Re: problems getting stderr and stdout from process object

Daniel Pitts <>
Mon, 28 Sep 2009 09:44:50 -0700
Peter Duniho wrote:

On Thu, 24 Sep 2009 11:52:19 -0700, Daniel Pitts
<> wrote:

Actually, what may be happening is that the process itself isn't
finishing because it is blocking on something else altogether. It is
also possible that the stderr buffer is full, so the process blocks
until *it* completes.

In general, you will need to read the stdout and stderr from separate
threads. This is a flaw in the API in my opinion.

I'm curious: how would you fix it?

You cannot buffer the output for either stdout or stderr indefinitely.
It would not even be practical or robust to buffer until a memory
allocation simply fails. So the question becomes, what to do when the
buffer becomes filled? You have two obvious choices: discard data, or
block output until room is made.

Do you see some other practical alternative?

The problem I have isn't with the blocking-on-buffer-fill behavior, but
with the fact that you *must* have two threads running to use this API

I can think of three alternatives off the top of my head:

   1. Don't use two InputStream instances, but instead use a new kind of
IO class designed to handle interleaved data. It would allow better
correlation between events in each "stream", and it would allow you to
read the streams in the current thread, without spawning a new one.

   2. Offer some sort of "select()" based waiting for the streams. This
allows one thread to handle multiple streams.

   3. 1 and 2 combined.

It would be important for the select() to also support "OutputStream"
readiness, because you could otherwise end up with a deadlock (the
process is supposed to receive more input, but the output buffer is
full, so the whole thing might block)

As far as those two choices go, I'm quite pleased that the design choice
made was to block output, rather than to discard data.

I suppose that now, with NIO (which came well after Process), the API
could provide a SelectableChannel implementation, allowing a single
thread to process more than one stream. But, the main motivation for
that NIO feature is to avoid the creation of thousands of threads when
you have that many streams to deal with; a process is only going to have
at most these two output streams, so all the work to implement a
SelectableChannel just to avoid the creation of one extra thread seems
like overkill to me.

Creating one thread is more than just run-time overhead. There is a
development cost with multi-threading. You are more prone to deadlocks,
synchronization problems, and much more, when you create a new Thread.
Yes, those problems can be avoided, but its *much* more to think about.

I think providing SelectableChannels and/or an InterleavedInputStream
would provide the least error-prone API.

Daniel Pitts' Tech Blog: <>

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