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Subject: Re: [OMPI devel] RFC: Add an __attribute__((destructor)) function to opal
From: Paul Hargrove (phhargrove_at_[hidden])
Date: 2014-07-15 20:06:17


The priority appears to have been added in gcc 4.3.
You'll note it is not described in
https://gcc.gnu.org/onlinedocs/gcc-4.2.0/gcc/Function-Attributes.html

I also don't think the presence of the priority argument fixes anything...

An OpenMPI code author cannot change the "priority" of a ctor or dtor in a
precompiled third-party library (libpmi comes to mind). Nor can one know
what value the third part chose (in order to be higher or lower than
theirs). You cannot even be assured the third-party didn't set priority to
INT_MIN or INT_MAX (or whatever).

That text also says nothing about dl_open() and dl_close() which must be
considered in Open MPI.

Before assuming constructor/destructor attributes are going to save the
world, wash your dog, and pick up the dry cleaning, one should probably
verify some minimal level of support on non-gnu tool-chains including
vendor compilers (PGI, XLC, etc) and system linkers (Darwin and Solaris).

-Paul

On Tue, Jul 15, 2014 at 4:52 PM, Joshua Ladd <jladd.mlnx_at_[hidden]> wrote:

> According to http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
>
> *"constructor *
> * destructor *
> * constructor (*priority*)** destructor (priority)* *The constructor
> attribute causes the function to be called automatically before execution
> enters main (). Similarly, the destructor attribute causes the function to
> be called automatically after main () completes or exit () is called.
> Functions with these attributes are useful for initializing data that is
> used implicitly during the execution of the program. *
>
> *You may provide an optional integer priority to control the order in
> which constructor and destructor functions are run. A constructor with a
> smaller priority number runs before a constructor with a larger priority
> number; the opposite relationship holds for destructors. So, if you have a
> constructor that allocates a resource and a destructor that deallocates the
> same resource, both functions typically have the same priority. The
> priorities for constructor and destructor functions are the same as those
> specified for namespace-scope C++ objects (see C++ Attributes
> <http://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html#C_002b_002b-Attributes>).
> *
>
> *These attributes are not currently implemented for Objective-C."*
>
>
>
> On Tue, Jul 15, 2014 at 5:20 PM, Paul Hargrove <phhargrove_at_[hidden]> wrote:
>
>>
>> On Tue, Jul 15, 2014 at 12:49 PM, Pritchard, Howard r <howardp_at_[hidden]>
>> wrote:
>>
>>> I don't think there's anything wrong with using ctor/dtors in shared
>>> libraries,
>>> but one does need to make sure that in these functions there's no
>>> assumptions
>>> about ordering of them wrt to other ctors/dtors.
>>>
>>
>> The ELF specification is clear that the order of execution of DT_INIT and
>> DT_FINI entries is undefined.
>> The .ctors and .dtors sections typically used by the GNU toolchain are, I
>> believe, not part of any formal linker specification.
>> So, I agree w/ Howard that one must take care not to assume anything
>> about order.
>>
>> -Paul
>>
>>
>> --
>> Paul H. Hargrove PHHargrove_at_[hidden]
>> Future Technologies Group
>> Computer and Data Sciences Department Tel: +1-510-495-2352
>> Lawrence Berkeley National Laboratory Fax: +1-510-486-6900
>>
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>>
>
>
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>

-- 
Paul H. Hargrove                          PHHargrove_at_[hidden]
Future Technologies Group
Computer and Data Sciences Department     Tel: +1-510-495-2352
Lawrence Berkeley National Laboratory     Fax: +1-510-486-6900