Streaming data from STDOUT

ruby

Solution

Your code is working fine. You're only missing one piece and that is flushing the output buffer of your `threader`.

The problem is STDOUT is almost always buffered, and in this case the buffer will not be flushed, unless explicitly told to, until after threader exits.

That is why the reader will not see anything, and then suddenly it will get a burst of output when threader exits and STDOUT gets flushed.

So the simple fix for this test is:

10.times do
  $stdout.puts "test"
  $stdout.flush
  sleep 1
end

Note however that because you are reading using NONBLOCK your reader will busy loop(!) consuming 100% CPU. What you really should do to prevent busylooping is wait for incoming data before reading.

This can be done with `IO.select`:

...
loop do
  IO.select([stdout]) # <- waits for data (any data, even 1 byte)
  data = stdout.read_nonblock(8)
  print data
end  
...

Problem

So I have the following code: reader.rb ``` require 'open4' def streamer(stdout) begin loop do data = stdout.read_nonblock(8) print data end rescue Errno::EAGAIN retry rescue EOFError puts 'End of file' end end pid, stdin, stdout, stderr = Open4::popen4 "ruby threader.rb" stdout.fcntl(Fcntl::F_SETFL, Fcntl::O_NONBLOCK) streamer(stdout) ``` threader.rb ``` 10.times do $stdout.puts "test" sleep 1 end ``` One ruby script is simple a spinner that puts to stdout every second. The other is meant to run that script and I would like to capture the data as it comes in. So I want the stream to read from stdout non-blocking. I cannot seem to get this to work. I think I am setting the fctnl `O_NONBLOCK` flag properly but perhaps I am not.

Original source