# File I/O from threads

**URL:** <https://www.ruby-forum.com/t/file-i-o-from-threads/166701>\
**Category:** Ruby\
**Created:** [April 28, 2009, 5:19pm UTC](https://www.ruby-forum.com/t/file-i-o-from-threads/166701 "2009-04-28T17:19:46Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![maartenm](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/5e9695/32.png) [@maartenm](https://www.ruby-forum.com/u/maartenm)\
**Post date:** [April 28, 2009, 5:19pm UTC](https://www.ruby-forum.com/t/file-i-o-from-threads/166701/1 "2009-04-28T17:19:46Z")

</div>

Hi, I was writing a quick Relay class and ran into a problem.  
Objects of this class act between two ports and log every TCP/IP that is  
sent between those ports - according to a specific protocol (in this  
simple example the first 8 characters contain the length of the data  
that will follow)

These are the important methods of the class. All objects are local to  
the object and not shared between other objects.

def start  
@server = TCPServer.new(l\_h, l\_p)  
@client = TCPSocket.new(r\_h, r\_p)

```
loop do
  Thread.start(@server.accept) do |s|
      # handshake(s)
      client_to_server(s)
      server_to_client(s)
    end
end

```

end

def client\_to\_server(s)  
Thread.new do  
loop do  
len = @client.recv(8)  
msg = @client.recv(len.to\_i)  
@logfile.puts("Received " + msg)  
s.write(len)  
s.write(msg)  
end  
end  
end

def server\_to\_client(s)  
Thread.new do  
loop do  
len = s.recv(8)  
msg = s.recv(len.to\_i)  
@logfile.puts("Sent " + msg)  
@client.write(len)  
@client.write(msg)  
end  
end  
end

My problem is that the file operations (@logfile.puts) are not performed  
immediately. When I check the file while communication is passing by, it  
is incomplete, sometimes even empty. Only when I kill the connection  
contained in @server.accept is the file properly flushed and is all  
traffic put into it.  
I don’t understand why this happens. Why does @logfile.puts not happen  
immediately while @client.write and s.write do?  
I can’t see why this would have anything to do with thread safety.

Putting a mutex lock/unlock around the file operation didn’t really do  
anything (I didn’t really expect that to work either)

Can anybody tell me what is happening here?

---

<div class="post-metadata">

**Author:** ![maartenm](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/5e9695/32.png) [@maartenm](https://www.ruby-forum.com/u/maartenm)\
**Post date:** [April 28, 2009, 5:51pm UTC](https://www.ruby-forum.com/t/file-i-o-from-threads/166701/2 "2009-04-28T17:51:02Z")

</div>

On 28.04.2009 17:19, Maarten M. wrote:

> def start  
> end  
> end  
> end  
> immediately while @client.write and s.write do?  
> I can’t see why this would have anything to do with thread safety.
> 
> Putting a mutex lock/unlock around the file operation didn’t really do  
> anything (I didn’t really expect that to work either)
> 
> Can anybody tell me what is happening here?

You see the effect of IO buffering which indeed does not have anything  
to do with multi threading. You basically have two options to remedy  
that

1. use @client.sync = true for automatic flushing after every write  
operation

2. manually invoke #flush

Independent from that you should take measures to make writing to the  
logfile thread safe. One option would be to have a reserved writer  
thread which pulls log messages from a Queue and writes them to the  
file.

Another alternative is your approach (synchronization).

Kind regards

robert

---

<div class="post-metadata">

**Author:** ![maartenm](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/5e9695/32.png) [@maartenm](https://www.ruby-forum.com/u/maartenm)\
**Post date:** [April 28, 2009, 6:57pm UTC](https://www.ruby-forum.com/t/file-i-o-from-threads/166701/3 "2009-04-28T18:57:22Z")

</div>

Robert K. wrote:

> You see the effect of IO buffering

Thanks!

Maarten