[lttng-dev] tracepoint provider: static versus dynamic linking (TRACEPOINT_PROBE_DYNAMIC_LINKAGE)

[David Bryant]

Hello,

What are the pros and cons of static versus dynamic linking of the 
tracepoint provider? I can't find much information looking around.

Are their any licensing related issues? For example, is closed-source 
software not allowed to use static linking?

Is it just an optimisation - allowing the program to run without probes 
connected?

Thanks,
Dave


Notes from my digging of how it works:

Looking at the preprocessor output of sample.c without/with 
TRACEPOINT_PROBE_DYNAMIC_LINKAGE defined I see:

$ diff -u sample.i.static sample.i.dynamic
--- sample.i.static    2012-11-08 13:29:04.049059515 +1030
+++ sample.i.dynamic    2012-11-08 13:29:04.169062438 +1030
@@ -746,7 +746,7 @@
  struct tracepoint __tracepoint_sample_component___message 
__attribute__((section("__tracepoints"))) = {
      __tp_strtab_sample_component___message,
      0,
      ((void *)0),
-    &__tracepoint_provider_sample_component,
+    ((void *)0),
      "char *, text",
  };

(line breaks added by me).

So the "int * tracepoint::tracepoint_provider_ref" member is set to 
((void *)0) in struct tracepoint:

struct tracepoint {
  const char *name;
  int state;
  struct tracepoint_probe *probes;
  int *tracepoint_provider_ref;         /* XXX */
  const char *signature;
  char padding[16];
};

So the static version is not dependent on the symbol 
"__tracepoint_provider_sample_component". Running 
LD_PRELOAD=libprovider.so ./sample (somehow?) reinstates that symbol. 
(Can't quite see how that works).

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