess.Popen._get_handles under Windows

Alexey Borzenkov snaury at gmail.com
Thu Aug 23 07:41:40 CEST 2007

On 8/23/07, Mark Hammond <mhammond at skippinet.com.au> wrote:
> > Further investigations showed that it seems to be some strange OS
> > quirk/bug,
> I'm not quite with you here - what strange OS bug do you think you have
> found?  I expect that such a bug would be well documented somewhere, even if
> not directly by MS.
> MSDN documents that without that flag, hStdInput will be "the keyboard
> buffer" while the output handles will default to "the console window's
> buffer" - which sounds significantly different to your expectation that the
> existing handles will be used as the default (unless I misunderstand).
> Sadly, your mail isn't clear enough for me to be sure about what semantics
> you are seeing and exactly what you expect, and I don't have time to
> experiment.  Maybe a clear indication of the OS bug you are referring to,
> and some complete examples which demonstrate the problems you are having
> would help.

Hmm, sorry if I wasn't clear. Here's a better example:

    #include <stdio.h>
    #include <windows.h>

    int main(int argc, char** argv)
        HANDLE hStdOut = GetStdHandle(STD_OUTPUT_HANDLE);
        fprintf(stderr, "hStdOut: %08X\n", hStdOut);
        DWORD dwWritten;
        WriteFile(hStdOut, "Something", 9, &dwWritten, NULL);
        return 0;

    import subprocess

in cmd.exe shell:
    hStdOut: 00000007

    hStdOut: 000006E0

    (1.txt file now contains string "Something")

    hStdOut: 00000007

    hStdOut: 00000004

    (1.txt file is now completely empty, i.e. hStdOut is invalid)

If what you say was true and stdout would default to console window's
buffer, then I would at least see "Something" in the console. On the
contrary output just stops working at all.

And what I expect (and always expected) is output redirection to be
inherited by child processes. The best example is when I'm writing a
batch file:

    dosomething.exe /something
    dosomething2.exe /somethingmore

and then execute it as batchfile.cmd>somefile.txt I always get this
behavior, i.e. all standard output (except standard error) is
redirected to somefile.txt. I'm sure on *nixes it's all the same as
well. So what documentation for STARTUPINFO specifies is already very
suspicious and uncommon, while in reality it doesn't even work as

Best regards,

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