How to pass arguments to a method loaded from a static library in CPP

Question

I'm trying to write a program to use a static library of a C++ code into another C++ code. The first C++ code is hello.cpp:

#include <iostream>
#include <string.h>
using namespace std;
extern "C" void say_hello(const char* name) {
    cout << "Hello " <<  name << "!\n";
}
int main(){
    return 0;
}

The I made a static library from this code, hello.a, using this command:

g++ -o hello.a -static -fPIC hello.cpp -ldl 

Here's the second C++ code to use the library, say_hello.cpp:

#include <iostream>
#include <string>
#include <dlfcn.h>
using namespace std;
int main(){
    void* handle = dlopen("./hello.a", RTLD_LAZY);
    cout<<handle<<"\n";
    if (!handle) {
        cerr<<"Cannot open library: "<<dlerror()<<'\n';
        return 1;
    }
    typedef void (*hello_t)();
    dlerror(); // reset errors
    hello_t say_hello = (hello_t) dlsym(handle, "say_hello");
    const char *dlsym_error = dlerror();
    if (dlsym_error) {
        cerr<<"Cannot load symbol 'say_hello': "<<dlsym_error<<'\n';
        dlclose(handle);
        return 1;
    }
    say_hello("World");
    dlclose(handle);
    return 0;
}

Then I compiled say_hello.cpp using:

g++ -W -ldl say_hello.cpp -o say_hello 

and ran ./say_hello in the command line. I expected to get Hello World! as output, but I got this instead:

0x8ea4020
Hello ▒▒▒▒!

What is the problem? Is there any trick to make compatibility for method's argument like what we use in ctypes or what?

If it helps I use a lenny.

EDIT 1:

I have changed the code and used a dynamic library, 'hello.so', which I've created using this command:

g++ -o hello.so -shared -fPIC hello.cpp -ldl

The 6th line of the code changed to:

void* handle = dlopen("./hello.so", RTLD_LAZY);

When I tried to compile say_hello.cpp, I got this error:

say_hello.cpp: In function ‘int main()’:
say_hello.cpp:21: error: too many arguments to function

I also tried to compile it using this line:

g++ -Wall -rdynamic say_hello.cpp -ldl -o say_hello

But same error raised. So I removed the argument "World" and the it has been compiled with no error; but when I run the executable, I get the same output like I have mentioned before.

EDIT 2:

Based on @Basile Starynkevitch 's suggestions, I changed my say_hello.cpp code to this:

#include <iostream>
#include <string>
#include <dlfcn.h>
using namespace std;
int main(){
    void* handle = dlopen("./hello.so", RTLD_LAZY);
    cout<<handle<<"\n";
    if (!handle) {
        cerr<<"Cannot open library: "<<dlerror()<<'\n';
        return 1;
    }
    typedef void hello_sig(const char *);
    void* hello_ad = dlsym(handle, "say_hello");
    if (!hello_ad){
        cerr<<"dlsym failed:"<<dlerror()<<endl;
        return 1;
    }
    hello_sig* fun = reinterpret_cast<hello_sig*>(hello_ad);
    fun("from main");
    fun = NULL;
    hello_ad = NULL;
    dlclose(handle);
    return 0;
}

Before that, I used below line to make a .so file:

g++ -Wall -fPIC -g -shared hello.cpp -o hello.so

Then I compiled say_hello.cpp wth this command:

g++ -Wall -rdynamic -g say_hello.cc -ldl -o say_hello

And then ran it using ./say_hello. Now everything is going right. Thanks to @Basile Starynkevitch for being patient about my problem.

Answer 1

Functions never have null addresses, so dlsym on a function name (or actually on any name defined in C++ or C) cannot be NULL without failing:

hello_t say_hello = (hello_t) dlsym(handle, "say_hello");
if (!say_hello) {
    cerr<<"Cannot load symbol 'say_hello': "<<dlerror()<<endl;
    exit(EXIT_FAILURE);
};

And dlopen(3) is documented to dynamically load only dynamic libraries (not static ones!). This implies shared objects (*.so) in ELF format. Read Drepper's paper How To Use Shared Libraries

I believe you might have found a bug in dlopen (see also its POSIX dlopen specification); it should fail for a static library hello.a; it is always used on position independent shared libraries (like hello.so).

You should dlopen only position independent code shared objects compiled with

g++ -Wall -O -shared -fPIC hello.cpp -o hello.so 

or if you have several C++ source files:

g++ -Wall -O -fPIC src1.cc -c -o src1.pic.o
g++ -Wall -O -fPIC src2.cc -c -o src2.pic.o
g++ -shared src1.pic.o src2.pic.o -o yourdynlib.so

^{you could remove the -O optimization flag or add -g for debugging or replace it with -O2 if you want.}

and this works extremely well: my MELT project (a domain specific language to extend GCC) is using this a lot (generating C++ code, forking a compilation like above on the fly, then dlopen-ing the resulting shared object). And my manydl.c example demonstrates that you can dlopen a big lot of (different) shared objects on Linux (typically millions, and hundred of thousands at least). Actually the limitation is the address space.

^{BTW, you should not dlopen something having a main function, since main is by definition defined in the main program calling (perhaps indirectly) dlopen.}

Also, order of arguments to g++ matters a lot; you should compile the main program with

g++ -Wall -rdynamic say_hello.cpp -ldl -o say_hello 

^{The -rdynamic flag is required to let the loaded plugin (hello.so) call functions from inside your say_hello program.}

For debugging purposes always pass -Wall -g to g++ above.

^{BTW, you could in principle dlopen a shared object which don't have PIC (i.e. was not compiled with -fPIC); but it is much better to dlopen some PIC shared object.}

Read also the Program Library HowTo and the C++ dlopen mini-howto (because of name mangling).

---

example

File helloshared.cc (my tiny plugin source code in C++) is

#include <iostream>
#include <string.h>
using namespace std;
extern "C" void say_hello(const char* name) {
    cout << __FILE__ << ":" << __LINE__ << " hello " 
         <<  name << "!" << endl;
}

and I am compiling it with:

g++ -Wall -fPIC -g -shared helloshared.cc -o hello.so

The main program is in file mainhello.cc :

#include <iostream>
#include <string>
#include <dlfcn.h>
#include <stdlib.h>
using namespace std;
int main() {
    cout << __FILE__ << ":" << __LINE__ << " starting." << endl;
    void* handle = dlopen("./hello.so", RTLD_LAZY);
    if (!handle) {
        cerr << "dlopen failed:" << dlerror() << endl;
        exit(EXIT_FAILURE);
    };
    // signature of loaded function
    typedef void hello_sig_t(const char*);
    void* hello_ad = dlsym(handle,"say_hello");
    if (!hello_ad) {
        cerr << "dlsym failed:" << dlerror() << endl;
        exit(EXIT_FAILURE);
    }
    hello_sig_t* fun = reinterpret_cast<hello_sig_t*>(hello_ad);
    fun("from main");
    fun = NULL; hello_ad = NULL;
    dlclose(handle);
    cout << __FILE__ << ":" << __LINE__ << " ended." << endl;
    return 0;
}

which I compile with

g++ -Wall -rdynamic -g mainhello.cc -ldl -o mainhello

Then I am running ./mainhello with the expected output:

mainhello.cc:7 starting.
helloshared.cc:5 hello from main!
mainhello.cc:24 ended.

Please notice that the signature hello_sig_t in mainhello.cc should be compatible (homomorphic, i.e. the same as) with the function say_hello of the helloshared.cc plugin, otherwise it is undefined behavior (and you probably would have a SIGSEGV crash).