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From: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <59ab84cc-ec75-b55b-a6db-519dc4589727@linaro.org>
Date: Fri, 15 Dec 2017 09:50:41 -0600
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Subject: Re: [Qemu-devel] [RFC PATCH 0/5] Scoped locks using
 attribute((cleanup))
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To: Eric Blake <eblake@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Peter Maydell <peter.maydell@linaro.org>
Cc: "Emilio G . Cota" <cota@braap.org>, Fam Zheng <famz@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>

On 12/12/2017 02:41 PM, Eric Blake wrote:
> On 12/11/2017 03:32 PM, Paolo Bonzini wrote:
>> On 11/12/2017 15:11, Eric Blake wrote:
>>> I don't know if there is a way to make gcc insert stack-unwind
>>> directives that are honored across longjmp (I know C++ does it for
>>> exceptions; so there may be a way, and I just don't know it).
>>
>> Probably -fexceptions.
>>
> 
> Well, that's what 'info gcc' mentions:
> 
> 'cleanup (CLEANUP_FUNCTION)'
>      The 'cleanup' attribute runs a function when the variable goes out
>      of scope.  This attribute can only be applied to auto function
>      scope variables; it may not be applied to parameters or variables
>      with static storage duration.  The function must take one
>      parameter, a pointer to a type compatible with the variable.  The
>      return value of the function (if any) is ignored.
> 
>      If '-fexceptions' is enabled, then CLEANUP_FUNCTION is run during
>      the stack unwinding that happens during the processing of the
>      exception.  Note that the 'cleanup' attribute does not allow the
>      exception to be caught, only to perform an action.  It is undefined
>      what happens if CLEANUP_FUNCTION does not return normally.
> 
> but adding -fexceptions to my sample program does NOT make a difference
> (apparently, unwind cleanup triggered by C++ exceptions is NOT the same
> as unwinding done by longjmp()).

longjmp isn't an exception, and so doesn't run stack cleanups.  You'd need to
use _Unwind_Throw ... except for the fact that C doesn't have a mechanism by
which we can catch it.

We would really need to use C++ and throw to get those semantics.


r~