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#### async_write_at (2 of 4 overloads)

Start an asynchronous operation to write a certain amount of data at the specified offset.

    template<
        typename AsyncRandomAccessWriteDevice,
        typename ConstBufferSequence,
        typename CompletionCondition,
        typename WriteHandler>
    void-or-deduced async_write_at(
        AsyncRandomAccessWriteDevice & d,
        uint64_t offset,
        const ConstBufferSequence & buffers,
        CompletionCondition completion_condition,
        WriteHandler handler);

This function is used to asynchronously write a certain number of bytes of data to a random access device at a specified offset. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

- All of the data in the supplied buffers has been written. That is, the bytes transferred is equal to the sum of the buffer sizes.
- The completion_condition function object returns 0.

This operation is implemented in terms of zero or more calls to the device's async_write_some_at function, and is known as a composed operation. The program must ensure that the device performs no overlapping write operations (such as async_write_at, the device's async_write_some_at function, or any other composed operations that perform writes) until this operation completes. Operations are overlapping if the regions defined by their offsets, and the numbers of bytes to write, intersect.

###### Parameters

d

  The device to which the data is to be written. The type must support the AsyncRandomAccessWriteDevice concept.

offset

  The offset at which the data will be written.

buffers

  One or more buffers containing the data to be written. Although the buffers object may be copied as necessary, ownership of the underlying memory blocks is retained by the caller, which must guarantee that they remain valid until the handler is called.

completion_condition

  The function object to be called to determine whether the write operation is complete. The signature of the function object must be:

      std::size_t completion_condition(
        // Result of latest async_write_some_at operation.
        const boost::system::error_code& error,

        // Number of bytes transferred so far.
        std::size_t bytes_transferred
      );

  A return value of 0 indicates that the write operation is complete. A non-zero return value indicates the maximum number of bytes to be written on the next call to the device's async_write_some_at function.

handler

  The handler to be called when the write operation completes. Copies will be made of the handler as required. The function signature of the handler must be:

      void handler(
        // Result of operation.
        const boost::system::error_code& error,

        // Number of bytes written from the buffers. If an error
        // occurred, this will be less than the sum of the buffer sizes.
        std::size_t bytes_transferred
      );

  Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using `boost::asio::io_service::post()`.

###### Example

To write a single data buffer use the `buffer` function as follows:

    boost::asio::async_write_at(d, 42,
        boost::asio::buffer(data, size),
        boost::asio::transfer_at_least(32),
        handler);

See the `buffer` documentation for information on writing multiple buffers in one go, and how to use it with arrays, boost::array or std::vector.