C++ dlopen mini HOWTOAaronIsottonaaron@isotton.com2006-03-161.102006-03-16AIChanged the license from the GFDL to the GPL. Fixed usage
of dlerror; thanks to Carmelo Piccione. Using a virtual destructor
in the example; thanks to Joerg Knobloch. Added Source Code
section. Minor fixes.1.032003-08-12AIAdded reference to the GLib Dynamic Module
Loader. Thanks to G. V. Sriraam for the pointer.1.022002-12-08AIAdded FAQ. Minor changes1.012002-06-30AIUpdated virtual destructor explanation. Minor changes.1.002002-06-19AIMoved copyright and license section to the
beginning. Added terms section. Minor changes.0.972002-06-19JYGEntered minor grammar and sentence level changes.0.962002-06-12AIAdded bibliography. Corrected explanation of extern
functions and variables.0.952002-06-11AIMinor improvements.How to dynamically load C++ functions and classes using
the dlopen API.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 .Copyright and LicenseThis document, C++ dlopen mini HOWTO, is
copyrighted (c) 2002-2006 by Aaron Isotton.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU General Public License, Version
2, as published by the Free Software Foundation.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.
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".Many unnamed others pointing out errors or giving tips to
improve this howto. You know who you are!Feedback
Feedback is most certainly welcome for this document. Send
your additions, comments and criticisms to the following email
address: aaron@isotton.com.
Terms Used in this Documentdlopen APIThe dlclose,
dlerror,
dlopen and
dlsym functions as described in the
dlopen(3) man page.Notice that we use
dlopen to refer to
the individual dlopenfunction, and
dlopen API to refer
to the entire API.The ProblemAt 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.Name ManglingIn 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.ClassesAnother 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.The Solutionextern "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.Loading FunctionsIn 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.Loading a Functionmain.cpp:
#include
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)();
// reset errors
dlerror();
hello_t hello = (hello_t) dlsym(handle, "hello");
const char *dlsym_error = dlerror();
if (dlsym_error) {
cerr << "Cannot load symbol 'hello': " << dlsym_error <<
'\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:
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-externfunction
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.Loading ClassesLoading 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 hello
function; then, we can create and destroy as many
instances as we wish.Loading a ClassHere we use a generic polygon
class as interface and the derived class
triangle as implementation.main.cpp:
#include
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;
}
// reset errors
dlerror();
// load the symbols
create_t* create_triangle = (create_t*) dlsym(triangle, "create");
const char* dlsym_error = dlerror();
if (dlsym_error) {
cerr << "Cannot load symbol create: " << dlsym_error << '\n';
return 1;
}
destroy_t* destroy_triangle = (destroy_t*) dlsym(triangle, "destroy");
dlsym_error = dlerror();
if (dlsym_error) {
cerr << "Cannot load symbol destroy: " << dlsym_error << '\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:triangle.cpp:
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 , in
section 20.Source CodeYou can download all the source code presented in this howto as an
archive: .Frequently Asked QuestionsI'm using Windows and I can't find the
dlfcn.h header file! What's the problem?The problem is that Windows doesn't have the
dlopen API, and thus there is no
dlfcn.h header. There is a similar API
around the LoadLibrary function, and
most of what is written here applies to it, too. Please refer to the
Microsoft Developer Network
Website for more information.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.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.See AlsoThe 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.ISO14882The 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.ISO14482ISO/IEC 14482-1998 — The
C++ Programming Language. Available as
PDF and as printed book from .STR2000StroustrupBjarneThe C++ Programming Language, Special
Edition.
ISBN 0-201-70073-5.
Addison-Wesley.