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Clarify UDP errors and recvmmsg #3061

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Clarify UDP errors and recvmmsg #3061

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12 changes: 7 additions & 5 deletions docs/src/udp.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -73,7 +73,8 @@ Data types
* `nread`: Number of bytes that have been received.
0 if there is no more data to read. Note that 0 may also mean that an
empty datagram was received (in this case `addr` is not NULL). < 0 if
a transmission error was detected.
a transmission error was detected; if using :man:`recvmmsg(2)` no more
chunks will be received and the buffer can be freed safely.
* `buf`: :c:type:`uv_buf_t` with the received data.
* `addr`: ``struct sockaddr*`` containing the address of the sender.
Can be NULL. Valid for the duration of the callback only.
Expand All @@ -84,10 +85,11 @@ Data types
on error.

When using :man:`recvmmsg(2)`, chunks will have the `UV_UDP_MMSG_CHUNK` flag set,
those must not be freed. There will be a final callback with `nread` set to 0,
`addr` set to NULL and the buffer pointing at the initially allocated data with
the `UV_UDP_MMSG_CHUNK` flag cleared and the `UV_UDP_MMSG_FREE` flag set.
The callee can now safely free the provided buffer.
those must not be freed. If no errors occur, there will be a final callback with
`nread` set to 0, `addr` set to NULL and the buffer pointing at the initially
allocated data with the `UV_UDP_MMSG_CHUNK` flag cleared and the `UV_UDP_MMSG_FREE`
flag set. If a UDP socket error occurs, `nread` will be < 0. In either scenario,
the callee can now safely free the provided buffer.

.. versionchanged:: 1.40.0 added the `UV_UDP_MMSG_FREE` flag.

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