mem_algo_common.hpp

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      assert((needs_backwards % backwards_multiple) == 0);
      needs_backwards_lcmed = detail::get_rounded_size(needs_backwards, lcm);
      lcm_out = lcm;
      needs_backwards_lcmed_out = needs_backwards_lcmed;
      return true;
   }

   static multiallocation_iterator allocate_many
      ( MemoryAlgorithm *memory_algo
      , const std::size_t *elem_sizes
      , std::size_t n_elements
      , std::size_t sizeof_element)
   {
      return this_type::priv_allocate_many(memory_algo, elem_sizes, n_elements, sizeof_element);
   }

   static void* allocate_aligned
      (MemoryAlgorithm *memory_algo, std::size_t nbytes, std::size_t alignment)
   {
      
      //Ensure power of 2
      if ((alignment & (alignment - std::size_t(1u))) != 0){
         //Alignment is not power of two
         BOOST_ASSERT((alignment & (alignment - std::size_t(1u))) == 0);
         return 0;
      }

      std::size_t real_size;
      if(alignment <= Alignment){
         return memory_algo->priv_allocate(allocate_new, nbytes, nbytes, real_size).first;
      }

      if(nbytes > UsableByPreviousChunk)
         nbytes -= UsableByPreviousChunk;
      
      //We can find a aligned portion if we allocate a block that has alignment
      //nbytes + alignment bytes or more.
      std::size_t minimum_allocation = max_value
         (nbytes + alignment, std::size_t(MinBlockUnits*Alignment));
      //Since we will split that block, we must request a bit more memory
      //if the alignment is near the beginning of the buffer, because otherwise,
      //there is no space for a new block before the alignment.
      // 
      //            ____ Aligned here
      //           |
      //  -----------------------------------------------------
      // | MBU | 
      //  -----------------------------------------------------
      std::size_t request = 
         minimum_allocation + (2*MinBlockUnits*Alignment - AllocatedCtrlBytes
         //prevsize - UsableByPreviousChunk
         );

      //Now allocate the buffer
      void *buffer = memory_algo->priv_allocate(allocate_new, request, request, real_size).first;
      if(!buffer){
         return 0;
      }
      else if ((((std::size_t)(buffer)) % alignment) == 0){
         //If we are lucky and the buffer is aligned, just split it and
         //return the high part
         block_ctrl *first  = memory_algo->priv_get_block(buffer);
         std::size_t old_size = first->m_size;
         const std::size_t first_min_units =
            max_value(ceil_units(nbytes) + AllocatedCtrlUnits, std::size_t(MinBlockUnits));
         //We can create a new block in the end of the segment
         if(old_size >= (first_min_units + MinBlockUnits)){
            block_ctrl *second =  reinterpret_cast<block_ctrl *>
               (reinterpret_cast<char*>(first) + Alignment*first_min_units);
            first->m_size  = first_min_units;
            second->m_size = old_size - first->m_size;
            BOOST_ASSERT(second->m_size >= MinBlockUnits);
            memory_algo->priv_mark_new_allocated_block(first);
            //memory_algo->priv_tail_size(first, first->m_size);
            memory_algo->priv_mark_new_allocated_block(second);
            memory_algo->priv_deallocate(memory_algo->priv_get_user_buffer(second));
         }
         return buffer;
      }

      //Buffer not aligned, find the aligned part.
      // 
      //                    ____ Aligned here
      //                   |
      //  -----------------------------------------------------
      // | MBU +more | ACB |
      //  -----------------------------------------------------
      char *pos = reinterpret_cast<char*>
         (reinterpret_cast<std::size_t>(static_cast<char*>(buffer) +
            //This is the minimum size of (2)
            (MinBlockUnits*Alignment - AllocatedCtrlBytes) +
            //This is the next MBU for the aligned memory
            AllocatedCtrlBytes +
            //This is the alignment trick
            alignment - 1) & -alignment);

      //Now obtain the address of the blocks
      block_ctrl *first  = memory_algo->priv_get_block(buffer);
      block_ctrl *second = memory_algo->priv_get_block(pos);
      assert(pos <= (reinterpret_cast<char*>(first) + first->m_size*Alignment));
      assert(first->m_size >= 2*MinBlockUnits);
      assert((pos + MinBlockUnits*Alignment - AllocatedCtrlBytes + nbytes*Alignment/Alignment) <=
             (reinterpret_cast<char*>(first) + first->m_size*Alignment));
      //Set the new size of the first block
      std::size_t old_size = first->m_size;
      first->m_size  = (reinterpret_cast<char*>(second) - reinterpret_cast<char*>(first))/Alignment;
      memory_algo->priv_mark_new_allocated_block(first);

      //Now check if we can create a new buffer in the end
      //
      //              __"second" block
      //             |      __Aligned here
      //             |     |      __"third" block
      //  -----------|-----|-----|------------------------------
      // | MBU +more | ACB | (3) | BCU |
      //  -----------------------------------------------------
      //This size will be the minimum size to be able to create a
      //new block in the end.
      const std::size_t second_min_units = max_value(std::size_t(MinBlockUnits),
                        ceil_units(nbytes) + AllocatedCtrlUnits );

