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#### async_read_at (4 of 4 overloads)

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

    template<
        typename AsyncRandomAccessReadDevice,
        typename Allocator,
        typename CompletionCondition,
        typename ReadHandler>
    void-or-deduced async_read_at(
        AsyncRandomAccessReadDevice & d,
        uint64_t offset,
        basic_streambuf< Allocator > & b,
        CompletionCondition completion_condition,
        ReadHandler handler);

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

- The completion_condition function object returns 0.

This operation is implemented in terms of zero or more calls to the device's async_read_some_at function.

###### Parameters

d

  The device from which the data is to be read. The type must support the AsyncRandomAccessReadDevice concept.

offset

  The offset at which the data will be read.

b

  A `basic_streambuf` object into which the data will be read. Ownership of the streambuf is retained by the caller, which must guarantee that it remains valid until the handler is called.

completion_condition

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

      std::size_t completion_condition(
        // Result of latest async_read_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 read operation is complete. A non-zero return value indicates the maximum number of bytes to be read on the next call to the device's async_read_some_at function.

handler

  The handler to be called when the read 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 copied into the buffers. If an error
        // occurred, this will be the number of bytes successfully
        // transferred prior to the error.
        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()`.