Boost C++ Libraries: Ticket #3166: Possible Bug in Boost.Interprocess vectorstream

component changed; owner set

anonymous — Thu, 11 Jun 2009 14:07:03 GMT

owner set to Ion Gaztañaga
component None → interprocess

anonymous — Thu, 11 Jun 2009 14:13:02 GMT

This could be resolved by overloading xsputn in the vectorstream class in order to update the internal high water mark pointer after calling std::basic_streambuf<char_type, traits>::xsputn.

virtual std::streamsize xsputn(const char_type* s, std::streamsize n)
{
	std::streamsize ret = base_type::xsputn(s, n);
	if (mp_high_water < base_t::pptr()) mp_high_water = base_t::pptr();
	return ret;
}

chsalvia@… — Thu, 11 Jun 2009 15:48:01 GMT

Essentially, this bug happens because the vector stream class has a member function vector(), which returns a const reference to the internal vector. But, if the the vectorstream is an output stream, a call to vector() also resizes the vector:

 //!Returns a const reference to the internal vector.
   //!Does not throw.
   const vector_type &vector() const
   {
      if (this->m_mode & std::ios_base::out){
         if (mp_high_water < base_t::pptr()){
            //Restore the vector's size if necessary
            mp_high_water = base_t::pptr();
         }
         m_vect.resize(mp_high_water - (m_vect.size() ? &m_vect[0] : 0));
         const_cast<basic_vectorbuf * const>(this)->initialize_pointers();
      }
      return m_vect;
   }

Before resizing, the mp_high_water mark is set to the current position in the output sequence if it is less than pptr(). But, if a previous call to basic_vectorbuf::seekoff sets the stream position to 0, then the mp_high_water mark will not be changed, and the vector will be incorrectly resized.

chsalvia@… — Thu, 11 Jun 2009 16:22:47 GMT

Actually, the best way to fix this is probably to update the high water mark when a call to seekoff() is made. That way, overloading xsputn is unnecessary, and sputc will work properly as well.

In basic_vectorbuf<CharVector, Traits>::seekoff(), the following line could be added to fix the problem:

if (mp_high_water < base_type::pptr()) mp_high_water = base_type::pptr();

Ion Gaztañaga — Thu, 25 Jun 2009 16:19:26 GMT

Fixed for Boost 1.40

status changed; resolution set

Ion Gaztañaga — Thu, 25 Jun 2009 16:23:28 GMT

status new → closed
resolution → fixed