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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



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


>            static_cast<stream_offset>(static_cast<std::streamoff>(pos)
Raised on Trac:
Best regards,
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