[Tutor] Fwd: Difference between popens

[Ivan Furone]

2006/6/9, Bernard Lebel <3dbernard at gmail.com>:
>
> Hi,
>
> I'd like to know what are the differences at the various os.popenX
> flavors. I read the documentation and I can see they return file
> objects..... so what can you do with these file objects? I mean, why
> would you need a set of file objects rather than another?
>
> Sorry the difference is very not clear to me.
>
>
> Thanks
> Bernard
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>

Hello,

The common idea behind the various popen implementation is to provide the
creation of pipes.A pipe is a flavor of a file object.A pipe serves the job
to address data from a process to another,but what if sometimes you want to
execute a command
out of the parent process on a child process?You should be able to issue the
command,catch the result and even to know which way the thing has ended,at
times.
If you are familiar with the concept of 'streams',the "input","output" and
"error" stream are the resource of communication provided by Unix to handle
these tasks,respectively.Nevertheless,at times you don't need to gather data
from all the streams,but just to control two of them out of three,for
example.The reason could be that of sparing system resources,time or just
that you don't need or want to.popen2,popen3 and popen4 are designed to
catch exactly the streams you want to control.So:

popen2:
returns the file objects corresponding to the child stdin and stdout;

popen3:
returns the file objects corresponding to the child stdin,stdout and stderr;

popen4:
returns the file objects corresponding to the child stdout and stderr (together
as a single object) and stdin.

The file objects,you see,are what you normally make use to manipulate data
within processes.
They are memory structures which assume peculiar names based on their type :
Queues,Semaphores,Pipes and so on...
And they can persist a) in the scope of a process b) of a machine session c)
on the filesystem (the strongest kind of persistence).
Choosing the right one for each task a matter depending on what you want to
do with them.
I recognize I'm not being exhaustive but hope that helps.

Cheers,
Ivan
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