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Date: Thu, 8 Apr 2021 13:17:35 -0400
From: Rich Felker <>
To: Florian Weimer <>
Cc: Andrey Bugaevskiy <>,
Subject: Re: errno and swapcontext in a multithreaded setup

On Thu, Apr 08, 2021 at 07:04:09PM +0200, Florian Weimer wrote:
> * 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.

Indeed, this is not functionality that has ever been defined on any
implementation I'm aware of. There are a lot of other things that
could go wrong. Even if you don't explicitly use TLS, if your code is
built with stack protector it may access the canary value from TLS,
and the value may be individual to each thread (musl doesn't currently
do this, but could in the future as further hardening).


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