Building a shared library using gcc

Question

SOLVED. See below for the corrections (labeled FIXED).

I'm having trouble creating a shared library using gcc.

I created a little sample project that closely mirrors the structure of the actual project I'm working on. I've made it available as a tar.gz archive here:

http://209.59.216.197/libtest.tar.gz

FIXED: I've made the fixed version available here:
http://209.59.216.197/libtest_fixed.tar.gz

In this sample project, I have an application (app) that loads a shared library that I wrote (libshared.so) at runtime and calls a function that the shared library defines: function_inside_shared_lib().

In turn, this shared library uses a function defined inside a static library (libstatic.a): function_inside_static_lib().

The problem is when I build the shared library, the symbol "function_inside_shared_lib" does not get exported. I examined the shared library using "nm" and the symbol wasn't there. I am wondering if the command I am using to create the shared library is correct:

g++ -g -ggdb -fPIC -rdynamic -I ../static -c shared.cpp -o shared.o
g++ -g -ggdb -fPIC -rdynamic -shared -L ../static -lstatic -o libshared.so

FIXED: The correct commands are:

g++ -g -ggdb -fPIC -rdynamic -I../static -c shared.cpp -o shared.o
g++ -g -ggdb -fPIC -rdynamic -shared -L../static -o libshared.so shared.o -lstatic

I tried these commands with and without -rdynamic, as well as with and without -fPIC. The results are always the same.

I am using Ubuntu 10.04 (64-bit) with g++ version 4.4.3.

The full sample project follows. (Or you can download the archive using the link at the top of my post).

serg@rodent:~/libtest$ ls
app shared static

Here are the three components:

Component 1: A static library that defines a function called function_inside_static_lib().

This consists of the following:

serg@rodent:~/libtest$ cd static/  
serg@rodent:~/libtest/static$ ls  
static.cpp  static.h

static.h

// Header file for the static library

int function_inside_static_lib(int arg1, int arg2);

static.cpp

// Source file for the static library

#include <iostream>
using namespace std;

#include "static.h"

int function_inside_static_lib(int arg1, int arg2)
{
    cout << "In function_inside_static_lib()" << endl;

    // Return the sum
    int result = arg1 + arg2;
    return result;
}

---

Component 2: A shared library that uses the static library and defines a new function.

serg@rodent:~/libtest$ cd shared
serg@rodent:~/libtest/shared$ ls
shared.cpp

shared.cpp

// The shared library only has one source file.

// The shared library uses the static one.
#include "static.h"

#include <iostream>
using namespace std;

int function_inside_shared_lib(int arg1, int arg2)
{
    cout << "In function_inside_shared_lib()" << endl;

    cout << "Calling function_inside_static_lib()" << endl;
    int result = function_inside_static_lib(arg1, arg2);

    return result;
}

---

Component 3: An application that uses the shared library.

serg@rodent:~/libtest$ cd app
serg@rodent:~/libtest/app$ ls
app.cpp

app.cpp

FIXED: Because C++ symbols get mangled, the correct function name to search for is _Z26function_inside_static_libii instead of function_inside_static_lib

// The application loads the shared library at runtime.

#include <dlfcn.h>
#include <iostream>

using namespace std;

int main(int argc, char **argv)
{
    void *handle;
    int (*function_inside_shared_lib)(int, int);
    char *error;

    int arg1 = 3;
    int arg2 = 7;

    cout << "app: loading the shared library." << endl;
    handle = dlopen ("libshared.so", RTLD_LAZY);
    if (!handle) {
        cout << "Error: Failed to open shared library." << endl;
        cout << dlerror() << endl;
        return -1;
    }

    cout << "app: Looking for function_inside_shared_lib" << endl;

    // The next line is now FIXED:
    function_inside_shared_lib = (int (*)(int, int))dlsym(handle, "_Z26function_inside_static_libii");

    if ((error = dlerror()) != NULL)  {
        cout << "Error: Could not find the function." << endl;
        cout << error << endl;
        return -1;
    }

    cout << "app: Calling function_inside_shared_lib(" << arg1 << ", " << arg2 << ")" << endl;
    int result = (*function_inside_shared_lib)(arg1, arg2);

    cout << "app: The result is " << result << endl;

    dlclose(handle);
    return 0;
}

---

Here are the commands I'm using to build all of these components. Note that I want debugging symbols to be available in the final resulting app. Ideally, I want to be able to do a backtrace inside the app and see symbols from both the shared library and the static library.

