It seems like stdin must be closed for popen3 to send stdin to the
sub-process. How can I do something like this:

require 'open3'

stdin, stdout, stderr, wait_thr = Open3.popen3("dc")
stdin.puts "1 2 + p"
stdin.flush # does not not help
stdin.close # without this it will just hang on the read
res = stdout.read
puts "1+2 is #{res}"
#stdin.reopen # is it possible to re-open the stdin somehow?
stdin.puts "#{res} 2 * p"
stdin.close
res = stdout.read
puts "(1+2)*2 is #{res}"
...

Is there a way to flush or re-open stdin? Is there some other variation
of popen3 which can be used?

Thanks!

//Petter

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On 26.02.2017 17:36, Petter Gustad wrote:

It seems like stdin must be closed for popen3 to send stdin to the sub-process. How can I do something like this:

require 'open3'

stdin, stdout, stderr, wait_thr = Open3.popen3("dc")
stdin.puts "1 2 + p"
stdin.flush # does not not help
stdin.close # without this it will just hang on the read
res = stdout.read
puts "1+2 is #{res}"
#stdin.reopen # is it possible to re-open the stdin somehow?
stdin.puts "#{res} 2 * p"
stdin.close
res = stdout.read
puts "(1+2)*2 is #{res}"
...

Is there a way to flush or re-open stdin? Is there some other variation
of popen3 which can be used?

You are missing one important point: stdout.read must block until dc
closes its output because there is no limit set on the read amount. And
that will only happen if dc's input has been closed because, obviously,
dc needs to be able to write results as long as input is coming. If you replace that line with

res = stdout.gets

it will work the way you expect. You could also use stdout.sync=true to
avoid to have to use #flush all the time.

http://ruby-doc.org/core-2.4.0/IO.html#method-i-sync-3D

Kind regards

robert

--
remember.guy do |as, often| as.you_can - without end http://blog.rubybestpractices.com/

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On 26.02.2017 22:46, Petter Gustad wrote:

Again, thanks!

You're welcome! Just one more caveat, which might not apply in this
case but a general advice for #popen and the likes: buffers between
processes are limited. If you handle everything single threaded there
is potential for deadlock. This will always happen if you send more
data longer than the buffer size to the pipe and the other process will
also send more than buffer size data. Then both are blocked on IO.

Kind regards

robert


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Robert Klemme <shortcutter@googlemail.com> writes:

On 26.02.2017 17:36, Petter Gustad wrote:

It seems like stdin must be closed for popen3 to send stdin to the
sub-process. How can I do something like this:

require 'open3'

stdin, stdout, stderr, wait_thr = Open3.popen3("dc")
stdin.puts "1 2 + p"
stdin.flush # does not not help
stdin.close # without this it will just hang on the read
res = stdout.read
puts "1+2 is #{res}"
#stdin.reopen # is it possible to re-open the stdin somehow?
stdin.puts "#{res} 2 * p"
stdin.close
res = stdout.read
puts "(1+2)*2 is #{res}"
...

Is there a way to flush or re-open stdin? Is there some other variation
of popen3 which can be used?

You are missing one important point: stdout.read must block until dc
closes its output because there is no limit set on the read amount.
And that will only happen if dc's input has been closed because,
obviously, dc needs to be able to write results as long as input is
coming. If you replace that line with

res = stdout.gets

it will work the way you expect. You could also use stdout.sync=true
to avoid to have to use #flush all the time.

http://ruby-doc.org/core-2.4.0/IO.html#method-i-sync-3D

Thanks! Actually I was thinking readline, where I was writing read, and
of course the correct ruby method is gets...

require 'open3'

stdin, stdout, stderr, wait_thr = Open3.popen3("dc")
stdin.puts "1 2 + p"
res = stdout.gets
puts "1+2 is #{res}"
stdin.puts "#{res} 2 * p"
res = stdout.gets
puts "(1+2)*2 is #{res}"

The above results in:

1+2 is 3
(1+2)*2 is 6

Again, thanks!

Best regards
//Petter

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