From: Derek Lewis <lewisd@...00f.net>
Date: 2003-12-13T02:55:29+09:00
Subject: Re: Bug with fifos in Linux?

On Sat, 13 Dec 2003, ts wrote:

> >>>>> "D" == Derek Lewis <lewisd@f00f.net> writes:
>
> D> I don't know if this is a new bug, known bug, or some sort of feature...
>
>  Something which is intentional can't be called a bug :-)
>
> D> When I try to open a fifo, the entire ruby interpreter locks until
> D> something else opens the other "end" of the fifo.  Here's some example
> D> code:
>
>  You can use IO::NONBLOCK with IO::WRONLY or IO::RDONLY

Should that be File::NONBLOCK and File::RDONLY?  I just changed my code to
use those, but it's still behaving strangly.

Some more sample code based on the previous:

FIFO_FILE='/tmp/rubyfifo'
`mkfifo "#{FIFO_FILE}"` unless test(?e, FIFO_FILE)

thread = Thread.new {
   puts "Opening fifo for writing"
   File.open(FIFO_FILE, File::NONBLOCK | File::WRONLY) { |file|
      puts "Starting write loop"
      while true
         puts "Thread is running"
         sleep 1
         file.puts "Thread is running"
      end
   }
}


puts "Opening fifo for reading"
File.open(FIFO_FILE, File::NONBLOCK | File::RDONLY) { |file|
   puts "Reading from fifo"
   sleep 2
   while !file.eof
      line = file.gets("\n")
      puts "Read from fifo: #{line}"
   end
   puts "exited read loop"
}

I'm still never getting the "Starting write loop" output, but I do get the
"Reading from fifo":

lewisd@derlewi:/tmp$ ruby fifo.rb
Opening fifo for writing
Opening fifo for reading
Reading from fifo
exited read loop
lewisd@derlewi:/tmp$

Also, if I comment out the entire thread bit, I get this output:
lewisd@derlewi:/tmp$ ruby fifo.rb
Opening fifo for reading
Reading from fifo
exited read loop
lewisd@derlewi:/tmp$

With a 2 second delay between "Reading..." and "exited...", as expected.

If, before I start the ruby program, I run this:
$ echo "lala" > /tmp/rubyfifo
The echo blocks, as the fifo hasn't been opened for reading yet, and when
I run the ruby program, I get this:
lewisd@derlewi:/tmp$ ruby fifo.rb
Opening fifo for reading
Reading from fifo
Read from fifo: lala
exited read loop
lewisd@derlewi:/tmp$

If I remove the "File::NONBLOCK" from the open command, it waits until I
open the fifo, as expected.

I'm not sure how to read from a fifo reliably, without leaving out the
NONBLOCK, which will then block the rest of the program. :(


>
>
> Guy Decoux
>
>

Derek Lewis

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