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

[Woegerer, Paul]

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):

diff --git a/include/lttng/tracepoint.h b/include/lttng/tracepoint.h
index 8b08914..532f1d2 100644
--- a/include/lttng/tracepoint.h
+++ b/include/lttng/tracepoint.h
@@ -261,7 +261,7 @@ int __tracepoint_registered
 struct tracepoint_dlopen tracepoint_dlopen
        __attribute__((weak, visibility("hidden")));
 
-static void lttng_ust_notrace __attribute__((constructor))
+static void lttng_ust_notrace __attribute__((constructor (1000)))
 __tracepoints__init(void);
 static void
 __tracepoints__init(void)
@@ -300,7 +300,7 @@ __tracepoints__init(void)
                                __start___tracepoints_ptrs);
 }
 
-static void lttng_ust_notrace __attribute__((destructor))
+static void lttng_ust_notrace __attribute__((destructor (1000)))
 __tracepoints__destroy(void);
 static void
 __tracepoints__destroy(void)
diff --git a/include/lttng/ust-tracepoint-event.h b/include/lttng/ust-tracepoint-event.h
index 74d06e1..7ad3144 100644
--- a/include/lttng/ust-tracepoint-event.h
+++ b/include/lttng/ust-tracepoint-event.h
@@ -638,7 +638,7 @@ static struct lttng_probe_desc _TP_COMBINE_TOKENS(__probe_desc___, TRACEPOINT_PR
 
 /* Reset all macros within TRACEPOINT_EVENT */
 #include <lttng/ust-tracepoint-event-reset.h>
-static void lttng_ust_notrace __attribute__((constructor))
+static void lttng_ust_notrace __attribute__((constructor (1000)))
 _TP_COMBINE_TOKENS(__lttng_events_init__, TRACEPOINT_PROVIDER)(void);
 static void
 _TP_COMBINE_TOKENS(__lttng_events_init__, TRACEPOINT_PROVIDER)(void)
@@ -649,7 +649,7 @@ _TP_COMBINE_TOKENS(__lttng_events_init__, TRACEPOINT_PROVIDER)(void)
        assert(!ret);
 }
 
-static void lttng_ust_notrace __attribute__((destructor))
+static void lttng_ust_notrace __attribute__((destructor (1000)))
 _TP_COMBINE_TOKENS(__lttng_events_exit__, TRACEPOINT_PROVIDER)(void);
 static void
 _TP_COMBINE_TOKENS(__lttng_events_exit__, TRACEPOINT_PROVIDER)(void)



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

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