The Problem

C++ dlopen mini HOWTO
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# 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.

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