#### read_at (4 of 8 overloads)

Attempt to read a certain amount of data at the specified offset before returning.

    template<
        typename SyncRandomAccessReadDevice,
        typename MutableBufferSequence,
        typename CompletionCondition>
    std::size_t read_at(
        SyncRandomAccessReadDevice & d,
        uint64_t offset,
        const MutableBufferSequence & buffers,
        CompletionCondition completion_condition,
        asio::error_code & ec,
        constraint_t< is_completion_condition< CompletionCondition >::value >  = 0);

This function is used to read a certain number of bytes of data from a random access device at the specified offset. The call will block 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.

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

###### Parameters

d

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

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 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 read_some_at function.

ec

  Set to indicate what error occurred, if any.

###### Return Value

The number of bytes read. If an error occurs, returns the total number of bytes successfully transferred prior to the error.