Dynamic Libraries in Linux

lodziarz · 03-22-2008, 03:00 PM

Hi!

I'm beginner in dynamic libraries thing. I want to build app, which will have 2 parts: main part and modules. Main part has to connect to server and in loop reads all traffic from it. Then it passes all what it reads to modules, and modules react for that. Modules has to have ability to send something to server also. Another important thing - it has to be multi-threaded. Main code reads line from server and runs X threads (X - modules count)... 1 thread = 1 module. The question is: how to do that? I've just started work with dynamic libraries, I modified easiest example found here: http://bogomip.net/blog/dynamic-libr...n-c-and-linux/ but I can't compile it... I don't know, what I'm doing wrong...

tools.cpp:

Code:

#include <cstdio>
#include <cstdlib>
#include <string>
#include <dlfcn.h>

using namespace std;

int main() {
	void *handle;
	void (*ReadRaw)(string);
	char *error;

	handle = dlopen("modules/SpamCatch/SpamCatch.so", RTLD_LAZY);
	if(handle == NULL) {
		fprintf(stderr, "Error: open/load error of dynamic.so failed: %s\n", dlerror());
		exit(1);
	}

	ReadRaw = (void(*)(string)) dlsym(handle, "ReadRaw");
	if((error = dlerror()) != NULL) {
		fprintf(stderr, "Error: symbol lookup in dynamic.so failed: %s\n", dlerror());
		exit(2);
	}

	ReadRaw("Here will be incoming line from server");

	dlclose(handle);

	return 0;
}

SpamCatch.cpp:

Code:

#include <cstdio>
#include <string>
#include <iostream>

using namespace std;

extern "C" void _init();
extern "C" void ReadRaw(string);
extern "C" void _fini();

void _init() {
	printf("Initialization Phase\n");
}

void ReadRaw(string bleh) {
	cout << "Text: " << bleh << endl;
}

void _fini() {
	printf("Deconstruction Phase\n");
}

Makefile:

Code:

# Makefile

APP = tools
CC = g++
LD = ld

all: $(APP) modules

SpamCatch.o: modules/SpamCatch/SpamCatch.cpp
	$(CC) -fPIC -I. -c modules/SpamCatch/SpamCatch.cpp -o modules/SpamCatch/SpamCatch.o

SpamCatch.so: SpamCatch.o
	$(LD) -shared -o modules/SpamCatch/SpamCatch.so modules/SpamCatch/SpamCatch.o

$(APP): tools.cpp
	$(CC) -o $(APP) tools.cpp -ldl -lpthread

modules: SpamCatch.so

clean:
	@rm -f modules/SpamCatch/*.o modules/SpamCatch/*.so $(APP)

Compiles ok... but when I run:

Code:

$ ./tools
Error: open/load error of dynamic.so failed: modules/SpamCatch/SpamCatch.so: undefined symbol: __dso_handle

Maybe you have another ideas to solve this problem? Or maybe you can advice some tricks/ideas how to build those soft in easiest way?

Thanks for all your help!

Greetings,
Michal

osor · 03-22-2008, 10:08 PM

I haven’t didn’t look at anything except the Makefile, but I think it would be easier for you to do:

Code:

LD = g++

and btw, CC is traditionally the abbreviation for the C Compiler.

paulsm4 · 03-22-2008, 11:01 PM

Hi -

I would just do this:

Code:

MODULES_DIR = modules/SpamCatch
...
libSpamCatch.so: SpamCatch.o
	$(LD) -shared -o $(MODULES_DIR)/libSpamCatch.so $(MODULES_DIR)/SpamCatch.o

$(APP): tools.cpp
	$(CC) -o $(APP) tools.cpp -L$(MODULES_DIR) -lSpamCatch -lpthread

It's worth noting:

1. You don't need to explicitly call "dlopen" or "dlsym" (the compiler, linker and runtime will do the right things for you)

2. Linux shared libraries do *not* contain any state.

In other words, the fact that you're doing threading is more or less irrelevant to the fact that you're also using shared libraries. This sometimes comes as a surprise to people coming from the Windows world...

Here's a good article on the subject:
http://tldp.org/HOWTO/Program-Librar...libraries.html

'Hope that helps .. pSM

lodziarz · 03-23-2008, 05:30 AM

yep, but then adding new module needs to recompile whole program... that's what I wanted to avoid... Main loop needs to be running without any break and it has to be connected to the server... and in meanwhile I'll code new modules, compile them and load them dynamically by commands comming from server main loop is connected to... f.e. I code & compile module x.so and I send command from server LOAD x.so... and it loads it and uses as rest... this same with unloading.

Thanks for all your help.

Greetings,
Michal

//EDIT: Problem with __dso_handle solved: used http://www.linuxquestions.org/questi...ream-c-331113/
But still question stays... am I solving that right way?

paulsm4 · 03-23-2008, 02:44 PM

Nope - adding a "brand new module" involves compiling *something* (your new code), but only involves relinking (vis. your actual program).

orgcandman · 03-24-2008, 02:42 PM

check out: http://aconole.brad-x.com/programs/nosak-demo.tar.bz2

I have a quick 'n dirty solution already written (albeit in C). Has a dispatcher thread, and a "sniffing" thread. The application loads a plugin from the plugins/ directory. In this case, it loads the ethernet plugin.

Just a demo of how to do something that you've described here that I started writing a while ago but never completed.