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Date: Tue, 05 Aug 2014 21:06:16 +0200
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: Re: PATCH: don't call cleanup handlers after a regular
return from the thread start function
Hello,
Am Dienstag, den 05.08.2014, 13:09 -0400 schrieb Rich Felker:
> > Linux manpages are more explicit and state:
> >
> > Clean-up handlers are not called if the thread terminates by
> > performing a return from the thread start function.
> >
> > This patch aligns musl to that behavior.
>
> Could you clarify why this patch is necessary? I think such a return
> is explicitly UB.
>
> "The effect of the use of return, break, continue, and goto to
> prematurely leave a code block described by a pair of
> pthread_cleanup_push() and pthread_cleanup_pop() functions calls is
> undefined."
yes
The linux man page (glibc I suppose) has no such mention, only
disallows longjmp and the phrase I cited above. I think this
establishes an extension for POSIX on Linux systems.
But I have another reason for wanting that, future compatibility with
C threads. Programs that are written for C threads will not be aware
of such interdictions. Concretely in our case of my C thread v3 patch
a user can longjmp from a once-init-handler (written by her or him)
through pthread_once (in libc, for musl with pthread_cleanup_push) to
the thread start function (again user code) and then return from
there. (All of this seems to be allowed by POSIX)
I consider to not execute the cleanup handlers a little bit more
friendly than executing them.
Another possibility would be to split the behavior
- abort for pthreads (this is nicer than executing the handlers or
than silently ignore them)
- ignore for C threads
Jens
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