#### async_read_at (2 of 4 overloads)

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

    template<
        typename AsyncRandomAccessReadDevice,
        typename MutableBufferSequence,
        typename CompletionCondition,
        typename ReadToken = default_completion_token_t<        typename AsyncRandomAccessReadDevice::executor_type>>
    DEDUCED async_read_at(
        AsyncRandomAccessReadDevice & d,
        uint64_t offset,
        const MutableBufferSequence & buffers,
        CompletionCondition completion_condition,
        ReadToken && token = default_completion_token_t< typename AsyncRandomAccessReadDevice::executor_type >(),
        constraint_t< is_completion_condition< CompletionCondition >::value >  = 0);

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

- The supplied buffers are full. That is, the bytes transferred is equal to the sum of the buffer sizes.
- The completion_condition function object returns 0.

###### 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.

buffers

  One or more buffers into which the data will be read. The sum of the buffer sizes indicates the maximum number of bytes to read from the device. 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 completion 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 asio::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.

token

  The completion token that will be used to produce a completion handler, which will be called when the read completes. Potential completion tokens include `use_future`, `use_awaitable`, `yield_context`, or a function object with the correct completion signature. The function signature of the completion handler must be:

      void handler(
        // Result of operation.
        const asio::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 completion handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using `async_immediate`.

###### Completion Signature

    void(asio::error_code, std::size_t)

###### Example

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

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

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

###### Per-Operation Cancellation

This asynchronous operation supports cancellation for the following `cancellation_type` values:

- `cancellation_type::terminal`
- `cancellation_type::partial`

if they are also supported by the `AsyncRandomAccessReadDevice` type's async_read_some_at operation.