1: Building the static library. I think I'm fine with this step:

serg@rodent:~/libtest/static$ g++ -g -ggdb -c static.cpp -o static.o # See the FIXED version just below
serg@rodent:~/libtest/static$ ar rcs libstatic.a static.o
serg@rodent:~/libtest/static$ ls
libstatic.a static.cpp static.h static.o

FIXED: The first command above must include -fPIC as well. The correct command is

g++ -g -ggdb -fPIC -c static.cpp -o static.o

2: Building the shared library. I'm pretty sure this is where I'm going wrong.

serg@rodent:~/libtest/shared$ g++ -g -ggdb -fPIC -rdynamic -I ../static -c shared.cpp -o shared.o
serg@rodent:~/libtest/shared$ g++ -g -ggdb -fPIC -rdynamic -shared -L ../static -lstatic -o libshared.so # See just below for FIXED version serg@rodent:~/libtest/shared$ ls
libshared.so shared.cpp shared.o

FIXED: The second command above should be:

g++ -g -ggdb -fPIC -rdynamic -shared -L../static -o libshared.so shared.o -lstatic

At this point, if I run nm to examine the symbols inside libshared.so, I don't see function_inside_shared_lib() anywhere, even with the -a and -D options for nm. (However, I do see it inside shared.o).

EDIT: With the fix above, the symbol appears as _Z26function_inside_shared_libii.

3: Building the app:

First, copy the shared library into the app folder:

serg@rodent:~/libtest$ cp shared/libshared.so app/
serg@rodent:~/libtest$ cd app
serg@rodent:~/libtest/app$ ls
app.cpp libshared.so

Now compile:

serg@rodent:~/libtest/app$ g++ -g -ggdb -ldl -L. -lshared app.cpp -o app
serg@rodent:~/libtest/app$ ls
app app.cpp libshared.so

If I try to run:

serg@rodent:~/libtest/app$ ./app
app: loading the shared library.
app: Looking for function_inside_shared_lib
Error: Could not find the function.
/home/serg/libtest/app/libshared.so: undefined symbol: function_inside_shared_lib

This makes sense because I could not see function_inside_shared_lib() using nm either, which means I'm probably building the shared library incorrectly in step 2.

How can I fix my command in the second step so that function_inside_shared_lib gets exported correctly?

Also feel free to give me any other advice if you notice that I'm doing anything odd. I'm still a beginner.

Answer 1

So, in step 2 you don't specify shared.o. So, instead of:

g++ -g -ggdb -fPIC -rdynamic -shared -L ../static -lstatic -o libshared.so

You should do:

g++ -g -ggdb -fPIC -rdynamic -shared -L ../static shared.o -lstatic -o libshared.so

Also it is important that shared.o is before -lstatic. Otherwise the linker will not find the function and let it 'undefined'.

Answer 2

There's a few errors here:

libshared.so is empty

Your Makefile doesn't actually link in the shared.o , it just creates an empty shared library. Change

g++ -g -ggdb -fPIC -rdynamic -shared -Lstatic -lstatic -o shared/libshared.so 

to

g++ -g -ggdb -fPIC -rdynamic -shared -Lstatic -o shared/libshared.so shared/shared.o -lstatic

The -lstatic have to come after shared/shared.o as you have to specify static libraries in reverse order of their dependencies.

-fPIC is needed on all object files in a shared library

You create a shared library that links in a static library. That static library also have to be compiled with -fPIC, otherwise you're creating a shared library where some parts of it cannot be relocated. Change

g++ -g -ggdb -c static/static.cpp -o static/static.o

to

g++ -fPIC -g -ggdb -c static/static.cpp -o static/static.o

C++ symbols get mangled

As you're creating a shared library from C++ code, function names and similar gets mangeled This means there is no function name matching the string "function_inside_static_lib" which you try to dynamically load. run nm on the static library, you'll see it's actually named "_Z26function_inside_static_libii" . You can run nm -C to pretty print C++ names.

This means your code in app.cpp have to be:

 function_inside_shared_lib = (int (*)(int, int))dlsym(handle, "_Z26function_inside_static_libii");

This is one of the reasons it's often preferrable to export functions from shared objects using C instead of C++ if you want to dynamically (dlopen) fetch something from a shared library. The C++ name mangling have in the past varied from compiler to compiler, though these days they seem to have all agreed to a standard that won't be changing. With C it's simpler, and the symbols in the shared library will be the same as you gave them in your source code.