Demo of bug about init priority in both gcc and clang

bar.cpp

#include "bar.h"

#include <memory>

template <typename T>
struct Constructor
{
    Constructor(T&& fn)
    {
        fn();
    }
    Constructor() = delete;
    ~Constructor() = default;
};

void on_load();

// Putting this init before barGlobalPtr
static Constructor<void(*)(void)> init(on_load);

#ifdef BUG
std::unique_ptr<bar> barGlobalPtr(new bar(1));
#else
std::unique_ptr<bar> barGlobalPtr
    __attribute__((init_priority(101))) (new bar(1));
#endif


//void on_load() __attribute__((constructor));
void on_load()
{
    barGlobalPtr.reset(new bar(2));
}

void bar_func(void)
{
    printf("bar global ptr %p, value: %d\n", barGlobalPtr.get(), barGlobalPtr->m);
}

bar.h

#pragma once
#include <cstdio>

struct bar
{
    bar(int i)
     :m(i)
    {
        printf("%s: %p, %d\n", __func__, this, m);
    }
    ~bar()
    {
        printf("%s: %p, %d\n", __func__, this, m);
    }
    int m;
};

extern "C" void bar_func(void);

main.cpp

#include "bar.h"

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

using bar_func_type = void(*)(void);

int main(void)
{
    auto h = dlopen("./libbar.so", RTLD_LAZY);
    if (!h)
    {
        fprintf(stderr, "%s\n", dlerror());
        exit(EXIT_FAILURE);
    }

    bar_func_type f = nullptr;
    auto r = dlsym(h, "bar_func");
    if (!r)
    {
        fprintf(stderr, "%s\n", dlerror());
        goto out;
    }

    *reinterpret_cast<void**>(&f) = r; 

    f();

out:
    dlclose(h);

    return 0;
}

Makefile

CC=g++
#CC=clang++
CFLAGS=-c -Wall -fpic
BUGFLAG=-DBUG
SOURCES=main.cpp bar.cpp

all: main

main: main.o libbar.so
	$(CC) -o main main.o -ldl

libbar.so: bar.cpp
	$(CC) $(BUGFLAG) -shared -fPIC -o $@ $<

.cpp.o:
	$(CC) $(CFLAGS) $< -o $@


clean:
	@rm -f main main.o bar.o libbar.so


###
#
# To show the bug:
#
#     make
#     ./main
# 
# Output:
#
#     bar: 0x55e42b836570, 2
#     bar: 0x55e42b8369a0, 1
#     bar global ptr 0x55e42b8369a0, value: 1
#     ~bar: 0x55e42b8369a0, 1
# 
# To show the fix of the bug:
#
#     make BUGFLAG=
#     ./main
# 
# Output:
#
#     bar: 0x5592bb9df570, 1
#     bar: 0x5592bb9df9a0, 2
#     ~bar: 0x5592bb9df570, 1
#     bar global ptr 0x5592bb9df9a0, value: 2
#     ~bar: 0x5592bb9df9a0, 2