Prerequisites

First, we need to get the library we want to load. Here we will make a small example library.

Loading dynamic libraries at runtime is an operation, where at runtime, you use the OS' API to load symbols from a shared library. This library loads named functions, that being, functions whose names are not under the influence of name mangling. Because of this, your functions have to be exported with C semantics.

To create a library, simply write some C or C++ code like this:

Header:

#pragma once

extern "C"
{
    void begin();
}

Source file:

#include "libtest.hpp"
#include <iostream>

void begin()
{
    std::cout << "begin" << std::endl;
}

As you can see, the library we made is a C++ one. To compile with C semantics, you need to wrap your functions under a extern "C" block. Otherwise name mangling will be enabled.

Make sure that, when compiling for a Unix system, you have enabled -fPIC in your compiler flags. This way you can have PIC(Position Independent Code).

On Windows, make sure that all the functions you want to export are prefixed with __declspec(dllexport), otherwise they will not be exported from the DLL.

Using the library

C++ API

The C++ API provides some good syntactic sugar, like templates, that generally make your life better. The library looks like this:

namespace URLL
{
    void* dlopen(const char* location) noexcept;

    template<typename T, typename... T2>
    void* dlsym(void* handle, const char* name, std::function<T(T2...)>& function) noexcept;

    template<typename T>
    void* dlsym_val(void* handle, const char* name, T* var) noexcept;

    template<typename T>
    void* dlsym_func(void* handle, const char* name, T& var) noexcept;

    void* dlsym(void* handle, const char* name) noexcept;

    // returns 0 on success, everything else must be an error
    int dlclose(void* handle) noexcept;

    // returns a string with the corresponding error, if there is no error it returns null
    char* dlerror() noexcept;
}

First, you need to include the urll.h header file. After that you need to load a library from a file. This is how the library is intended to be used:

1. Call dlopen with a string argument, set to the path to the library you want to load and save the returned handle pointer to a variable
2. Call one of the dlsym functions to load a symbol, and check if it has been successfully loaded, by checking if the return value is not nullptr
3. Use your functions
4. When you're done with using the functions, simply call dlclose
5. If there is any error along the way, call dlerror to get an error message

All functions are under the URLL namespace. Here is an example of the C++ API:

void load()
{
    auto* handle = URLL::dlopen("https://madladsquad.com/libLIBRARY.so");
    if (handle == nullptr)
    {
         std::cout << dlerror() << std::endl;
         return;
    }

    std::function<void(void)> func;
    if (URLL::dlsym(handle, "begin", func) == handle)
        func();
    URLL::dlclose(handle);
}

The code is completely compatible with the library example we listed above.

On the line where we use dlsym, we use the std::function support and templates. This, as said in the last page, can be enabled using the URLL_USE_FUNCTIONAL macro.

We also have a generic dlsym function, that just returns a void*, that you can use to manually cast to the type of your variable/function. Additionally, we also provide support for C function pointers, using the dlsym_func function and support for variables using the dlsym_var function.

C API

The C API is much more bare-bones, it looks like this:

void* urll_dlopen(const char* location);

void* urll_dlsym(void* handle, const char* name);
void* urll_dlsym_func(void* handle, const char* name, void** function);
void* urll_dlsym_var(void* handle, const char* name, void* var);

int urll_dlclose(void* handle);

char* urll_dlerror();

notice how if we compare with the C++ API, the only differences in most cases are that the names are prefixed with urll_ to signify the missing of the namespaces feature in C.

Here is an example that uses the C API. Remember that, for the C API, you need to include the curll.h header:

void load()
{
    void* handle = urll_dlopen("https://madladsquad.com/libLIBRARY.so");
    if (handle == NULL)
    {
        printf("%s\n", urll_dlerror());
        return;
    }
    void (*test)(void) = NULL;

    if (urll_dlsym_func(handle, "begin", &test))
    {
        test();
    }
    urll_dlclose(handle);
}

Here, to load a function, we call dlsym_func. For loading variables we also have dlsym_var, and a generic dlsym, that just returns a void* which you can manually cast your variables/functions.

Compiling your application

This library depends on libdl when compiling on Unix based systems, so make sure you are linking against it.

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