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### Buffer-oriented asynchronous write stream requirements

In the table below, `a` denotes an asynchronous write stream object, `cb` denotes an object satisfying constant buffer sequence requirements, and `h` denotes an object satisfying write handler requirements.

Table 4. Buffer-oriented asynchronous write stream requirements

operation | type | semantics, pre/post-conditions
--- | --- | ---
`a.get_io_service();` | `io_service&` | Returns the `io_service` object through which the `async_write_some` handler `h` will be invoked.
`a.async_write_some(cb, h);` | `void` | Initiates an asynchronous operation to write one or more bytes of data to the stream `a`. 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` 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.