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subprocess -- broken pipe error

Hi,

Can someone explain what a broken pipe is? The following produces a
broken pipe error:

----------
import subprocess as sub

p = sub.Popen(["ls", "-al", "../"], stdin=sub.PIPE, stdout=sub.PIPE )

print p.stdout.read()
#outputs the files correctly

p.stdin.write(" ls\n")
#IOError: [Errno 32] Broken pipe
-----------

Jul 2 '07
11 21541
7stud wrote:
On Jul 2, 1:58 pm, Bjoern Schliessmann <usenet-
mail-0306.20.chr0n.. .@spamgourmet.c omwrote:
>7stud wrote:
>>Thanks for the response. So are you saying that the only way you
can get data out of a pipe is when the subprocess has terminated?
No, not only because Pipes aren't related to processes in any
special way.

He said that you can't write to a pipe whose reader has already
terminated.

What he said was:
>...once the subprocess terminates (which it must have done for
p.stdout.read( ) to return a result)

And based on the results of the examples I posted in my last post, it
seems to confirm that no data travels through a pipe until a program
on one side of the pipe has terminated.
No, you plonker!

No data is produced *by .read()* until the writer has closed it.

I really don't remember anyone in recent history as eager to willfully
misunderstand any attempted assistance. Please try to read what is
written more carefully. It's most annoying when "the better the advice
the worse it's wasted", as the Scots say.

Please forgive my brusqueness.

regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://del.icio.us/steve.holden
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Jul 2 '07 #11
7stud wrote:
On Jul 2, 2:12 pm, Steve Holden <s...@holdenweb .comwrote:
>a) Who told you pipes should be unbuffered by default, and b) what difference
does that make anyway?

a) The docs.

b) If the pipes were buffered then writing a small amount of data like
"text3" to the pipe would cause the other side to hang forever thereby
providing a possible explanation for the results.
>>It just hangs, and then when I hit Ctrl+C and look in the file, the
data isn't in there.
Of course it does, for the reasons mentioned above. file.read() only
returns when it has consumed *all* the data from the file (which means
the write must close the file for the reader to be able to return).

That doesn't seem like a very good explanation, since the only thing
written to the file(i.e. stdin) was "text3", and the write() was
unbuffered, so the read() could consume all the data without the
write() closing the file--there was no more data.
[sigh].

So please explain how the receiving process mysteriously manages to look
inside your producer process to know that it is never going to produce
any more data. Let's (briefly) look at the docs for read():

"""
read( [size])

Read at most size bytes from the file (less if the read hits EOF before
obtaining size bytes). If the size argument is negative or omitted, read
all data until EOF is reached. ...
"""

I believe you omitted the argument. As I have explained, the read() call
therefore waits until the writer has closed the file. Which is what
makes the EOF indication appear.

And please stop dragging buffering into this as a red herring. You do
know what buffering *is*, I take it? The read() call buffers even an
unbuffered source, by definition.

regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://del.icio.us/steve.holden
--------------- Asciimercial ------------------
Get on the web: Blog, lens and tag the Internet
Many services currently offer free registration
----------- Thank You for Reading -------------

Jul 2 '07 #12

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