      //Check if we can create a new block (of size MinBlockUnits) in the end of the segment
      if((old_size - first->m_size) >= (second_min_units + MinBlockUnits)){
         //Now obtain the address of the end block
         block_ctrl *third = new (reinterpret_cast<char*>(second) + Alignment*second_min_units)block_ctrl;
         second->m_size = second_min_units;
         third->m_size  = old_size - first->m_size - second->m_size;
         BOOST_ASSERT(third->m_size >= MinBlockUnits);
         memory_algo->priv_mark_new_allocated_block(second);
         memory_algo->priv_mark_new_allocated_block(third);
         memory_algo->priv_deallocate(memory_algo->priv_get_user_buffer(third));
      }
      else{
         second->m_size = old_size - first->m_size;
         assert(second->m_size >= MinBlockUnits);
         memory_algo->priv_mark_new_allocated_block(second);
      }

      memory_algo->priv_deallocate(memory_algo->priv_get_user_buffer(first));
      return memory_algo->priv_get_user_buffer(second);
   }

   static bool try_shrink 
      (MemoryAlgorithm *memory_algo, void *ptr
      ,const std::size_t max_size,   const std::size_t preferred_size
      ,std::size_t &received_size)
   {
      (void)memory_algo;
      //Obtain the real block
      block_ctrl *block = memory_algo->priv_get_block(ptr);
      std::size_t old_block_units = block->m_size;

      //The block must be marked as allocated
      BOOST_ASSERT(memory_algo->priv_is_allocated_block(block));

      //Check if alignment and block size are right
      assert_alignment(ptr);

      //Put this to a safe value
      received_size = (old_block_units - AllocatedCtrlUnits)*Alignment + UsableByPreviousChunk;

      //Now translate it to Alignment units
      const std::size_t max_user_units       = floor_units(max_size - UsableByPreviousChunk);
      const std::size_t preferred_user_units = ceil_units(preferred_size - UsableByPreviousChunk);

      //Check if rounded max and preferred are possible correct
      if(max_user_units < preferred_user_units)
         return false;

      //Check if the block is smaller than the requested minimum
      std::size_t old_user_units = old_block_units - AllocatedCtrlUnits;

      if(old_user_units < preferred_user_units)
         return false;

      //If the block is smaller than the requested minimum
      if(old_user_units == preferred_user_units)
         return true;

      std::size_t shrunk_user_units = 
         ((BlockCtrlUnits - AllocatedCtrlUnits) > preferred_user_units)
         ? (BlockCtrlUnits - AllocatedCtrlUnits)
         : preferred_user_units;

      //Some parameter checks
      if(max_user_units < shrunk_user_units)
         return false;

      //We must be able to create at least a new empty block
      if((old_user_units - shrunk_user_units) < BlockCtrlUnits ){
         return false;
      }

      //Update new size
      received_size = shrunk_user_units*Alignment + UsableByPreviousChunk;
      return true;
   }

   static bool shrink 
      (MemoryAlgorithm *memory_algo, void *ptr
      ,const std::size_t max_size,   const std::size_t preferred_size
      ,std::size_t &received_size)
   {
      //Obtain the real block
      block_ctrl *block = memory_algo->priv_get_block(ptr);
      std::size_t old_block_units = block->m_size;

      if(!try_shrink 
         (memory_algo, ptr, max_size, preferred_size, received_size)){
         return false;
      }

      //Check if the old size was just the shrunk size (no splitting)
      if((old_block_units - AllocatedCtrlUnits) == ceil_units(preferred_size - UsableByPreviousChunk))
         return true;

      //Now we can just rewrite the size of the old buffer
      block->m_size = (received_size-UsableByPreviousChunk)/Alignment + AllocatedCtrlUnits;
      BOOST_ASSERT(block->m_size >= BlockCtrlUnits);

      //We create the new block
      block_ctrl *new_block = reinterpret_cast<block_ctrl*>
                  (reinterpret_cast<char*>(block) + block->m_size*Alignment);
      //Write control data to simulate this new block was previously allocated
      //and deallocate it
      new_block->m_size = old_block_units - block->m_size;
      BOOST_ASSERT(new_block->m_size >= BlockCtrlUnits);
      memory_algo->priv_mark_new_allocated_block(block);
      memory_algo->priv_mark_new_allocated_block(new_block);
      memory_algo->priv_deallocate(memory_algo->priv_get_user_buffer(new_block));
      return true;
   }

   private:
   static multiallocation_iterator priv_allocate_many
      ( MemoryAlgorithm *memory_algo
      , const std::size_t *elem_sizes
      , std::size_t n_elements
      , std::size_t sizeof_element)
   {
      //Note: sizeof_element == 0 indicates that we want to
      //allocate n_elements of the same size "*elem_sizes"

