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<title>6.9.2 Flow Control Issues
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<H2><A NAME=
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6.9.2 Flow Control Issues
Any time you are working with any form of inter-process communication,
control flow needs to be carefully thought out. This remains the case
with the file objects provided by this module (or the
<tt class=
"module"><a href=
"module-os.html">os
</a></tt>
When reading output from a child process that writes a lot of data to
standard error while the parent is reading from the child's standard
output, a deadlock can occur. A similar situation can occur with other
combinations of reads and writes. The essential factors are that more
than
<tt class=
"constant">_PC_PIPE_BUF
</tt> bytes are being written by one process in
a blocking fashion, while the other process is reading from the other
process, also in a blocking fashion.
There are several ways to deal with this situation.
The simplest application change, in many cases, will be to follow this
model in the parent process:
<div class=
"verbatim"><pre>
r, w, e = popen2.popen3('python slave.py')
with code like this in the child:
<div class=
"verbatim"><pre>
# note that each of these print statements
# writes a single long string
print
>>sys.stderr,
400 * 'this is a test\n'
os.close(sys.stderr.fileno())
print
>>sys.stdout,
400 * 'this is another test\n'
In particular, note that
<code>sys.stderr
</code> must be closed after
writing all data, or
<tt class=
"method">readlines()
</tt> won't return. Also note
that
<tt class=
"function">os.close()
</tt> must be used, as
<code>sys.stderr.close()
</code>
won't close
<code>stderr
</code> (otherwise assigning to
<code>sys.stderr
</code>
will silently close it, so no further errors can be printed).
Applications which need to support a more general approach should
integrate I/O over pipes with their
<tt class=
"function">select()
</tt> loops, or use
separate threads to read each of the individual files provided by
whichever
<tt class=
"function">popen*()
</tt> function or
<tt class=
"class">Popen*
</tt> class was
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<span class=
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2.4.2, documentation updated on
28 September
2005.
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