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

[Mathieu Desnoyers]

Hi David,

Sorry for the late reply,

* David Bryant (david.bryant at quantum.com) wrote:
> Hello,
>
> What are the pros and cons of static versus dynamic linking of the  
> tracepoint provider? I can't find much information looking around.

static linking: no need to use LD_PRELOAD.

dynamic linking: can ship tracepoint provider as separate .so, packaging
separately from the instrumented application.

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

The headers used to build the tracepoint providers are MIT-licensed, so
there is absolutely no licensing issue here with static linking of
tracepoint providers with closed-source apps. However, liblttng-ust per
se is _always_ dynamically linked.

Thanks,

Mathieu

>
> 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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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com