import fcntl, os

_FD_CLOEXEC = getattr(fcntl, 'FD_CLOEXEC', 1)

if hasattr(os, 'O_CLOEXEC'):
    o_cloexec_works = None
else:
    o_cloexec_works = False

def open_cloexec(filename, open_flags):
    global o_cloexec_works
    if hasattr(os, 'O_CLOEXEC'):
        open_flags |= os.O_CLOEXEC
    fd = os.open(filename, open_flags)
    if hasattr(os, 'O_CLOEXEC') and o_cloexec_works:
        set_flags = False
    else:
        fcntl_flags = fcntl.fcntl(fd, fcntl.F_GETFD)
        if hasattr(os, 'O_CLOEXEC'):
            if o_cloexec_works is None:
                # Linux < 2.6.23 ignores silently the O_CLOEXEC flag
                o_cloexec_works = (fcntl_flags & _FD_CLOEXEC) == _FD_CLOEXEC
            set_flags = not o_cloexec_works
        else:
            set_flags = True
    if set_flags:
        fcntl_flags |= _FD_CLOEXEC
        fcntl.fcntl(fd, fcntl.F_SETFD, fcntl_flags)

print("Works?", o_cloexec_works)
open_cloexec('/', os.O_RDONLY)
print("Works?", o_cloexec_works)