Buffer-oriented asynchronous random-access write device requirements

In the table below, a denotes an asynchronous write stream object, o denotes an offset of type boost::uint64_t, cb denotes an object satisfying constant buffer sequence requirements, and h denotes an object satisfying write handler requirements.

Table 2. Buffer-oriented asynchronous random-access write device requirements

operation	type	semantics, pre/post-conditions
`a.get_io_service();`	`io_service&`	Returns the `io_service` object through which the `async_write_some_at` handler `h` will be invoked.
`a.async_write_some_at(o, cb, h);`	`void`	Initiates an asynchronous operation to write one or more bytes of data to the device `a` at offset `o`. The operation is performed via the `io_service` object `a.get_io_service()` and behaves according to asynchronous operation requirements. The constant buffer sequence `cb` specifies memory where the data to be written is located. The `async_write_some_at` operation shall always write a buffer in the sequence completely before proceeding to the next. The implementation shall maintain one or more copies of `cb` until such time as the write 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 `cb` is destroyed, or — the handler for the asynchronous write operation is invoked, whichever comes first. If the total size of all buffers in the sequence `cb` is `0`, the asynchronous write operation shall complete immediately and pass `0` as the argument to the handler that specifies the number of bytes written.