[Gllug] pipes and C++

[Bruce Richardson]

On Sun, Sep 12, 2010 at 09:49:58PM +0100, Adrian McMenamin wrote:
> I have been chaining three C++ programs I have written together -
> working on the principle that each should do one thing - so
> 
> A - generates some xml
> B - processes said xml
> C - outputs graphics
> 
> A | B | C
> 
> All works well - but now I want to have them repeat this every few
> seconds ie for every three seconds
> 
> A --r 3 | B --r 3 | C --r 3
> 
> But while it is easy to get the first program to churn out the xml I
> cannot work out how to get the second element in the chain to recognise
> the end of the output - (I am guessing that previously B didn't really
> start until A was dead?)

No, that's not the case.  In fact, A and B run in parallel.  B reads
from STDIN, which, since it directly connected to A's STDOUT, blocks
until A writes to it.  Here's a little test you can run with two
scripts:

-------------- a.sh -----------------
#!/bin/sh
logfile=/tmp/test.log
for output in 1 2 3 4 5 6 7 8 9 0; do
  echo A: $output
  echo A: $output >> $logfile
done

-------------- b.sh -----------------
#!/bin/sh
logfile=/tmp/test.log
while read input; do
  echo B: $input
  echo B: $input >> $logfile
done


Now, if you run

	a.sh | b.sh

and examine /tmp/test.log, what you will *not* find is all of a.sh's
output followed by all of b.sh's output.


Going back to your problem, in the first example, when A terminates it
closes STDOUT.  Since the shell has diverted A's STDOUT to B's STDIN,
this closes B's STDIN.  In your second example, A is not terminating and
so STDOUT is not closed and so STDIN for B is not closed.  Simple as
that.  You could close STDIN from within A, but then you will break the
pipe between A and B and A will have no way to re-establish this, as it
knows nothing of A; it only knows about STDOUT, which the shell has
linked to B's STDIN.

> 
> Is my naive way of doing this simply not going to work?

No, it can't.  (How did you get anywhere with C/C++ without learning
about STDIN, STDOUT and pipes?  You should remedy that.)  Since you're
working with XML, can't process B recognise when its input is finished
because it's received an entire XML document?  Surely unambiguous
communication is what XML is all about?  (Stop laughing, you at the
back).  A should produce as many documents as it is told to, then close
STDOUT.  B should keep parsing documents (and doing whatever it does to
produced output after each completed document) until STDIN closes.

-- 
Bruce

What would Edward Woodward do?
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