[lttng-dev] instrumenting shared library

[Anand Neeli]

Hi Daniel,
Thanks for your reply.
sorry for my ignorance but how are you generating libtp.so in your example?

1) Also will this solution scale if my binary depends on lots of libraries
which are instrumented?

2) i'm fine with linking statically, can we avoid LD_PRELOAD?

3) Is there any page which explains on why and how to use
TRACEPOINT_PROVIDER, TRACEPOINT_CREATE_PROBES, TRACEPOINT_INCLUDE,
TRACEPOINT_DEFINE, TRACEPOINT_PROBE_DYNAMIC_LINKAGE


my tp.h is, are there any errors in this?
#############
#define TRACEPOINT_PROVIDER sample_component

#undef TRACEPOINT_INCLUDE
#define TRACEPOINT_INCLUDE "./tp.h"

#if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
#define _TP_H

#include <lttng/tracepoint.h>

TRACEPOINT_EVENT(
        sample_component,
        event,
        TP_ARGS(char *, text),
        TP_FIELDS(
                ctf_string(message, text)
        )
)
TRACEPOINT_LOGLEVEL(
        sample_component,
        event,
        TRACE_WARNING)

#endif /* _TP_H */
#include <lttng/tracepoint-event.h>
#############



Thanks,
Anand Neeli


On Thu, Apr 3, 2014 at 9:35 PM, Thibault, Daniel <
Daniel.Thibault at drdc-rddc.gc.ca> wrote:

> ----------------------------------------------------------------------
> Date: Thu, 3 Apr 2014 16:55:38 +0530
> From: Anand Neeli <anand.neeli at gmail.com>
> To: "lttng-dev at lists.lttng.org" <lttng-dev at lists.lttng.org>
>
> > I have a library which is instrumented and tracepoints are added. And I
> > have 2 binary's (or daemons) which try to include this library. (one
> binary
> > has instrumented code with tracepoints and other binary doesn't use
> > tracing)
> >
> > I found that i need to instrument the daemon which includes the
> > instrumented library for the code to compile.
> >
> > Can't I have a daemon (without tracing functionality) which can include
> an
> > instrumented library (i don't want to use tracing functionality in this
> > case)
> >
> > I'm only able to build if both library and daemon which includes it are
> > instrumented.
> >
> > Can anyone please give me more details on how to do this?
> > This is useful for huge code bases where you can selectively add tracing
> > support to few libraries or daemons.
> >
> > Anand Neeli
>
>    It can be done.  An uninstrumented app can rely on an instrumented
> shared object; whether the latter produces lttng events then depends on
> whether or not the tracepoint provider shared object is loaded when the app
> is invoked (using LD_PRELOAD).
>
>    Here is sample_neeli.c:
> #####
> #include <stdio.h> //For printf
> #include <unistd.h>
>
> #include "cobject.h"
> int main(int argc, char **argv)
> {
>         int i = 0;
>         char themessage[20]; //Can hold up to "Hello World 9999999\0"
>
>         fprintf(stderr, "sample_neeli starting\n");
>         for (i = 0; i < 10000; i++) {
>                 //We either prepare themessage in this module, or rely on
> an
>                 //external shared object
>                 cobject_message(themessage, i);
>                 usleep(1);
>         }
>         fprintf(stderr, "sample_neeli done\n");
>         return 0;
> }
> #####
> cobject.c:
> #####
> #include <stdio.h> //For printf
> #include <unistd.h>
>
> #include "tp.h"
>
> void cobject_message(char *themessage, int i)
> {
>         tracepoint(sample_component, event, "cobject_message");
>         sprintf(themessage, "CObject %u", i);
> }
> #####
> cobject.h:
> #####
> extern void cobject_message(char *themessage, int i);
> #####
> The tracepoint provider code should be easy to guess.
>
> The relevant Makefile fragments:
> #####
> CC = gcc
> LIBDL = -ldl # On Linux
> #LIBDL = -lc # On BSD
> LOCAL_CPPFLAGS += -I. $(AM_CPPFLAGS)
> LDFLAGS += -L/usr/local/lib $(AM_LDFLAGS)
> SOFLAGS = -fPIC
> TP_DEFINE = -D TRACEPOINT_DEFINE
> TP_DEFINE_DYNAMIC = $(TP_DEFINE) -D TRACEPOINT_PROBE_DYNAMIC_LINKAGE
> SOVERSION_MAJOR = 1
> SOVERSION_MINOR = 0
>
> libcobject.so: libcobject.o
>         @echo "~~~~~~Packaging $@:"
>         $(CC) -shared -Wl,-soname,$@.$(SOVERSION_MAJOR) -Wl,-no-as-needed \
>            -o $@.$(SOVERSION_MAJOR).$(SOVERSION_MINOR) $^
>         ln -sf $@.$(SOVERSION_MAJOR).$(SOVERSION_MINOR)
> $@.$(SOVERSION_MAJOR)
>         ln -sf $@.$(SOVERSION_MAJOR) $@
>
> libcobject.o: cobject.c cobject.h tp.h
>         @echo "~~~~~~Compiling $@:"
>         $(CC) $(CPPFLAGS) $(LOCAL_CPPFLAGS) $(SOFLAGS)
> $(TP_DEFINE_DYNAMIC) \
>            -c -o $@ $<
>
> neeli_dynamic_lib:: neeli_dynamic_lib.o
>         @echo "~~~~~~Linking sample_$@:"
>         $(CC) -Wl,-no-as-needed -o $@ -L. -lcobject $^ $(LDFLAGS) \
>            $(LIBDL) -Wl,-rpath,'$$ORIGIN',--enable-new-dtags
>         @echo "   Use '[LD_PRELOAD=./libtp.so] ./$@' to run $@"
>
> neeli_dynamic_lib.o: sample_neeli.c cobject.h
>         @echo "~~~~~~Compiling $@:"
>         $(CC) $(CPPFLAGS) $(LOCAL_CPPFLAGS) -c -o $@ $<
> #####
>
>    Running './neeli_dynamic_lib' yields no events.  Running
> 'LD_PRELOAD=./libtp.so ./neeli_dynamic_lib' yields the libcobject.so events.
>
> Daniel U. Thibault
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