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### Buffer-oriented asynchronous read stream requirements

In the table below, `a` denotes an asynchronous read stream object, `mb` denotes an object satisfying mutable buffer sequence requirements, and `h` denotes an object satisfying read handler requirements.

Table 3. Buffer-oriented asynchronous read stream requirements

operation | type | semantics, pre/post-conditions
--- | --- | ---
`a.io_service();` | `io_service&` | Returns the `io_service` object through which the `async_read_some` handler `h` will be invoked.

`a.async_read_some(mb, h);`

`void`

Initiates an asynchronous operation to read one or more bytes of data from the stream `a`. The operation is performed via the `io_service` object `a.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` 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.