C++ dlopen mini HOWTO

Aaron Isotton

<aaron@isotton.com>

2003-08-12
Revision History                                                             
Revision 1.03            2003-08-12            Revised by: AI                
Added reference to the GLib Dynamic Module Loader. Thanks to G. V. Sriraam   
for the pointer.                                                             
Revision 1.02            2002-12-08            Revised by: AI                
Added FAQ. Minor changes                                                     
Revision 1.01            2002-06-30            Revised by: AI                
Updated virtual destructor explanation. Minor changes.                       
Revision 1.00            2002-06-19            Revised by: AI                
Moved copyright and license section to the beginning. Added terms section.   
Minor changes.                                                               
Revision 0.97            2002-06-19            Revised by: JYG               
Entered minor grammar and sentence level changes.                            
Revision 0.96            2002-06-12            Revised by: AI                
Added bibliography. Corrected explanation of extern functions and variables. 
Revision 0.95            2002-06-11            Revised by: AI                
Minor improvements.                                                          


How to dynamically load C++ functions and classes using the dlopen API.

-----------------------------------------------------------------------------
Table of Contents
1. Introduction
    1.1. Copyright and License
    1.2. Disclaimer
    1.3. Credits / Contributors
    1.4. Feedback
    1.5. Terms Used in this Document
   
   
2. The Problem
    2.1. Name Mangling
    2.2. Classes
   
   
3. The Solution
    3.1. extern "C"
    3.2. Loading Functions
    3.3. Loading Classes
   
   
4. Frequently Asked Questions
5. See Also
Bibliography

1. Introduction

  A question which frequently arises among Unix C++ programmers is how to
load C++ functions and classes dynamically using the dlopen API.

In fact, that is not always simple and needs some explanation. That's what
this mini HOWTO does.

An average understanding of the C and C++ programming language and of the
dlopen API is necessary to understand this document.

This HOWTO's master location is [http://www.isotton.com/howtos/
C++-dlopen-mini-HOWTO/] http://www.isotton.com/howtos/C++-dlopen-mini-HOWTO/.
-----------------------------------------------------------------------------

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

dlopen API
    The dlclose, dlerror, dlopen and dlsym functions as described in the
    dlopen(3) man page.
   
    Notice that we use "dlopen" to refer to the individual dlopen function,
    and "dlopen API" to refer to the entire API.
   

-----------------------------------------------------------------------------
2. The Problem

At some time you might have to load a library (and use its functions) at
runtime; this happens most often when you are writing some kind of plug-in or
module architecture for your program.

In the C language, loading a library is very simple (calling dlopen, dlsym
and dlclose is enough), with C++ this is a bit more complicated. The
difficulties of loading a C++ library dynamically are partially due to name
mangling, and partially due to the fact that the dlopen API was written with
C in mind, thus not offering a suitable way to load classes.

Before explaining how to load libraries in C++, let's better analyze the
problem by looking at name mangling in more detail. I recommend you read the
explanation of name mangling, even if you're not interested in it because it
will help you understanding why problems occur and how to solve them.
-----------------------------------------------------------------------------

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. The Solution

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.


Example 1. Loading a Function

main.cpp:
#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);                                                         
}                                                                            

hello.cpp:
#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.

Warning 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.


Example 2. Loading a Class

Here we use a generic polygon class as interface and the derived class
triangle as implementation.

main.cpp:
#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);                                                       
}                                                                            

polygon.hpp:
#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                                                                       

triangle.cpp:
#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/] http://www.parashift.com/
    c++-faq-lite/, in section 20.
   

-----------------------------------------------------------------------------
4. Frequently Asked Questions

4.1. I'm using Windows and I can't find the dlfcn.h header file on my PC!
    What's the problem?
4.2. Is there some kind of dlopen-compatible wrapper for the Windows
    LoadLibrary API?

4.1. I'm using Windows and I can't find the dlfcn.h header file on my PC!
What's the problem?

The problem is, as usual, Windows. There is no dlfcn.h header on Windows,and
there is no dlopen API. There is a similar API around the LoadLibrary
function, and most of what is written here applies to it, too. Alternatively,
you can use libltdl (included in libtool) to "emulate" dlopen on a variety of
platforms.

You should also read section 4, "Dynamically Loaded (DL) Libraries", of the 
Program Library HOWTO for more techniques to load libraries and create
classes independently of your platform.

4.2. Is there some kind of dlopen-compatible wrapper for the Windows
LoadLibrary API?

I don't know of any, and I don't think there'll ever be one supporting all of
dlopen's options.

There are alternatives though: libtltdl (a part of libtool), which wraps a
variety of different dynamic loading APIs, among others dlopen and
LoadLibrary. Another one is the Dynamic Module Loading functionality of GLib.
You can use one of these to ensure better possible cross-platform
compatibility. I've never used any of them, so I can't tell you how stable
they are and whether they really work.
-----------------------------------------------------------------------------

5. See Also

  * The dlopen(3) man page. It explains the purpose and the use of the dlopen
    API.
   
  * The article   Dynamic Class Loading for C++ on Linux by James Norton
    published on the Linux Journal.
   
  * Your favorite C++ reference about extern "C", inheritance, virtual
    functions, new and delete. I recommend [STR2000].
   
  * [ISO14882]
   
  * The Program Library HOWTO, which tells you most things you'll ever need
    about static, shared and dynamically loaded libraries and how to create
    them. Highly recommended.
   
  * The Linux GCC HOWTO to learn more about how to create libraries with GCC.
   

-----------------------------------------------------------------------------
Bibliography

  ISO14482 ISO/IEC 14482-1998 ?? The C++ Programming Language. Available as
PDF and as printed book from [http://webstore.ansi.org/] http://
webstore.ansi.org/.


  STR2000 Bjarne Stroustrup The C++ Programming Language, Special Edition.
ISBN 0-201-70073-5. Addison-Wesley.
