position_to_offset() is broken for Microsoft Visual C++/Dinkumware

[gareth.sylvester-bradley@…]

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 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)));

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.

[Daniel James]

(In [60581]) In position_to_offset, only cast to stream_offset after calculating _Myoff. Refs #3969.

Thanks to Garth Sylvester-Bradley.

[Daniel James]

(In [60666]) Merge iostreams.

• Fix write_device_impl<ostream_tag>. Fixes #3839
• Fix error checks after calling SetFilePointer. Fixes #3953
• Gzip filter shouldn't require its source to be peekable. Fixes #3723.
• In position_to_offset, only cast to stream_offset after calculating _Myoff. Fixes #3969.
• ptrdiff_t is in std. Fixes #2505.