[GitHub] [arrow] westonpace commented on issue #35226: Any Possible issue in creating std::span from buffer memory directly

["westonpace (via GitHub)" <gi...@apache.org>]

westonpace commented on issue #35226:
URL: https://github.com/apache/arrow/issues/35226#issuecomment-1516558160

   Null values are not guaranteed to be 0.  We use a separate bitmask validity vector to determine if an element is null or not.  If an element is null then it's value is undefined and could be anything.
   
   However, creating spans from buffers is fine.
   
   Those spans will not keep the buffer alive though.  So you need to avoid something like this:
   
   ```
   std::span GetMyData() {
     std::shared_ptr<Array> m_array = GetSomeArray();
     auto ptr = reinterpret_cast<const double*>(m_array->data()->buffers[1]->data()) + m_array->offset();
     std::span view(ptr, m_array->length);
     return view;
   }
   ```
   
   Also, that reinterpret cast is only valid if `m_array->data()->buffers[1]->data()` is 8-byte aligned.  This is true for any buffer allocated by Arrow-C++ and is supposed to be true for any buffer sent over IPC but it is not currently guaranteed for buffers received over Flight due to https://github.com/apache/arrow/issues/32276.


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