Help with shared libraries

Andreas Pfeil
Wed Jan 4 01:06:00 GMT 2006

Shane Dawalt wrote:
>  I want to create an application that has modules.  Whenever I wish to 
> expand it, I simply create another module.  I don't want to have to 
> recompile the blasted application.
>  This led me to thinking about shared libraries.  My idea is to create 
> any number of shared libraries, each being a "module." (A module 
> contains a single class.)  I drop the .so file into a special directory 
> that my app looks at for shared library files.  My app loads each 
> "module" it finds at startup. Each module provides a well-known function 
> which my main app calls to get an idea of what the module can do.  My 
> app then instances the proper class from one of the modules and 
> continues on from there.
>  First, is there any complete information on how to really create and 
> use share libraries?
>  Second, does this description sound even reasonable for the use of 
> shared libraries?
>  If not, can someone point me at something that is reasonable for 
> on-the-fly module loading?
well, a shared library is not much different from any static form of a 
library or object file, except that it is linked with the -shared option.

>  If I am on the right track, then I've been successful at creating the 
> shared library.  I know I must use dlopen().  But I cannot understand 
> what dlsym() is suppose to be used for.  And, how would I specify to 
> dlsym() the constructor of my class that's in the shared library?  I 
> cannot fathom what its symbol's string representation might be.  (I'm 
> using gcc 3.2.3 if it makes any difference.)
dlopen() opens the object file, meaning, that it allows you to read from 
  it. However, it does not give you access to a special symbol inside of 
this object file yet.
This is what you use dlsym() for. dlsym() returns a pointer to the 
correct symbol (e.g. in your case the function). Now this is a bit of a 
bugger, as your program cannot know the module's name and constructor, etc.
That's why one way to solve this, is to implement a registration 
function, e.g. module_init() inside of your shared object, which will 
somehow give your program access to the module.
So if you have something like this:

class module_table{
	register_constructor(std::string name, void* fct);

then your module_init() function would look like

void module_init(module_table *theTable){
	theTable->register_constructor("myName", myConstructor);

or similar measures, if your using an object factory to create instances 
of your module.

You can also have this registration done by dlopen(), using either _init 
and _fini (deprecated according to my dlopen()(3) manpage, or something 
like a __attribute__(constructor) function. - However, I don't know what 
this is...

Hope, that helped.

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