[lttng-dev] Emitting events from shared object constructors is (currently) not possible

[Woegerer, Paul]

On 11/22/2012 04:39 PM, Mathieu Desnoyers wrote:
> * Woegerer, Paul (Paul_Woegerer at mentor.com) wrote:
>> I recently stumbled over the following lttng-ust limitation:
>>
>> I tried to emit events from a shared objects constructor function:
>>
>> __attribute__((constructor))
>> void func_constructor()
>> {
>>    tracepoint( foo, bar ); 
>> }
>>
>> Unfortunately this doesn't work because the constructor functions of
>> LTTng itself (__tracepoints__init and the ones from
>> ust-tracepoint-event.h) are not yet run.
>>
>> Trying to solve this problem with explicit constructor priority on my
>> side alone doesn't work:
>>
>> __attribute__((constructor (1111)))
>> void func_constructor()
>> {
>>    tracepoint( foo, bar ); 
>> }
>>
>> That's because whatever I specify as priority is irrelevant. LTTng's
>> constructors without priority will always come last.
>>
>> After also adding priority to the lttng constructor I was able to make
>> it work (see patch below):
> 
> Hi Paul,
> 
> I notice that you probably embed your probes along with your code within
> the same shared object/executable, right ?
> 
> Unfortunately, this approach does not work if the probes are placed in a
> different shared object than the program code. Here is an example:

Hi Mathieu,

In case the probes would be defined in different shared object from the
one where they are used we wouldn't have a problem at all because the
dynamic linker would invoke all the constructor functions for the probe
shared object before the constructor functions of the shared object that
has a dependency to the probe shared object, right ?

And even if this is not the case I could explicitly dlopen the probe
shared object from within my constructor function and thus make sure
that the constructor function of the probe shared object gets called
before I emit an event from my constructor function.

--
Paul

-- 
Paul Woegerer | SW Development Engineer
http://go.mentor.com/sourceryanalyzer

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