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Date: Thu, 08 Apr 2021 19:04:09 +0200
From: Florian Weimer <fweimer@...hat.com>
To: "Andrey Bugaevskiy" <bugaevskiy@...dex-team.ru>
Cc: <musl@...ts.openwall.com>
Subject: Re: errno and swapcontext in a multithreaded setup
* Andrey Bugaevskiy:
> I'm using makecontext/swapcontext to migrate contexts between threads
> and this sometimes leads to getting incorrect errno values.
>
> Investigating further I've noticed that __errno_location
> is marked __attribute__((const)).
> This causes optimizers to assume that errno address never changes
> in the scope of the function which is not the case in my scenario.
The optimizers make the same assumption for actual thread-local
variables, not just __errno_location. Migrating contexts between
threads results in undefined behavior.
Thanks,
Florian
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