I got curious when this happened:
1.9.3p286 :032 > File.open('/etc/passwd').fileno
=> 5
I know that file descriptors (FD) 0, 1, and 2 are assigned to STDIN, STDOUT, and STDERR. I also know that FDs are assigned in order. So what happened to 3 and 4?
1.9.3p286 :015 > STDIN.fileno
=> 0
1.9.3p286 :034 > IO.for_fd(3)
ArgumentError: The given fd is not accessible because RubyVM reserves it
from (irb):34:in `for_fd'
from (irb):34
from /Users/zlu/.rvm/rubies/ruby-1.9.3-p286/bin/irb:16:in `<main>'
Evidently some ruby process is using it, but what process?
zlu@zlu-mba:~$ lsof -d 3,4 | grep ruby
ruby 4980 zlu 3 PIPE 0xffffff8029fa7860 16384 ->0xffffff8029fa4370
ruby 4980 zlu 4 PIPE 0xffffff8029fa4370 16384 ->0xffffff8029fa7860
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
And
zlu@zlu-mba:~$ ps -p 4980
PID TTY TIME CMD
4980 ttys003 0:00.47 irb
So Ruby’s RVM is using pipes with FD 3 and 4. As far as what these pipes are used for, that’ll be another discussion.