On 12 September 2013 00:06, Antoine Pitrou <report@bugs.python.org> wrote:
>> Shortly, is_valid_fd always returns true because fd < 0 is always
>> false as my tests with Visual Studio 2012
>
> Well, that's not all there is, is_valid_fd() does other checks before returning true.

Given the function:

is_valid_fd(int fd)
{
    int dummy_fd;
    if (fd < 0 || !_PyVerify_fd(fd))
        return 0;
    dummy_fd = dup(fd);
    if (dummy_fd < 0)
        return 0;
    close(dummy_fd);
    return 1;
}

for fd values of 0, 1 or 2

1. fd < 0 is always false
2. _PyVerify_fd(fd) is always true. Given the current definition:
#define _PyVerify_fd(fd) (_get_osfhandle(fd) >= 0)
for those values of fd _get_osfhandle(fd) >= 0, always.
3. for those fd values, dup() never returns fd < 0