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### Datagram socket service requirements

A datagram socket service must meet the requirements for a socket service, as well as the additional requirements listed below.

In the table below, `X` denotes a datagram socket service class for protocol `Protocol`, `a` denotes a value of type `X`, `b` denotes a value of type `X::implementation_type`, `e` denotes a value of type `Protocol::endpoint`, `ec` denotes a value of type `error_code`, `f` denotes a value of type `socket_base::message_flags`, `mb` denotes a value satisfying mutable buffer sequence requirements, `rh` denotes a value meeting `ReadHandler` requirements, `cb` denotes a value satisfying constant buffer sequence requirements, and `wh` denotes a value meeting `WriteHandler` requirements.

Table 8. DatagramSocketService requirements

expression | return type | assertion/note pre/post-condition
--- | --- | ---
`a.receive(b, mb, f, ec);` | `size_t` | pre: `a.is_open(b)`. Reads one or more bytes of data from a connected socket `b`. The mutable buffer sequence `mb` specifies memory where the data should be placed. The operation shall always fill a buffer in the sequence completely before proceeding to the next.
If successful, returns the number of bytes read. Otherwise returns `0`.

`a.async_receive(b, mb, f, rh);` | `void` | pre: `a.is_open(b)`. Initiates an asynchronous operation to read one or more bytes of data from a connected socket `b`. 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 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 operation is invoked,
whichever comes first.
If the operation completes successfully, the `ReadHandler` object `rh` is invoked with the number of bytes transferred. Otherwise it is invoked with `0`.

`a.receive_from(b, mb, e, f, ec);` | `size_t` | pre: `a.is_open(b)`. Reads one or more bytes of data from an unconnected socket `b`. The mutable buffer sequence `mb` specifies memory where the data should be placed. The operation shall always fill a buffer in the sequence completely before proceeding to the next.
If successful, returns the number of bytes read. Otherwise returns `0`.

`a.async_receive_from(b, mb, e, f, rh);` | `void` | pre: `a.is_open(b)`. Initiates an asynchronous operation to read one or more bytes of data from an unconnected socket `b`. 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 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 operation is invoked,
whichever comes first.
The program must ensure the object `e` is valid until the handler for the asynchronous operation is invoked.
If the operation completes successfully, the `ReadHandler` object `rh` is invoked with the number of bytes transferred. Otherwise it is invoked with `0`.

`a.send(b, cb, f, ec);` | `size_t` | pre: `a.is_open(b)`. Writes one or more bytes of data to a connected socket `b`. The constant buffer sequence `cb` specifies memory where the data to be written is located. The operation shall always write a buffer in the sequence completely before proceeding to the next.
If successful, returns the number of bytes written. Otherwise returns `0`.

`a.async_send(b, cb, f, wh);` | `void` | pre: `a.is_open(b)`. Initiates an asynchronous operation to write one or more bytes of data to a connected socket `b`. 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 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 operation is invoked,
whichever comes first.
If the operation completes successfully, the `WriteHandler` object `wh` is invoked with the number of bytes transferred. Otherwise it is invoked with `0`.

    const typename Protocol::endpoint& u = e;
    a.send_to(b, cb, u, f, ec);
| `size_t` | pre: `a.is_open(b)`. Writes one or more bytes of data to an unconnected socket `b`. The constant buffer sequence `cb` specifies memory where the data to be written is located. The operation shall always write a buffer in the sequence completely before proceeding to the next.
If successful, returns the number of bytes written. Otherwise returns `0`.

    const typename Protocol::endpoint& u = e;
    a.async_send(b, cb, u, f, wh);
| `void` | pre: `a.is_open(b)`. Initiates an asynchronous operation to write one or more bytes of data to an unconnected socket `b`. 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 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 operation is invoked,
whichever comes first.
If the operation completes successfully, the `WriteHandler` object `wh` is invoked with the number of bytes transferred. Otherwise it is invoked with `0`.