Buffer-oriented asynchronous random-access read device requirements

In the table below, a denotes an asynchronous random access read device object, o denotes an offset of type boost::uint64_t, mb denotes an object satisfying mutable buffer sequence requirements, and h denotes an object satisfying read handler requirements.

Table 1. Buffer-oriented asynchronous random-access read device requirements

operation	type	semantics, pre/post-conditions
`a.get_io_service();`	`io_service&`	Returns the `io_service` object through which the `async_read_some_at` handler `h` will be invoked.
`a.async_read_some_at(o, mb, h);`	`void`	Initiates an asynchronous operation to read one or more bytes of data from the device `a` at the offset `o`. The operation is performed via the `io_service` object `a.get_io_service()` and behaves according to asynchronous operation requirements. The mutable buffer sequence `mb` specifies memory where the data should be placed. The `async_read_some_at` operation shall always fill a buffer in the sequence completely before proceeding to the next. The implementation shall maintain one or more copies of `mb` until such time as the read operation no longer requires access to the memory specified by the buffers in the sequence. The program must ensure the memory is valid until: — the last copy of `mb` is destroyed, or — the handler for the asynchronous read operation is invoked, whichever comes first. If the total size of all buffers in the sequence `mb` is `0`, the asynchronous read operation shall complete immediately and pass `0` as the argument to the handler that specifies the number of bytes read.