OT: processes, terminals and file descriptors on *nix (was: Re: attach to process by pid?)

[Dan Stromberg]

On Fri, Mar 11, 2011 at 3:49 PM, Alexander Kapps <alex.kapps at web.de> wrote:

> On 11.03.2011 03:18, Nobody wrote:
>
>> On Thu, 10 Mar 2011 23:55:51 +0100, Alexander Kapps wrote:
>>
>>  I think he wants to attach to another process's stdin/stdout and
>>>>> read/write from/to them.
>>>>> I don't know if this is possible but it would be a great addition for
>>>>> psutil.
>>>>>
>>>>
>>>> It's not even a meaningful concept, let alone possible.
>>>>
>>>
>>> Unless I misunderstand something,
>>>
>>
>> You do ...
>>
>
> Many thanks for the correction and lesson (to Grand Edwards too)!
>
> I still try to digest your explanations. I thought, that processes just do
> something like dup()'ing the file descriptors of their terminal but after
> some strace experiments, I think that is totally wrong.
>
Usually, file descriptors are inherited across fork() (and maybe Windows'
startprocess), as is the contents of any buffers corresponding to them - so
when flushing you can get two copies of the same output, or two copies of
the same input.

There's no need to dup to just inherit a filedescriptor from a parent
process to a child.


> I'd like to learn more about this (how processes, their controlling
> terminals and the std file descriptors relate)
>
I'm not sure controlling terminals are that relevant.

The standard filedescriptors are just 0, 1, and 2 for stdin, stdout and
stderr.  They're pretty much like any other filedescriptors, except they
tend to be open already when a program starts.  Oh, and the buffering might
be different from one filedes to another, of course.


> Do you know any links to deeper material (tried Google but what I've found
> is to shallow)
>

Check this out; you can pass a filedescriptor through a unix domain socket,
though I've never tried it myself in C, let alone Python:
http://stackoverflow.com/questions/909064/portable-way-to-pass-file-descriptor-between-different-processes
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