1.1. Copyright and License

This document, C++ dlopen mini HOWTO, is copyrighted (c) 2002 by Aaron Isotton. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts.

1.2. Disclaimer

No liability for the contents of this document can be accepted. Use the concepts, examples and information at your own risk. There may be errors and inaccuracies, that could be damaging to your system. Proceed with caution, and although this is highly unlikely, the author(s) do not take any responsibility.

All copyrights are held by their by their respective owners, unless specifically noted otherwise. Use of a term in this document should not be regarded as affecting the validity of any trademark or service mark. Naming of particular products or brands should not be seen as endorsements.

1.3. Credits / Contributors

In this document, I have the pleasure of acknowledging (in alphabetic order):

• Joy Y Goodreau <joyg (at) us.ibm.com> for her editing.

• D. Stimitis <stimitis (at) idcomm.com> for pointing out a few issues with the formatting and the name mangling, as well as pointing out a few subtleties of extern "C".

1.4. Feedback

Feedback is most certainly welcome for this document. Send your additions, comments and criticisms to the following email address: <aaron@isotton.com>.

1.5. Terms Used in this Document

2.1. Name Mangling

In every C++ program (or library, or object file), all non-static functions are represented in the binary file as symbols. These symbols are special text strings that uniquely identify a function in the program, library, or object file.

In C, the symbol name is the same as the function name: the symbol of strcpy will be strcpy, and so on. This is possible because in C no two non-static functions can have the same name.

Because C++ allows overloading (different functions with the same name but different arguments) and has many features C does not — like classes, member functions, exception specifications — it is not possible to simply use the function name as the symbol name. To solve that, C++ uses so-called name mangling, which transforms the function name and all the necessary information (like the number and size of the arguments) into some weird-looking string which only the compiler knows about. The mangled name of foo might look like foo@4%6^, for example. Or it might not even contain the word "foo".

One of the problems with name mangling is that the C++ standard (currently [ISO14882]) does not define how names have to be mangled; thus every compiler mangles names in its own way. Some compilers even change their name mangling algorithm between different versions (notably g++ 2.x and 3.x). Even if you worked out how your particular compiler mangles names (and would thus be able to load functions via dlsym), this would most probably work with your compiler only, and might already be broken with the next version.

2.2. Classes

Another problem with the dlopen API is the fact that it only supports loading functions. But in C++ a library often exposes a class which you would like to use in your program. Obviously, to use that class you need to create an instance of it, but that cannot be easily done.

3.1. extern "C"

C++ has a special keyword to declare a function with C bindings: extern "C". A function declared as extern "C" uses the function name as symbol name, just as a C function. For that reason, only non-member functions can be declared as extern "C", and they cannot be overloaded.

Although there are severe limitations, extern "C" functions are very useful because they can be dynamically loaded using dlopen just like a C function.

This does not mean that functions qualified as extern "C" cannot contain C++ code. Such a function is a full-featured C++ function which can use C++ features and take any type of argument.

3.2. Loading Functions

In C++ functions are loaded just like in C, with dlsym. The functions you want to load must be qualified as extern "C" to avoid the symbol name being mangled.

#include <iostream>
#include <dlfcn.h>


int main() {
    using std::cout;
    using std::cerr;

    cout << "C++ dlopen demo\n\n";

    // open the library
    cout << "Opening hello.so...\n";
    void* handle = dlopen("./hello.so", RTLD_LAZY);
    
    if (!handle) {
        cerr << "Cannot open library: " << dlerror() << '\n';
        return 1;
    }
    
    // load the symbol
    cout << "Loading symbol hello...\n";
    typedef void (*hello_t)();
    hello_t hello = (hello_t) dlsym(handle, "hello");
    if (!hello) {
        cerr << "Cannot load symbol 'hello': " << dlerror() <<
            '\n';
        dlclose(handle);
        return 1;
    }
    
    // use it to do the calculation
    cout << "Calling hello...\n";
    hello();
    
    // close the library
    cout << "Closing library...\n";
    dlclose(handle);
}

#include <iostream>

extern "C" void hello() {
    std::cout << "hello" << '\n';
}

The function hello is defined in hello.cppas extern "C"; it is loaded in main.cpp with the dlsym call. The function must be qualified as extern "C" because otherwise we wouldn't know its symbol name.

