footle

A colleague was asking today how he could see if there’s data waiting on STDIN in ruby (on Linux). On OS X, this is pretty straightforward:

if $stdin.stat.size > 0
  puts "got something: #{STDIN.read}"
else
  puts "nada"
end

That doesn’t work in Linux, though. After some digging, I found this post, which lead to:

require 'fcntl'

flags = STDIN.fcntl(Fcntl::F_GETFL, 0)
flags |= Fcntl::O_NONBLOCK
STDIN.fcntl(Fcntl::F_SETFL, flags)

begin
  puts "got something: #{STDIN.read}"
rescue Errno::EAGAIN
  puts "nada"
end

Which works on both platforms, but is (a) ugly (catching an exception), and (b) requires you to actually try to read from STDIN.

In playing around with that, though, I noticed that STDIN.fcntl(Fcntl::F_GETFL, 0) returned 0 if there was something on STDIN, and something non-zero (2 in OSX and 32770 in Linux) if STDIN was empty. So now the code is simple again:

require 'fcntl'

if STDIN.fcntl(Fcntl::F_GETFL, 0) == 0
  puts "got something: #{STDIN.read}"
else
  puts "nada"
end

I’m not sure how reliable that is on other platforms (it won’t work on Windows—fcntl is a Unix-y thing). If I’m understanding fcntl.h correctly, a return value of 2/32770 (0×8002) seems to indicate that the io stream in question is open for writing. I’m not sure if that is intended or just a side-effect. Anyone know?

This entry was posted on Thursday, August 21st, 2008 at 5:24 pm and is filed under geek, ruby. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.