Compiling a DLL with gcc

Question

Sooooo I'm writing a script interpreter. And basically, I want some classes and functions stored in a DLL, but I want the DLL to look for functions within the programs that are linking to it, like,

       program                dll
----------------------------------------------------
send code to dll----->    parse code
                              |
                              v
                          code contains a function,
                          that isn't contained in the DLL
                              |
list of functions in   <------/
program
      |
      v
corresponding function,
user-defined in the
program--process the
passed argument here
      |
      \-------------->    return value sent back
                          to the parsing function

I was wondering basically, how do I compile a DLL with gcc? Well, I'm using a windows port of gcc. Once I compile a .dll containing my classes and functions, how do I link to it with my program? How do I use the classes and functions in the DLL? Can the DLL call functions from the program linking to it? If I make a class { ... } object; in the DLL, then when the DLL is loaded by the program, will object be available to the program? Thanks in advance, I really need to know how to work with DLLs in C++ before I can continue with this project.

"Can you add more detail as to why you want the DLL to call functions in the main program?"

I thought the diagram sort of explained it... the program using the DLL passes a piece of code to the DLL, which parses the code, and if function calls are found in said code then corresponding functions within the DLL are called... for example, if I passed "a = sqrt(100)" then the DLL parser function would find the function call to sqrt(), and within the DLL would be a corresponding sqrt() function which would calculate the square root of the argument passed to it, and then it would take the return value from that function and put it into variable a... just like any other program, but if a corresponding handler for the sqrt() function isn't found within the DLL (there would be a list of natively supported functions) then it would call a similar function which would reside within the program using the DLL to see if there are any user-defined functions by that name.

So, say you loaded the DLL into the program giving your program the ability to interpret scripts of this particular language, the program could call the DLLs to process single lines of code or hand it filenames of scripts to process... but if you want to add a command into the script which suits the purpose of your program, you could say set a boolean value in the DLL telling it that you are adding functions to its language and then create a function in your code which would list the functions you are adding (the DLL would call it with the name of the function it wants, if that function is a user-defined one contained within your code, the function would call the corresponding function with the argument passed to it by the DLL, the return the return value of the user-defined function back to the DLL, and if it didn't exist, it would return an error code or NULL or something). I'm starting to see that I'll have to find another way around this to make the function calls go one way only

Answer 1

You could always load the dll at runtime with load library

Answer 2

This link explains how to do it in a basic way.

In a big picture view, when you make a dll, you are making a library which is loaded at runtime. It contains a number of symbols which are exported. These symbols are typically references to methods or functions, plus compiler/linker goo.

When you normally build a static library, there is a minimum of goo and the linker pulls in the code it needs and repackages it for you in your executable.

In a dll, you actually get two end products (three really- just wait): a dll and a stub library. The stub is a static library that looks exactly like your regular static library, except that instead of executing your code, each stub is typically a jump instruction to a common routine. The common routine loads your dll, gets the address of the routine that you want to call, then patches up the original jump instruction to go there so when you call it again, you end up in your dll.

The third end product is usually a header file that tells you all about the data types in your library.

So your steps are: create your headers and code, build a dll, build a stub library from the headers/code/some list of exported functions. End code will link to the stub library which will load up the dll and fix up the jump table.

Compiler/linker goo includes things like making sure the runtime libraries are where they're needed, making sure that static constructors are executed, making sure that static destructors are registered for later execution, etc, etc, etc.

Now as to your main problem: how do I write extensible code in a dll? There are a number of possible ways - a typical way is to define a pure abstract class (aka interface) that defines a behavior and either pass that in to a processing routine or to create a routine for registering interfaces to do work, then the processing routine asks the registrar for an object to handle a piece of work for it.

Answer 3

On the detail of what you plan to solve, perhaps you should look at an extendible parser like lua instead of building your own.

To your more specific focus.
A DLL is (typically?) meant to be complete in and of itself, or explicitly know what other libraries to use to complete itself.

What I mean by that is, you cannot have a method implicitly provided by the calling application to complete the DLLs functionality.

You could however make part of your API the provision of methods from a calling app, thus making the DLL fully contained and the passing of knowledge explicit.

How do I use the classes and functions in the DLL?
Include the headers in your code, when the module (exe or another dll) is linked the dlls are checked for completness.

Can the DLL call functions from the program linking to it?
Yes, but it has to be told about them at run time.

If I make a class { ... } object; in the DLL, then when the DLL is loaded by the program, will object be available to the program?
Yes it will be available, however there are some restrictions you need to be aware about. Such as in the area of memory management it is important to either:

• Link all modules sharing memory with the same memory management dll (typically c runtime)
• Ensure that the memory is allocated and dealloccated only in the same module.
• allocate on the stack

Examples!
Here is a basic idea of passing functions to the dll, however in your case may not be most helpfull as you need to know up front what other functions you want provided.

// parser.h
struct functions {
  void *fred (int );
};

parse( string, functions );

// program.cpp
parse( "a = sqrt(); fred(a);", functions );

What you need is a way of registering functions(and their details with the dll.) The bigger problem here is the details bit. But skipping over that you might do something like wxWidgets does with class registration. When method_fred is contructed by your app it will call the constructor and register with the dll through usage off methodInfo. Parser can lookup methodInfo for methods available.

// parser.h
class method_base { };
class methodInfo {
   static void register(factory);
   static map<string,factory> m_methods;
}

// program.cpp
class method_fred : public method_base {
   static method* factory(string args);
   static methodInfo _methoinfo;
}
methodInfo method_fred::_methoinfo("fred",method_fred::factory);

Answer 4

This sounds like a job for data structures.

Create a struct containing your keywords and the function associated with each one.

struct keyword {
    const char *keyword;
    int (*f)(int arg);
};

struct keyword keywords[max_keywords] = {
    "db_connect", &db_connect,
}

Then write a function in your DLL that you pass the address of this array to:

plugin_register(keywords);

Then inside the DLL it can do:

keywords[0].f = &plugin_db_connect;

With this method, the code to handle script keywords remains in the main program while the DLL manipulates the data structures to get its own functions called.

Taking it to C++, make the struct a class instead that contains a std::vector or std::map or whatever of keywords and some functions to manipulate them.

Answer 5

Winrawr, before you go on, read this first:

Any improvements on the GCC/Windows DLLs/C++ STL front?

Basically, you may run into problems when passing STL strings around your DLLs, and you may also have trouble with exceptions flying across DLL boundaries, although it's not something I have experienced (yet).