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#### read_at (4 of 6 overloads)

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

    template<
        typename SyncRandomAccessReadDevice,
        typename Allocator>
    std::size_t read_at(
        SyncRandomAccessReadDevice & d,
        boost::uint64_t offset,
        basic_streambuf< Allocator > & b);

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:

- An error occurred.

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.

b

  The `basic_streambuf` object into which the data will be read.

###### Return Value

The number of bytes transferred.

###### Exceptions

boost::system::system_error

  Thrown on failure.

###### Remarks

This overload is equivalent to calling:

    boost::asio::read_at(
        d, 42, b,
        boost::asio::transfer_all());