### 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 5. Buffer-oriented asynchronous random-access write device requirements

operation | type | semantics, pre/post-conditions
--- | --- | ---
`a.get_executor()` | A type satisfying the Executor requirements. | Returns the associated I/O executor.

`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.