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

From: Justin McManus (justin_at_[hidden])
Date: 2020-09-05 18:57:25

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]