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From:   Florian Weimer <fw@deneb.enyo.de>
To:     enh <enh@google.com>
Cc:     "Michael Kerrisk \(man-pages\)" <mtk.manpages@gmail.com>,
        linux-man@vger.kernel.org
Subject: Re: [PATCH] pthread_kill.3: Update to match POSIX.
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Date:   Tue, 12 Nov 2019 22:52:41 +0100
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* enh:

> On Tue, Nov 12, 2019 at 1:38 PM Florian Weimer <fw@deneb.enyo.de> wrote:
>>
>> * enh:
>>
>> > POSIX removed ESRCH years ago.
>> >
>> > In resolving http://austingroupbugs.net/view.php?id=1214 it was made
>> > clear that callers can't rely on using signal 0 to test for the
>> > continued existence of a thread. Update the man page to make it clearer
>> > that this doesn't generally work (even if it sometimes seems to).
>> >
>> > See also the long explanation of why this is the case (and how to fix
>> > your code) here:
>> >
>> > https://android.googlesource.com/platform/bionic/+/master/docs/status.md#invalid-handling-targetsdkversion-o
>>
>> Well, if you fix the thread exit race (like musl did, and glibc should
>> as well, see bug 12889), you could get a reliable ESRCH as a side
>> effect.  Pity that POSIX doesn't allow that.
>
> this isn't about the tid stored *in* the object that the pthread_t points to.
>
> like i (briefly) said in the commit message, this is because a
> pthread_t is a pointer, so if you have an old pthread_t that's been
> recycled... boom!

Backing storage for a pthread_t object denoting a joinable thread
cannot be recycled, so that's not the case here.  POSIX mandates
returning success even if the implementation has detected that it must
not send the signal because the thread has already terminated.