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Dialog with a process via subprocess.Popen blocks forever

Bernhard Herzog bh at intevation.de
Fri Mar 2 12:31:43 EST 2007 
bayer.justin at googlemail.com writes:

> So, once I start the C Program from the shell, I immediately get its
> output in my terminal. If I start it from a subprocess in python and
> use python's sys.stdin/sys.stdout as the subprocess' stdout/stdin I
> also get it immediately.

If stdout is connected to a terminal, it's usually line buffered, so the
buffer is flushed whenever a newline is written.

> BUT If I use PIPE for both (so I can .write() on the stdin and .read()
> from the subprocess' stdout stream (better: file descriptor)) reading
> from the subprocess stdout blocks forever. If I write something onto
> the subprocess' stdin that causes it to somehow proceed, I can read
> from its stdout.

When stdout is not connected to a terminal, it's usually fully buffered,
so that nothing is actually written to the file until the buffer
overflows or until it's explictly flushed.

If you can modify the C program, you could force its stdout stream to be
line buffered.  Alternatively, you could call fflush on stdout whenever
you're about to read from stdin.  If you can't modify the C program you
may have to resort to e.g. pseudo ttys to trick it into believing that
its stdout is a terminal.

   Bernhard

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