破坏全局和静态对象的最佳方法？

Question

What is the best way to end the lifetime of an object with static storage duration?

Current implementation finds the caller of __run_exit_handlers which then will be used to determine the __exit_funcs .

However this would easily fail since offset to __run_exit_handlers can change easily even in glibc with the same version. Another thing that could be done is to resolve the address of __run_exit_handlers first then use it in finding the caller rather than using a hardcoded call offset.

Current Working Code :

#include <iostream>
#include <fstream>
#include <cstddef>
#include <cstdint>
#include <cstring>
#include <cstdio>

#include <execinfo.h>

struct A
{
    A(std::string pName)
        : mName(pName)
    {
        std::printf("%s %s\n", __PRETTY_FUNCTION__, mName.c_str());
    }

    ~A()
    {
        std::printf("%s %s\n", __PRETTY_FUNCTION__, mName.c_str());
    }
    volatile int i = 0;
    std::string mName;
};

A a{"a"};
A b{"b"};
A c{"c"};

class StaticDestroyer
{
public:
    StaticDestroyer()
    {
        std::ifstream maps("/proc/self/maps", std::ios::in);
        char line[1024];
        uint8_t* magic = nullptr;
        while (maps.getline(line, sizeof(line)))
        {
            char perms[4];
            uint8_t *magicbegin, *magicend;
            std::string lsv(line);
            if (std::string::npos == lsv.find("/libc-",0,6)) continue;
            std::sscanf(line, "%lx-%lx %4s", &magicbegin, &magicend, perms);
            if (perms[0]==114 && perms[2]==120)
            {
                 magic = findMagic(magicbegin, magicend);
                 break;
            }
        }

        if (magic==nullptr)
            throw std::runtime_error("magic not found!");

        mHead = *(HakaishinNode**)magic;
    }

    bool destroy(void* pTarget)
    {
        HakaishinNode *current = mHead;
        while (nullptr != current)
        {
            for (size_t i = current->idx-1 ; i>0; i--)
            {
                const  Hakaishin *const f = &current->fns[i];
                if (4 == f->type && pTarget == f->arg)
                {
                    void (*destruct) (void *arg, int status) = f->fn;
                    asm ("ror $2*8+1, %0\nxor %%fs:%c2, %0" : "=r" (destruct) : "0" (destruct), "i" (48));
                    destruct (f->arg, 1);
                    if (current->idx-1 != i) for (size_t j = i; j < current->idx ; j++) current->fns[j] = current->fns[j+1];
                    current->idx--;
                    return true;
                }
            }

            current = current->next;
        }
        return false;
    }
private:
    struct Hakaishin
    {
        long int type;
        void (*fn) (void *arg, int status);
        void *arg;
        void *dso_handle;
    };

    struct HakaishinNode
    {
        struct HakaishinNode *next;
        size_t idx;
        Hakaishin fns[32];
    };

    uint8_t* findMagic(uint8_t* magicbegin, uint8_t* magicend)
    {
        const void* const begin = magicbegin;
        int32_t ptr;
        while ((magicbegin+7) <= magicend)
        {
            if (magicbegin[0]==0x48 && (magicbegin[1]==0x8b || magicbegin[1]==0x8d))
            {
                std::memcpy(&ptr, magicbegin+3, sizeof(ptr));
                uint8_t* magicp = magicbegin+ptr+7;
                if (ptr==0x38a5c1) return magicp;
            }
            magicbegin++;
        }
        return nullptr;
    }

    HakaishinNode* mHead = nullptr;
};

A& getA()
{
    static A a{"getA"};
    return a;
}

A& getA2()
{
    static A a{"getA2"};
    return a;
}

int main()
{
    std::printf("entering...\n");
    StaticDestroyer d;
    d.destroy(&a);
    d.destroy(&b);
    auto& ga = getA();
    d.destroy(&ga);
    getA2();
    std::printf("returning...\n");
}

Output:

A::A(std::string) a
A::A(std::string) b
A::A(std::string) c
entering...
A::~A() a
A::~A() b
A::A(std::string) getA
A::~A() getA
A::A(std::string) getA2
returning...
A::~A() getA2
A::~A() c

Answer 1

Static objects will be destructed with the termination of the program.

If you like to manage the resources don't make it static or use a static pointer. Here you can allocate and de-allocate the corresponding resources. This approach comes very close to a singleton, which is considered to be an anti pattern .

Conclusion: If you need to manage a resource don't make it static.

Answer 2

The need to mess around with the default behavior of life-time in such a way indicates that you have a design flaw in your application.

So you should either consider restructuring your program to not use globals. Or at least change the way how you handle the globals. So if you really need globals and release them earlier, then switch to unique_ptr :

#include <iostream>
#include <functional>
#include <memory>

struct A
{
    A(std::string pName)
        : mName(pName)
    {
        std::printf("%s %s\n", __PRETTY_FUNCTION__, mName.c_str());
    }

    ~A()
    {
        std::printf("%s %s\n", __PRETTY_FUNCTION__, mName.c_str());
    }
    volatile int i = 0;
    std::string mName;
};

auto a = std::make_unique<A>("a");
auto b = std::make_unique<A>("b");
auto c = std::make_unique<A>("c");

auto& getA()
{
    static auto a = std::make_unique<A>("getA");
    return a;
}

auto& getA2()
{
    static auto a = std::make_unique<A>("getA2");
    return a;
}

int main() {
    std::printf("entering...\n");

    a = nullptr;
    b = nullptr;
    c = nullptr;

    getA();
    getA2();

    getA() = nullptr;

    std::printf("returning...\n");
}

That way you can release the objects managed by the unique_ptr earlier, but they will be released on exit automatically if you don't set them to nullptr manually.