Re: buffer_size behavior in boost::iostreams::filtering_istream

From: Justin McManus (justin_at_[hidden])
Date: 2020-09-07 16:41:52

Thanks very much for the replies on this post.

Following Paul's advice, I did some thorough testing with version 1.74.0
(the latest boost version, as of this writing). The new version of boost
seems to resolve the issue entirely. This latest version works without
error regardless of how I set the buffering parameters.

Regards,
Justin

On Sun, Sep 6, 2020 at 7:44 AM Paul A Bristow via Boost <
boost_at_[hidden]> wrote:

> It would be courteous (and probably wise) to update to latest Boost
> version before raising an issue?
>
> Both 1.65 and 1.55 are way out of date.
>
> Hundreds of bugs have been squashed since then!
>
> Users of Boost should plan their development process to update Boost
> version regularly so as not to
> get too far behind like this.
>
> Paul
>
> > -----Original Message-----
> > From: Boost <boost-bounces_at_[hidden]> On Behalf Of Richard Hodges
> via Boost
> > Sent: 6 September 2020 08:55
> > To: boost_at_[hidden] List <boost_at_[hidden]>
> > Cc: Richard Hodges <hodges.r_at_[hidden]>
> > Subject: Re: [boost] buffer_size behavior in
> boost::iostreams::filtering_istream
> >
> > Perhaps file an issue here?
> >
> > https://github.com/boostorg/iostreams/issues
> >
> >
> > On Sat, 5 Sep 2020 at 20:58, Justin McManus via Boost <
> boost_at_[hidden]>
> > wrote:
> >
> > > To follow up on my original post, I have two additional observations:
> > > 1.) I'm currently using boost version 1_65_1. In version 1_58_0, the
> > > code always read to the EOF without an issue, even with the default
> buffering.
> > > 2.) I tried making the buffer size arbitrarily large (1e8), but this
> > > had almost no effect at all on the behavior of the code. Since that
> > > buffer size is guaranteed to be large enough to hold any line in the
> > > input files I'm processing, it would seem that a limitation in the
> > > buffer size is not the underlying problem.
> > >
> > > On Sat, Sep 5, 2020 at 2:23 PM Justin McManus <justin_at_[hidden]>
> wrote:
> > >
> > > > I have some code that works as intended, but it requires setting a
> > > > buffer_size parameter to zero on a std::ifstream pushed onto a
> > > > filtering chain, and I'd like to understand why, to ensure I'm not
> > > > introducing a
> > > bug
> > > > or a hack.
> > > >
> > > > I have essentially the following code:
> > > >
> > > >
> > > ----------------------------------------------------------------------
> > > ----------------------------------
> > > > std::ifstream m_jf("json_filename", std::ios_base::in |
> > > > std::ios_base::binary); std::locale utf8_locale("en_US.UTF-8");
> > > > m_jf.imbue(utf8_locale);
> > > >
> > > > boost::iostreams::filtering_istream m_inbuf;
> > > > m_inbuf.push(boost::iostreams::bzip2_decompressor());
> > > > m_inbuf.push(m_jf);
> > > >
> > > > std::string m_line;
> > > > while (std::getline(m_inbuf, m_line)) {
> > > >   // Process the current line from the JSON file }
> > > >
> > > >
> > >
>
> ----------------------------------------------------------------------------------------------------
> ----
> > > >
> > > > What I find is that the std::getline call will fail before the code
> has
> > > > reached the EOF. It will always fail at the same line in a given JSON
> > > file,
> > > > but it will fail on different lines in different JSON files. It's
> > > perfectly
> > > > reproducible.
> > > >
> > > > However, if I change lines 4 and 5 to
> > > >      m_inbuf.push(boost::iostreams::bzip2_decompressor(), *0*);
> > > >      m_inbuf.push(m_jf, *0*);
> > > > then the problem goes away.
> > > >
> > > > My question is, Why does setting the buffer_size parameter to zero
> solve
> > > > the issue? What does this do, exactly? I saw the suggestion to set
> the
> > > > buffer size this way from an old post in 2009, and it appears to
> work,
> > > but
> > > > I'd like a deeper understanding of what's happening under the hood.
> If
> > > the
> > > > buffer size is set to zero, what does the underlying implementation
> do,
> > > and
> > > > how might this influence whether std::getline fails before the EOF?
> > > >
> > > > Thanks very much,
> > > > Justin
> > > >
> > > > --
> > > > Justin McManus, Ph.D.
> > > > Principal Scientist
> > > > Lead Computational Biologist and Statistical Geneticist
> > > > Kallyope, Inc.
> > > > 430 East 29th Street, Suite 1050
> > > > New York, NY 10016
> > > >
> > >
> > >
> > > --
> > > Justin McManus, Ph.D.
> > > Principal Scientist
> > > Lead Computational Biologist and Statistical Geneticist
> > > Kallyope, Inc.
> > > 430 East 29th Street, Suite 1050
> > > New York, NY 10016
> > > [phone]
> > >
> > > _______________________________________________
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> > >
> >
> >
> > --
> > Richard Hodges
> > hodges.r_at_[hidden]
> > office: [phone]
> > home: [phone]
> > mobile: [phone]
> >
> > _______________________________________________
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>
>
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-- 
Justin McManus, Ph.D.
Principal Scientist
Lead Computational Biologist and Statistical Geneticist
Kallyope, Inc.
430 East 29th Street, Suite 1050
New York, NY 10016
[phone]