From: Joshua Haberman <joshua@...>
Date: 2005-10-03T09:35:51+09:00
Subject: Re: state of blocking/nonblocking I/O

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Tanaka,

Thanks for your helpful answers!

On Oct 2, 2005, at 5:11 PM, Tanaka Akira wrote:

> In article <17383D97-991D-4947-AB88-85758AB964FB@reverberate.org>,
>   Joshua Haberman <joshua@reverberate.org> writes:
>> - You can try setting O_NONBLOCK on your IO objects with fcntl.  That
>> will help you in the case where you only have one Ruby thread -- now
>> read and write will raise Errno::EAGAIN if the fd isn't ready.
>>
>
> No.
>
> IO#write doesn't raise Errno::EAGAIN but retry until all data is  
> written.
>
> IO#read also retry since Ruby 1.9.
>
> So IO#write and IO#read may block calling thread.

Hrm, so I guess that if I want to do real nonblocking I/O in Ruby, I  
have to write IO#nonblock_read and IO#nonblock_write, that do not  
have this retry behavior?

>> One other question: are the buffered fread()/fwrite() functions
>> guaranteed to work correctly if O_NONBLOCK is set on the underlying
>> descriptor?  I have not been able to find a good answer to this.
>>
>
> fwrite(3) may lost data.
>
> So Ruby 1.8 may lost data.
>
> % ruby-1.8.3 -v
> ruby 1.8.3 (2005-09-21) [i686-linux]
> % ruby-1.8.3 -rfcntl -e '
> w = STDOUT
> w.fcntl(Fcntl::F_SETFL, w.fcntl(Fcntl::F_GETFL) | Fcntl::O_NONBLOCK)
> w << "a" * 4096
> w.flush
> w << "b"
> w.flush
> ' | ruby -e 'sleep 1; p STDIN.read.length'
> 4096

Ooh, that's bad.  What's the explanation for that?

>> # this will block our thread, even though the fd is set to  
>> nonblocking.
>> # however, if you eliminate the background thread, this call with
>> give you EAGAIN,
>> # which is what you want.
>> read_pipe.read
>>
>
> If you want to test some data available, use IO.select.

Yes, but IO.select can't tell me how *much* data I can read or  
write.  IO#read and IO#write can still block if I try to read or  
write too much data, which is what I want to avoid.

Thanks,
Josh

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