std::function from dlsym results in segmentation fault

Question

I want to dynamically link a shared library and assign a function from it to a std::function. Here is the code:

function.cpp:

#include <array>

#ifdef __cplusplus
extern "C" {
#endif

double function(std::array<double, 1> arg)
{
    return arg[0] * 2;
}

#ifdef __cplusplus
}
#endif

main.cpp:

#include <functional>
#include <iostream>
#include <fstream>
#include <array>
#include <functional>

#ifdef __linux__
#include <dlfcn.h>
#endif

int main()
{
void *handle;
double (*function)(std::array<double, 1>);
char *error;

handle = dlopen("/home/oleg/MyProjects/shared_library_test/libFunction.so", RTLD_LAZY);
if (!handle) 
{
    fprintf(stderr, "%s\n", dlerror());
    exit(EXIT_FAILURE);
}

dlerror();
*(void **) (&function) = dlsym(handle, "function");

if ((error = dlerror()) != NULL)
{
    fprintf(stderr, "%s\n", error);
    exit(EXIT_FAILURE);
}

std::cout << "Native function output: " << function(std::array<double, 1>{ 3.0 }) << std::endl;
dlclose(handle);

std::function<double(std::array<double, 1>)> std_function(*function);
std::cout << "std::function output: " <<  std_function(std::array<double, 1>{ 3.0 }) << std::endl;

exit(EXIT_SUCCESS);
}

Build shared library:

g++ -Wall -Wextra -g -std=c++17 -shared -o libFunction.so -fPIC function.cpp

Build main:

g++ -Wall -Wextra -g -std=c++17 main.cpp -ldl

Running the program leads to the following output:

Native function output: 6
Segmentation fault

So, as you can see, I successfully compile the library and load it in my main program. However, assigning the function pointer to std::function doesn't work.

Please, help!

Answer 1

You better do conversion in C++ style:

using function_ptr = double (*)(std::array<double, 1>);
function_ptr function = reinterpret_cast<function_ptr>( dlsym(handle, "function") );

But the culprit is that you cannot call this function directly or indirectly through std::function wrapper after you close shared library:

dlclose(handle);
// function cannot be used anymore

note it can be better to use RAII for this:

std::unique_ptr<void *,int(void*)> handle( dlopen("/home/oleg/MyProjects/shared_library_test/libFunction.so", RTLD_LAZY), dlclose );

then you do not need to call dlclose() manually

Note: it is a bad idea to call exit from main() in C++, use return instead, details can be found here Will exit() or an exception prevent an end-of-scope destructor from being called?