From: 7stud -- <dolgun@...>
Date: 2007-10-04T07:21:53+09:00
Subject: Re: Non-blocking 'gets' ?

Stephen Ware wrote:
> Hi guys, sorry if this is a stupid question, but I've been reading
> around online and in 'Programming Ruby,' and I can't find anything.
> 
> Is there a way to perform a non-blocking gets or getc?  As in a method
> that reads a character from stdin if there is one in the buffer or
> returns nil otherwise?  I'm writing a multi-threaded application that
> communicates over the internet using sockets, but I want the person who
> runs the script on the host machine to be able to enter commands via
> stdin.  The 'gets' command hangs all my threads until it returns.
>

The following is an example that uses one thread to execute some code in 
the background while another thread waits for user input.  After being 
prompted for input, if you wait 5 seconds before entering any input, you 
will see that the background thread's output is written to the file 
while the main thread waits for your input.

If instead, you enter your input right away, you will see that what you 
entered will be interleaved with the thread's output.


require "monitor"

f = File.new("aaa.txt", "w")
lock = Monitor.new

t = Thread.new do
  lock.synchronize do
    f.puts("thread output1")
  end

  sleep(3)

  lock.synchronize do
    f.puts("thread output2")
  end
end

print "Enter command: "
STDOUT.flush
input = gets

lock.synchronize do
  f.puts(input)
end

t.join
f.close

File.open("aaa.txt") do |file|
  print file.read
end

--output:--
(waiting 5 seconds):
Enter command: hello
thread ouput1
thread output2
hello

(entering input immediately when prompted):
Enter command: hello
thread output1
hello
thread output2


One thing I found puzzling is having to call STOUT.flush.  When I don't 
include that statement, then the print statement is skipped.  Yet, if I 
use puts instead of print, then the flush call isn't necessary.  Anyone 
know why?
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