fork/exec & close file descriptors

[Alain Ketterlin]

Skip Montanaro <skip.montanaro at gmail.com> writes:

> Reviving (and concluding) a thread I started a couple weeks ago, I asked:
>
>> The basic fork/exec dance is not a problem, but how do I discover
>> all the open file descriptors in the new child process to make sure
>> they get closed? Do I simply start at fd 3 and call os.close() on
>> everything up to some largish fd number?
>
> I wanted this again today (for different reasons than before).
> Googling for "python close all file descriptors" returned the os
> module docs as the first hit, and lo and behold, what do I see
> documented? os.closerange (new in 2.6):
>
>     os.closerange(fd_low, fd_high)
>     Close all file descriptors from fd_low (inclusive) to fd_high
>     (exclusive), ignoring errors.

The close(2) manpage has the following warning on my Linux system:

| Not checking the return value of close() is a common  but  nevertheless
| serious  programming error.  It is quite possible that errors on a pre‐
| vious write(2) operation are first reported at the final close().   Not
| checking the return value when closing the file may lead to silent loss
| of data.  This can especially be observed with NFS and with disk quota.
| 

(I haven't followed the thread, but if your problem is to make sure fds
are closed on exec, you may be better off using the... close-on-exec
flag. Or simply do the bookkeeping.)

-- Alain.