Issue #9420 has been updated by Nobuyoshi Nakada.

Description updated

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Feature #9420: warn and puts should be atomic
https://bugs.ruby-lang.org/issues/9420#change-44380

* Author: Eric Wong
* Status: Open
* Priority: Low
* Assignee: 
* Category: core
* Target version: current: 2.2.0
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Since we added use of writev in r44618, we should make warn/puts atomic.
It would make development of multiprocess/multithreaded processes easier,
I think it was intended to be atomic based on old comments in io.c:

```
-       /*
-        * xxx: use writev to avoid double write if available
-        * writev may help avoid context switch between "a" and "\n" in
-        * STDERR.puts "a" [ruby-dev:25080] (rebroken since native threads
-        * introduced in 1.9)
-        */
```



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