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doc: Add notes how addMembership should be managed #16240

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doc: Add notes how addMembership should be managed #16240

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20 changes: 20 additions & 0 deletions doc/api/dgram.md
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Original file line number Diff line number Diff line change
Expand Up @@ -95,6 +95,26 @@ Tells the kernel to join a multicast group at the given `multicastAddress` and
one interface and will add membership to it. To add membership to every
available interface, call `addMembership` multiple times, once per interface.

UDP works as a bit special with cluster. When `dgram` creates a socket in the
master process, each worker gets a copy as in `dup(2)``, except the actual
mechanism is `sendmsg(2)`` with a SCM_RIGHTS vector. Therefor, `.addMembership`
needs to be managed to be called only once.

```js
const cluster = require('cluster');
const dgram = require('dgram');

if (cluster.isMaster) {
cluster.fork(); // ok
cluster.fork(); // EADDRINUSE
} else {
const s = dgram.createSocket('udp4');
s.bind(1234, () => {
s.addMembership('224.0.0.114');
});
}
```

### socket.address()
<!-- YAML
added: v0.1.99
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