During startup, is_valid_fd() in Python/pylifecycle.c is called to validate stdin, stdout, and stderr. Performance isn't critical here, but every bit helps in reducing startup time. In my tests, implementing this check in Windows via GetFileType((HANDLE)_get_osfhandle(fd)) is 5-6 times faster than close(dup(fd)). For example:

I got the same performance enhancement by measuring QueryPerformanceCounter.

The _Py_BEGIN_SUPPRESS_IPH and _Py_END_SUPPRESS_IPH macros are used to temporarily disable the MSVC invalid parameter handler for the _get_osfhandle() call. The use of these macros can be removed from the cases that are limited to POSIX systems. For example:

    int res;
    _Py_BEGIN_SUPPRESS_IPH
    res = fcntl(fd, F_GETFD);
    _Py_END_SUPPRESS_IPH
    return res >= 0;

becomes simply:

    return fcntl(fd, F_GETFD) >= 0;

The comment that discusses the function should be updated to remove Windows from the platforms that use dup().

Was it verified that fcntl(fd, F_GETFD) addresses the case that forced macOS to use fstat() instead of dup()? If not, create_stdio() could fail with an EBADF error in macOS.

Was it verified that fcntl(fd, F_GETFD) addresses the case that forced macOS to use fstat() instead of dup()? If not, create_stdio() could fail with an EBADF error in macOS.

See the comment:

   [bpo-30225](https://bugs.python.org/issue30225): On macOS Tiger, when stdout is redirected to a pipe and the other
   side of the pipe is closed, dup(1) succeed, whereas fstat(1, &st) fails with
   EBADF. FreeBSD has similar issue ([bpo-32849](https://bugs.python.org/issue32849)).

In case of doubt, I prefer to continue using fcntl() which always works. On BSD systems, dup() seems to implement less checks.

In case of doubt, I prefer to continue using fcntl() which always works. On BSD systems, dup() seems to implement less checks.

@vstinner, the original change was to use fstat() instead of dup(). bpo-30225 makes no reference to fcntl(). @tiran added the fcntl() call in bpo-45915, with __APPLE__ in the allowed platforms, but there was no discussion that it actually checks the pipe state in macOS to handle the case of bpo-30225.

The comment that discusses the function should be updated to remove Windows from the platforms that use dup().

The out of date comment was not updated in the merge.

The out of date comment was not updated in the merge.

@corona10 wrote #30090