[Tutor] Question on os.popen

[SM]

Eryksun: Thanks for your reply. Yes, as I mentioned in my reply to Allen, I
used subprocess.check_output and it worked for me.
-SM


On Sun, Jan 19, 2014 at 11:33 PM, eryksun <eryksun at gmail.com> wrote:

> On Sun, Jan 19, 2014 at 6:36 PM, SM <sunithanc at gmail.com> wrote:
> >
> > This time it probably ran for a few more iterations than before and
> stopped
> > with the same error message. This time it also output the following
> > messages:
> >
> > IOError: [Errno 4] Interrupted system call
> > Attribute not found in file (tsk_fs_attrlist_get: Attribute 128 not
> found)
> > Attribute not found in file (tsk_fs_attrlist_get: Attribute 128 not
> found)
> > Attribute not found in file (tsk_fs_attrlist_get: Attribute 128 not
> found)
>
> I can't help with these (NTFS?) Attribute errors from "The Sleuth Kit"
> digital forensics tools.
>
> In Python 3.3, `IOError` is an alias for `OSError`, and EINTR (i.e.
> errno.errorcode[4]) is exposed directly as `InterruptedError`. So you
> must be running a previous version. I see you're using `print` as a
> function, so I'll guess you're using 3.2.
>
> In 3.2, `os.popen` is implemented via `subprocess.Popen`:
>
> http://hg.python.org/cpython/file/cef745775b65/Lib/os.py#l776
>
> For example, it uses the following for 'r' mode:
>
>     proc = subprocess.Popen(cmd,
>                             shell=True,
>                             stdout=subprocess.PIPE,
>                             bufsize=buffering)
>    return _wrap_close(io.TextIOWrapper(proc.stdout), proc)
>
> If you're sticking to `os.popen`, you'll need to retry the read in
> case of an interrupted system call.
>
> I recommend you switch to `Popen` directly and call `communicate`.
> This retries reading `stdout` using the helper function
> `_eintr_retry_call`:
>
> http://hg.python.org/cpython/file/cef745775b65/Lib/subprocess.py#l452
>
>     def _eintr_retry_call(func, *args):
>         while True:
>             try:
>                 return func(*args)
>             except (OSError, IOError) as e:
>                 if e.errno == errno.EINTR:
>                     continue
>                 raise
>
> More simply, use `subprocess.check_output`, which calls `communicate`
> for you. You can pass `shell=True` if you really need it.
>
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