There are two different forms of the extern "C" declaration: extern "C" as used above, and extern "C" { … } with the declarations between the braces. The first (inline) form is a declaration with extern linkage and with C language linkage; the second only affects language linkage. The following two declarations are thus equivalent: extern "C" int foo; extern "C" void bar(); and extern "C" { extern int foo; extern void bar(); } As there is no difference between an extern and a non-extern function declaration, this is no problem as long as you are not declaring any variables. If you declare variables, keep in mind that extern "C" int foo; and extern "C" { int foo; } are not the same thing. For further clarifications, refer to [ISO14882], 7.5, with special attention to paragraph 7, or to [STR2000], paragraph 9.2.4. Before doing fancy things with extern variables, peruse the documents listed in the see also section.

3.3. Loading Classes

Loading classes is a bit more difficult because we need an instance of a class, not just a pointer to a function.

We cannot create the instance of the class using new because the class is not defined in the executable, and because (under some circumstances) we don't even know its name.

The solution is achieved through polymorphism. We define a base, interface class with virtual members in the executable, and a derived, implementation class in the module. Generally the interface class is abstract (a class is abstract if it has pure virtual functions).

As dynamic loading of classes is generally used for plug-ins — which must expose a clearly defined interface — we would have had to define an interface and derived implementation classes anyway.

Next, while still in the module, we define two additional helper functions, known as class factory functions. One of these functions creates an instance of the class and returns a pointer to it. The other function takes a pointer to a class created by the factory and destroys it. These two functions are qualified as extern "C".

To use the class from the module, load the two factory functions using dlsym just as we loaded the the hello function; then, we can create and destroy as many instances as we wish.

#include "polygon.hpp"
#include <iostream>
#include <dlfcn.h>

int main() {
    using std::cout;
    using std::cerr;

    // load the triangle library
    void* triangle = dlopen("./triangle.so", RTLD_LAZY);
    if (!triangle) {
        cerr << "Cannot load library: " << dlerror() << '\n';
        return 1;
    }

    // load the symbols
    create_t* create_triangle = (create_t*) dlsym(triangle, "create");
    destroy_t* destroy_triangle = (destroy_t*) dlsym(triangle, "destroy");
    if (!create_triangle || !destroy_triangle) {
        cerr << "Cannot load symbols: " << dlerror() << '\n';
        return 1;
    }

    // create an instance of the class
    polygon* poly = create_triangle();

    // use the class
    poly->set_side_length(7);
        cout << "The area is: " << poly->area() << '\n';

    // destroy the class
    destroy_triangle(poly);

    // unload the triangle library
    dlclose(triangle);
}

#ifndef POLYGON_HPP
#define POLYGON_HPP

class polygon {
protected:
    double side_length_;

public:
    polygon()
        : side_length_(0) {}

    void set_side_length(double side_length) {
        side_length_ = side_length;
    }

    virtual double area() const = 0;
};

// the types of the class factories
typedef polygon* create_t();
typedef void destroy_t(polygon*);

#endif

#include "polygon.hpp"
#include <cmath>

class triangle : public polygon {
public:
    virtual double area() const {
        return side_length_ * side_length_ * sqrt(3) / 2;
    }
};


// the class factories

extern "C" polygon* create() {
    return new triangle;
}

extern "C" void destroy(polygon* p) {
    delete p;
}

There are a few things to note when loading classes:

• You must provide both a creation and a destruction function; you must not destroy the instances using delete from inside the executable, but always pass it back to the module. This is due to the fact that in C++ the operators new and delete may be overloaded; this would cause a non-matching new and delete to be called, which could cause anything from nothing to memory leaks and segmentation faults. The same is true if different standard libraries are used to link the module and the executable.

• The destructor of the interface class should be virtual in any case. There might be very rare cases where that would not be necessary, but it is not worth the risk, because the additional overhead can generally be ignored.

  If your base class needs no destructor, define an empty (and virtual) one anyway; otherwise you will have problems sooner or later; I can guarantee you that. You can read more about this problem in the comp.lang.c++ FAQ at http://www.parashift.com/c++-faq-lite/, in section 20.