reading from fifo with no writers blocks the whole process

[Tomasz Wrobel]

Hi,

in my program I'm creating a thread, in which I open a FIFO and read
data from it.

pipe_listener_thread = Thread.new(ARGV[0]) do |fifoname|
while true # loop to reopen fifo, when it ends because of no writers
fi = File.open(fifoname, "r")
while line = fi.gets
puts "msg: #{line}"
end
fi.close
end
end

Everything works fine, except that when the thread is waiting to open
the fifo, whole process - all threads seem to be blocked. Such thing
doesn't happen when waiting for data (inner loop) - where the thread
alone gets blocked.

Please tell, if there's something wrong with this behavior, or it should
work that way.

Regs,
Tomasz Wrobel

 

[Robert Klemme]

in my program I'm creating a thread, in which I open a FIFO and read
data from it.

pipe_listener_thread = Thread.new(ARGV[0]) do |fifoname|
while true # loop to reopen fifo, when it ends because of no writers
fi = File.open(fifoname, "r")
while line = fi.gets
puts "msg: #{line}"
end
fi.close
end
end

Everything works fine, except that when the thread is waiting to open
the fifo, whole process - all threads seem to be blocked. Such thing
doesn't happen when waiting for data (inner loop) - where the thread
alone gets blocked.

First of all you should change your code to use the block form of
File.open. This avoids keeping the file descriptor open in case of errors.

You can do even more, see
http://blog.rubybestpractices.com/posts/rklemme/001-Using_blocks_for_Robustness.html

Please tell, if there's something wrong with this behavior, or it should
work that way.

I believe this is perfectly ok. Since IO indicates eof if there is no
more data in the fifo, it makes sense to block on open as long as there
is nothing to read. Otherwise your program would mindlessly be looping
and burning CPU.

I do wonder though why reading from a named fifo ever reaches eof. You
would expect the reading to continue. OTOH that way you will be aware
if a program terminates which writes to the fifo and can react on it
appropriately.

Kind regards

robert