Polymorphically catching an exception in a -fno-rtti shared library on Mac OS X

Question

Hi,

I'm building a shared library with f-no-rtti. Internally, this library throws std:invalid_argument and catches std::exception, but the catch clause is never entered.

The following code reproduces the problem (g++ 4.2, Mac OS X 10.6):

// library.cpp: exports f(), compiled with -fno-rtti
#include <stdexcept>
#include <iostream>
extern "C" {
    void f() {
        try {
            throw std::invalid_argument("std::exception handler");
        } catch( std::exception& e) {
            std::cout << e.what() << "\n";
        } catch(...) {
            std::cout << "... handler\n";
        }
    }
}

---

// main.cpp: the main executable, dynamically loads the library
#include <dlfcn.h>
typedef void(*fPtr)();

int main() {
    void* handle = dlopen( "./libexception_problem.dylib", RTLD_LAZY );
    fPtr p_f = reinterpret_cast<fPtr>( dlsym( handle, "f" ) );
    p_f();
}

---

Output:

MacBook-Pro:teste pfranco$ # works fine with rtti
MacBook-Pro:teste pfranco$ g++ -c library.cpp && g++ -shared -o libexception_problem.dylib library.o && g++ main.cpp -o main && ./main
std::exception handler
MacBook-Pro:teste pfranco$ # breaks with -fno-rtti
MacBook-Pro:teste pfranco$ g++ -c -fno-rtti library.cpp && g++ -shared -o libexception_problem.dylib library.o && g++ -fno-rtti main.cpp -o main && ./main
... handler
MacBook-Pro:teste pfranco$ #-no_dead_strip_inits_and_terms doesn't change anything
MacBook-Pro:teste pfranco$ g++ -c -no_dead_strip_inits_and_terms -fno-rtti library.cpp && g++ -no_dead_strip_inits_and_terms -shared -o libexception_problem.dylib library.o && g++ -fno-rtti -no_dead_strip_inits_and_terms main.cpp -o main && ./main
... handler
MacBook-Pro:teste pfranco$ # linking against the shared library works, but this isn't always an option
MacBook-Pro:teste pfranco$ g++ -c -fno-rtti library.cpp && g++ -shared -o libexception_problem.dylib library.o && g++ -fno-rtti main.cpp -o main -L. -lexception_problem && ./main
std::exception handler

---

This only happens if the code that throws is in a shared library, and only if the caught type is a base class of the actual exception - catch(std::invalid_argument&) works fine, std::logic_error& doesn't.

Interestingly, this doesn't happen on Linux, even when running the exact same commands.

Questions:

1. Why does this happen? Is this a bug, undefined behavior or by design?
2. How could I make it work, short of linking against the library?

Thanks a lot.

Answer 1

From the info page for gcc (my highlighing).

-fno-rtti Disable generation of information about every class with virtual functions for use by the C++ runtime type identification features (dynamic_cast and typeid). If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but it will generate it as needed. The dynamic_cast operator can still be used for casts that do not require runtime type information, i.e. casts to void * or to unambiguous base classes.

RTTI is a core part of the language. If the compiler allows you to disable itm you're working outside of the rules of the language so aren't necessarily going to have everything work as you expect.