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#### async_read_at (3 of 4 overloads)

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

    template<
        typename AsyncRandomAccessReadDevice,
        typename Allocator,
        typename ReadHandler>
    void-or-deduced async_read_at(
        AsyncRandomAccessReadDevice & d,
        uint64_t offset,
        basic_streambuf< Allocator > & b,
        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:

- An error occurred.

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.

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()`.

###### Remarks

This overload is equivalent to calling:

    boost::asio::async_read_at(
        d, 42, b,
        boost::asio::transfer_all(),
        handler);