      //Calculate the total size of all requests
      std::size_t total_request_units = 0;
      std::size_t elem_units = 0;
      const std::size_t ptr_size_units = memory_algo->priv_get_total_units(sizeof(multi_allocation_next_ptr));
      if(!sizeof_element){
         elem_units = memory_algo->priv_get_total_units(*elem_sizes);
         elem_units = ptr_size_units > elem_units ? ptr_size_units : elem_units;
         total_request_units = n_elements*elem_units;
      }
      else{
         for(std::size_t i = 0; i < n_elements; ++i){
            elem_units = memory_algo->priv_get_total_units(elem_sizes[i]*sizeof_element);
            elem_units = ptr_size_units > elem_units ? ptr_size_units : elem_units;
            total_request_units += elem_units;
         }
      }

      multi_allocation_next_ptr first = 0, previous = 0;
      std::size_t low_idx = 0;
      while(low_idx < n_elements){
         std::size_t total_bytes = total_request_units*Alignment - AllocatedCtrlBytes + UsableByPreviousChunk;
         std::size_t min_allocation = (!sizeof_element)
            ?  elem_units
            :  memory_algo->priv_get_total_units(elem_sizes[low_idx]*sizeof_element);
         min_allocation = min_allocation*Alignment - AllocatedCtrlBytes + UsableByPreviousChunk;

         std::size_t received_size;
         std::pair<void *, bool> ret = memory_algo->priv_allocate
            (allocate_new, min_allocation, total_bytes, received_size, 0);
         if(!ret.first){
            break;
         }

         block_ctrl *block = memory_algo->priv_get_block(ret.first);
         std::size_t received_units = block->m_size;
         char *block_address = reinterpret_cast<char*>(block);

         std::size_t total_used_units = 0;
//         block_ctrl *prev_block = 0;
         while(total_used_units < received_units){
            if(sizeof_element){
               elem_units = memory_algo->priv_get_total_units(elem_sizes[low_idx]*sizeof_element);
               elem_units = ptr_size_units > elem_units ? ptr_size_units : elem_units;
            }
            if(total_used_units + elem_units > received_units)
               break;
            total_request_units -= elem_units;
            //This is the position where the new block must be created
            block_ctrl *new_block = reinterpret_cast<block_ctrl *>(block_address);
            assert_alignment(new_block);

            //The last block should take all the remaining space
            if((low_idx + 1) == n_elements ||
               (total_used_units + elem_units + 
               ((!sizeof_element)
                  ? elem_units
                  : memory_algo->priv_get_total_units(elem_sizes[low_idx+1]*sizeof_element))
               ) > received_units){
               //By default, the new block will use the rest of the buffer
               new_block->m_size = received_units - total_used_units;
               memory_algo->priv_mark_new_allocated_block(new_block);

               //If the remaining units are bigger than needed and we can
               //split it obtaining a new free memory block do it.
               if((received_units - total_used_units) >= (elem_units + MemoryAlgorithm::BlockCtrlUnits)){
                  std::size_t shrunk_received;
                  std::size_t shrunk_request = elem_units*Alignment - AllocatedCtrlBytes + UsableByPreviousChunk;
                  bool shrink_ok = shrink
                        (memory_algo
                        ,memory_algo->priv_get_user_buffer(new_block)
                        ,shrunk_request
                        ,shrunk_request
                        ,shrunk_received);
                  (void)shrink_ok;
                  //Shrink must always succeed with passed parameters
                  BOOST_ASSERT(shrink_ok);
                  //Some sanity checks
                  BOOST_ASSERT(shrunk_request == shrunk_received);
                  BOOST_ASSERT(elem_units == ((shrunk_request-UsableByPreviousChunk)/Alignment + AllocatedCtrlUnits));
                  //"new_block->m_size" must have been reduced to elem_units by "shrink"
                  BOOST_ASSERT(new_block->m_size == elem_units);
                  //Now update the total received units with the reduction
                  received_units = elem_units + total_used_units;
               }
            }
            else{
               new_block->m_size = elem_units;
               memory_algo->priv_mark_new_allocated_block(new_block);
            }

            block_address += new_block->m_size*Alignment;
            total_used_units += new_block->m_size;
            //Check we have enough room to overwrite the intrusive pointer
            assert((new_block->m_size*Alignment - AllocatedCtrlUnits) >= sizeof(multi_allocation_next_t));
            multi_allocation_next_ptr p = new(memory_algo->priv_get_user_buffer(new_block))multi_allocation_next_t(0);
      
            if(!first){
               first = p;
            }
            else{
               previous->next_ = p;
            }
            previous = p;
            ++low_idx;
            //prev_block = new_block;
         }
         //Sanity check
         BOOST_ASSERT(total_used_units == received_units);
      }

      if(low_idx != n_elements){
         while(first){
            multi_allocation_next_ptr prev = first;
            first = first->next_;
            memory_algo->priv_deallocate(detail::get_pointer(prev));
         }
         return multiallocation_iterator();
      }
      else{
         return multiallocation_iterator(first);
      }
   }
};

}  //namespace detail {
}  //namespace interprocess {
}  //namespace boost {

#include <boost/interprocess/detail/config_end.hpp>

#endif   //#ifndef BOOST_INTERPROCESS_DETAIL_MEM_ALGO_COMMON_HPP