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doc: fix up warning text about character devices #22569

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21 changes: 11 additions & 10 deletions doc/api/fs.md
Original file line number Diff line number Diff line change
Expand Up @@ -1464,23 +1464,24 @@ the specified file descriptor. This means that no `'open'` event will be
emitted. `fd` should be blocking; non-blocking `fd`s should be passed to
[`net.Socket`][].

The blocking `fd`, if pointing to a character device (such as keyboard or
sound card) can potentially block the main thread on stream close. This is
because these devices do not produce EOF character as part of their data
flow cycle, and thereby exemplify endless streams. As a result, they do not
respond to `stream.close()`. A workaround is to close the stream first
using `stream.close()` and then push a random character into the stream, and
issue a single read. This unblocks the reader thread, leads to the completion
of the data flow cycle, and the actual closing of the stream.
If `fd` points to a character device that only supports blocking reads
(such as keyboard or sound card), read operations do not finish until data is
available. This can prevent the process from exiting and the stream from
closing naturally.

```js
const fs = require('fs');
// Create a stream from some character device.
const stream = fs.createReadStream('/dev/input/event0');
setTimeout(() => {
stream.close(); // This does not close the stream.
stream.close(); // This may not close the stream.
// Artificially marking end-of-stream, as if the underlying resource had
// indicated end-of-file by itself, allows the stream to close.
// This does not cancel pending read operations, and if there is such an
// operation, the process may still not be able to exit successfully
// until it finishes.
stream.push(null);
stream.read(0); // Pushing a null and reading leads to close.
stream.read(0);
}, 100);
```

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