shared library how to link to a symbol? - c

Question

I have:
car.cc
#include "car.h"
#include <iostream>
using namespace std;
extern "C" Car* create_object()
{
return new Car;
}
Car::Car() {
this->maxGear = 2;
this->currentGear = 1;
this->speed = 0;
}
void Car::shift(int gear) {
if (gear < 1 || gear > maxGear) {
return;
}
currentGear = gear;
}
void Car::brake() {
speed -= (5 * this->getCurrentGear());
std::cout<<"THE SPEED IS:" <<speed<<std::endl;
}
extern "C" void destroy_object( Car* object )
{
delete object;
}
car.h
#ifndef VEHICLES_CAR_H
#define VEHICLES_CAR_H
// A very simple car class
class Car {
public:
Car();
void shift(int gear);
void accelerate();
void brake();
private:
int maxGear;
int currentGear;
int speed;
};
#endif /* VEHICLES_CAR_H */
test.cc
#include "/home/car.h"
#include <dlfcn.h>
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
using namespace std;
int main()
{
/* on Linux, use "./myclass.so" */
void* handle = dlopen("/usr/lib/libCarTest.so", RTLD_LAZY);
int (*result)(int);
if (!handle)
{
}
/*dlsym(handle,"accelerate");
cout<<"IN HERE: "<<endl;
dlsym(handle,"brake");
dlclose(handle);*/
Car* (*create)();
void (*destroy)(Car*);
dlerror();
create = (Car* (*)())dlsym(handle, "create_object");
destroy = (void (*)(Car*))dlsym(handle, "destroy_object");
Car* carr = (Car*)create();
carr->brake();
destroy( carr );
dlclose(handle);
/*
Car carr;
carr.brake();
* compilation g++ test.cpp -o tst /path/libcar.so
*/
return 0;
}
After creating libMyLib.so and install it in /usr/lib i've tried to compile test.cc using: g++ test.cc -o tst -ldl. WHY do i need to include -lMyLib? is there a way to compile the code without libMyLib.so? Secondly why dlsym(handle,"brake") is not working? If i change dlsym (Car* (*).... with dlsym(handle,"brake") i get nothing. why?
Appreciate

Answer 1

WHY do i need to include -lMyLib?
Because you need to link to the Car::brake method.
Secondly why dlsym(handle,"brake") is not working?
Because there is no brake symbol. The method Car::brake has a complicated mangled (implementation-defined) name. You can see this in the output of nm -D.
AFAIK, you can solve it by
making all the methods of Car virtual (they will be called through a pointer, so no linking will be needed)
doing it the old C way, ie. export a free function brake() that would call the Car::brake method from the .so
making all the public methods of Car inline and defining them in the header.
emulating the virtual table approach (as we do it in C)
Combining the last two approaches:
class Car {
public:
void brake() { brake_impl(this); }
private:
void (*brake_impl)(Car*);
void do_brake(); // this would be the actual implementation
Car() : brake_impl([] (Car* c){ c->do_brake(); }) { ... }
};
Of course you could split the implementation and the interface so it's not such a mess.