How to compile a cpp and then link it to a shared library

Question

I want to have the functions which are defined in another .cpp file become available in another simulation tool.

I found the following code in this question: -finstrument-functions doesn't work with dynamically loaded g++ shared objects (.so)

Trace.cpp

#include <stdio.h>
#ifdef __cplusplus
extern "C"
{
    void __cyg_profile_func_enter(void *this_fn, void *call_site)
        __attribute__((no_instrument_function));
    void __cyg_profile_func_exit(void *this_fn, void *call_site)
        __attribute__((no_instrument_function));
}
#endif

void __cyg_profile_func_enter(void* this_fn, void* call_site)
{
    printf("entering %p\n", (int*)this_fn);
}

void __cyg_profile_func_exit(void* this_fn, void* call_site)
{
    printf("exiting %p\n", (int*)this_fn);
}

Trace.cpp is compiled by doing:

g++ -g -finstrument-functions -Wall -Wl,-soname,libMyLib.so.0 -shared -fPIC -rdynamic MyLib.cpp MyLibStub.cpp Trace.cpp -o libMyLib.so.0.0  
ln -s libMyLib.so.0.0 libMyLib.so.0  
ln -s libMyLib.so.0.0 libMyLib.so  
g++ MainStatic.cpp -g -Wall -lMyLib -L./ -o MainStatic   
g++ MainDynamic.cpp -g -Wall -ldl -o MainDynamic

Note that I don't need: MyLib.cpp and MyLibStub.cpp.

Instead compiled Trace.cpp doing:

g++ -g -finstrument-functions -Wall -Wl,-soname,libMyLib.so.0 -shared -fPIC -rdynamic Trace.cpp -o libMyLib.so.0.0

What I've tried:

The shared object where I want to have Trace.cpp is obtained by:

opp_makemake -f --deep --no-deep-includes --make-so -I . -o veins -O out -I../../inet/src/util/headerserializers/sctp/headers -L../../inet/src -linet

I added -L and -l:

opp_makemake -f --deep --no-deep-includes --make-so -I . -o veins -L /home/user/Desktop/test/ -lMyLib -O out -I../../inet/src/util/headerserializers/sctp/headers -L../../inet/src -linet

and got:

/usr/bin/ld: cannot find -lMyLib

I also tried:

opp_makemake -f --deep --no-deep-includes --make-so -I . -o veins /home/user/Desktop/test/libMyLib.so.0.0 -O out -I../../inet/src/util/headerserializers/sctp/headers -L../../inet/src -linet

which compiled successfully but the application crashed:

Error during startup: Cannot load library '../../src//libveins.so': libMyLib.so.0: cannot open shared object file: No such file or directory.

Question:

How to compile Trace.cpp correctly?
How to link it with the rest of the shared library?

As you might notice I am not very experienced with compiling, linking and similar. So, any extra explanation is very welcome!

The answer to the question you asked in comments elsewhere is given in the questions you linked: "You have to get your own __cyg_profile_func_enter somewhere before the one from libc.so.6. One way to do that is to link it into your main executable. Or link it into a shared library that is directly linked to your executable (i.e. not dlopen()d one).". Other than that this question seems to be a generic "how do I link it?" which is covered by documentation on your chosen build tools. — Flexo, Apr 29 '16 at 17:50
hmm, ok.. that's what I am trying to figure out now. the tricky part is that the tool I am using generates makefiles using a scripts which in the end "merges" 3 simulation tools together... the final make is something like this: http://pastebin.com/EiXGiHbK ... so the real question is how do I put my implementation before the one from `libc.so.6`? :/ — cross, Apr 29 '16 at 18:25

score 1 · Accepted Answer · edited May 23 '17 at 11:52

As @Flexo restates what @EmployedRussian said in the linked question, the main point is to get your implementation of __cyg_profile_func_*** before the one provided by libc.so.6.

One method to do this, is to use the LD_PRELOAD environment variable. Here you can read what LD_PRELOAD does and how it works.

To use the LD_PRELOAD trick you will need to compile your implementation of the above-mentioned functions as a shared library.

You can do this by doing:

g++ -shared -fPIC myImp.cc -o myImp.so -ldl

Once you get the .so file, navigate to the directory where your executable is located and do:

LD_PRELOAD=<path/to/myImp.so>- ./<myExecutable>

For shared libraries, dynamic linking is used. Meaning:

resolving of some undefined symbols (is postponed) until a program is run.

By using LD_PRELOAD you resolve the symbols of your interest before letting the linked do that.

Here you have an implementation of myImp.cc, which I took from: https://groups.google.com/forum/#!topic/gnu.gcc.help/a-hvguqe10I

The current version lacks proper implementation for __cyg_profile_func_exit, and I have not been able to demangle the function names.

#ifdef __cplusplus
extern "C"
{

        #include <stdio.h>
        #include <stdlib.h>
        #include <sys/types.h>
        #include <sys/stat.h>
        #include <unistd.h>
    #include <dlfcn.h>


        void __cyg_profile_func_enter(void *this_fn, void *call_site)__attribute__((no_instrument_function));
        void __cyg_profile_func_exit(void *this_fn, void *call_site)__attribute__((no_instrument_function));

}

#endif
static FILE *fp;
int call_level=0;
void * last_fn;
void __cyg_profile_func_enter(void *this_fn, void *call_site)
{

    Dl_info di; 

        if (fp == NULL) fp = fopen( "trace.txt", "w" );
        if (fp == NULL) exit(-1);

        if ( this_fn!=last_fn) ++call_level;
        for (int i=0;i<=call_level;i++) 
    {
        fprintf(fp,"\t");
    }
        //fprintf(fp,  "entering %p\n", (int *)this_fn);
        fprintf(fp,  "entering %p", (int *)this_fn);
        if (dladdr(this_fn, &di)) {
          fprintf(fp,  " %s (%s)", di.dli_sname ? di.dli_sname : "<unknown>", di.dli_fname);
        }
        fputs("\n", fp);
        (void)call_site;
        last_fn = this_fn;
}

void __cyg_profile_func_exit(void *this_fn, void *call_site)
{
        --call_level;
        for (int i=0;i<=call_level;i++) fprintf(fp,"\t");
        fprintf(fp, "exiting %p\n", (int *)this_fn);
        (void)call_site;

}

Another option for function tracing which uses LD_PRELOAD is used by LTTng, in the section Function Tracing, but I have never used it...

How to compile a cpp and then link it to a shared library

1 Answers1

Linked