[boost] [iostreams][patch] position_to_offset() is broken for Microsoft Visual C++/Dinkumware

Subject: [boost] [iostreams][patch] position_to_offset() is broken for Microsoft Visual C++/Dinkumware
From: Sylvester-Bradley, Gareth (Gareth.Sylvester-Bradley_at_[hidden])
Date: 2010-03-02 07:20:10

The recent work on Boost.Iostreams by Richard Smith and Daniel James has
really improved this library. Thanks guys; this kind of maintenance is
really appreciated.

 

I have found a bug in position_to_offset() on Microsoft Visual
C++/Dinkumware demonstrated by the following test app -

 

#include <iostream>

#include <boost/iostreams/positioning.hpp>

 

int main( int, char*[] )

{

  const boost::int64_t twoGB = 2 << 30;

  std::streampos pos = boost::iostreams::offset_to_position( twoGB );

  pos -= 2;

  if ( twoGB - 2 != boost::iostreams::position_to_offset( pos ) )

  {

    std::cout << "Failed!" << std::endl;

    return -1;

  }

  return 0;

}

 

This has the potential to affect many parts of Boost.Iostreams but in
particular it means the "small seeks optimization" in
indirect_streambuf.hpp is broken.

 

I believe this is resolved by the following patch to
boost/iostreams/positioning.hpp.

 

105,106c105,106

<
static_cast<stream_offset>(static_cast<std::streamoff>(pos)) -

<
static_cast<stream_offset>(_FPOSOFF(streampos_to_fpos_t(pos)));

---
>            static_cast<stream_offset>(static_cast<std::streamoff>(pos)
-
>
_FPOSOFF(streampos_to_fpos_t(pos)));
 
Raised on Trac: https://svn.boost.org/trac/boost/ticket/3969
 
Best regards,
Gareth
 
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