Re: [PATCH 5/4] run-command: implement abort_async for pthreads

From: Erik Faye-Lund <kusmabite@gmail.com>
Date: Apr 1, 2011, 20:31 UTC
Message-ID: <AANLkTi=ec9azPovJoDVO6kyXEik4uFQu3dZNxAJyvDY=@mail.gmail.com>
In-Reply-To: <201104012218.07872.j6t@kdbg.org>

On Fri, Apr 1, 2011 at 10:18 PM, Johannes Sixt <j6t@kdbg.org> wrote:

Show 16 quoted lines

> On Freitag, 1. April 2011, Erik Faye-Lund wrote:
>> On Fri, Apr 1, 2011 at 9:42 PM, Johannes Sixt <j6t@kdbg.org> wrote:
>> > But this does not help the case at hand in any way. How would you
>> > interrupt a thread that is blocked in ReadFile()? The point of
>> > pthread_cancel() is that it interrupts blocked system calls
>>
>> There is no mention of such a guarantee in POSIX (section 2.9.5 Thread
>> Cancellation), so relying on that is undefined behavior.
>
> In the paragraph before the bulleted list at the end of "Cancellation Points":
>
> "...If a thread has cancelability enabled and a cancellation request is made
> with the thread as a target while the thread is suspended at a cancellation
> point, the thread shall be awakened and the cancellation request shall be
> acted upon..."
>

A blocking thread and a suspended are two different matters. A suspended thread is a thread that has been explicitly suspended by wait, waitpid, sleep, pause etc. These functions explicitly say that they suspend the thread ("shall suspend the calling thread until"), while read etc does not ("shall block the calling thread until").

Similarly, making a blocking read/write fail (or terminate mid-way) is not the same thing as awakening the thread.

I see how some people can read something like this into this section, but I think it's pretty clear - this is not what it's talking about. In fact, the more I read the relevant texts, the more convinced I get that implementations that does terminate read/write strictly speaking is in violation of the standard.