共享库中带有__attribute __（（constructor））的全局/静态变量初始化问题 [英] Global/Static variables initialization issue with __attribute__((constructor)) in shared library

问题描述

我遇到了在共享库中使用 __ attribute __（（constructor））初始化全局/静态变量的问题，某些变量似乎被初始化了两次。

下面是代码段：

shared.cpp

  struct MyStruct 
 {
 MyStruct（int s = 1）
：s（s）{
 printf（ ％s，this：％p，s =％d\n，__func__，this，s）; 
} 
〜MyStruct（）{
 printf（％s，this：％p，s =％d\n，__func__，this，s）; 
} 
 int s; 
}; 
 
 MyStruct * s1 = nullptr; 
 std :: unique_ptr< MyStruct> s2 = nullptr; 
 std :: unique_ptr< MyStruct> s3; 
 MyStruct s4； 
 
 void onLoad（）__attribute __（（constructor））; 
 void onLoad（）
 {
 s1 =新的MyStruct; 
 s2 = std :: make_unique< MyStruct>（）; 
 s3 = std :: make_unique< MyStruct>（）; 
 s4 = MyStruct（2）; 
 
 printf（& s1：％p，& s2：％p，& s3：％p\n，& s1，& s2，& s3）; 
 printf（ s1：％p，s2：％p，s3：％p\n，s1，s2.get（），s3.get（））; 
 printf（ s4：％p，s4.s：％d\n，& s4，s4.s）; 
} 
 
 extern C void foo（）
 {
 printf（& s1：％p，& s2：％p，& s3 ：％p\n，& s1，& s2，& s3）； 
 printf（ s1：％p，s2：％p，s3：％p\n，s1，s2.get（），s3.get（））; 
 printf（ s4：％p，s4.s：％d\n，& s4，s4.s）; 
} 
  

main.cpp

  #include< cstdio> 
 #include< dlfcn.h> 
 
使用Foo = void（*）（void）; 
 
 int main（）
 {
 printf（ Calling dlopen ... \n）; 
 void * h = dlopen（ ./ libshared.so，RTLD_NOW | RTLD_GLOBAL）; 
 Foo f = reinterpret_cast< Foo>（dlsym（h， foo））; 
 printf（ \nCalling foo（）... \n）; 
 f（）; 
返回0； 
} 
  

已编译

  $ g ++ -fPIC -shared -std = c ++ 14 shared.cpp -o libshared.so 
 $ g ++ -std = c ++ 14 -o main main.cpp -ldl 
  

输出：

 正在调用dlopen ... 
 MyStruct，这个：0x121b200，s = 1 
 MyStruct，这个：0x121b220，s = 1 
 MyStruct，这个：0x121b240，s = 1 
 MyStruct，这：0x7ffc19736910，s = 2 
〜MyStruct，这：0x7ffc19736910，s = 2 
& s1：0x7fb1fe487190，& s2：0x7fb1fe487198，& s1：0x7a 
 s1：0x121b200，s2：0x121b220，s3：0x121b240 
 s4：0x7fb1fe4871a8，s4.s：2 
 MyStruct，this：0x7fb1fe4871a8，s = 1 
 
调用foo（）... 
& s1：0x7fb1fe487190，& s2：0x7fb1fe487198，& s3：0x7fb1fe4871a0 
 s1：0x121b200，s2：（nil），s3：0x121b240 
 s4 0x7fb1fe4871a8，s4.s：1 
〜MyStruct，这：0x7fb1fe4871a8，s = 1 
〜MyStruct，这：0x121b240，s = 1 
  

s1 和<

但是 s2 和 s4 表现很奇怪。

• s2.get（）应该为 0x121b220 ，但是在 foo（）中，它变为 nullptr ;


• s4 的值显示为 s4.s：2 onLoad（）中的code>，但是之后使用默认值 s = 1 调用其构造函数，然后在 foo（）中，其值为 s = 1 。

将变量放入匿名名称空间中会得到相同的结果。

s2怎么了和 s4 ？

我的操作系统：Ubuntu 16.04.2，GCC：5.4.0

解决方案

根据此GCC错误报告和此后续文档tch ，看来您看到的是GCC中未指定的行为（不是bug）。

但是，顺序未指定使用静态存储持续时间的C ++对象的构造函数以及用属性 constructor 装饰的函数的构造函数。在混合声明中，属性 init_priority 可用于施加特定顺序。

在这种情况下，似乎避免了段错误，因为分配给未初始化的 std :: unique_ptr 可能导致 delete 为未初始化的指针成员调用。根据C ++规范，GCC的未指定行为会转换为未定义的行为（在这种情况下），因为它是从未初始化的变量读取的未定义的行为（未初始化的未签名字符除外）。

无论如何，要纠正此问题，您确实需要使用 __ attribute（（init_priority））在构造函数之前命令初始化静态声明的对象。

I meet an issue with global/static variables' initialization with __attribute__((constructor)) in shared library， that certain variables seem to be initialized twice.

Below are code snippets:

shared.cpp

struct MyStruct
{
  MyStruct(int s = 1)
  : s(s) {
    printf("%s, this: %p, s=%d\n", __func__, this, s);
  }
  ~MyStruct() {
    printf("%s, this: %p, s=%d\n", __func__, this, s);
  }
  int s;
};

MyStruct* s1 = nullptr;
std::unique_ptr<MyStruct> s2 = nullptr;
std::unique_ptr<MyStruct> s3;
MyStruct s4;

void onLoad() __attribute__((constructor));
void onLoad()
{
  s1 = new MyStruct;
  s2 = std::make_unique<MyStruct>();
  s3 = std::make_unique<MyStruct>();
  s4 = MyStruct(2);

  printf("&s1: %p, &s2: %p, &s3: %p\n", &s1, &s2, &s3);
  printf("s1: %p, s2: %p, s3: %p\n", s1, s2.get(), s3.get());
  printf("s4: %p, s4.s: %d\n", &s4, s4.s);
}

extern "C" void foo()
{
  printf("&s1: %p, &s2: %p, &s3: %p\n", &s1, &s2, &s3);
  printf("s1: %p, s2: %p, s3: %p\n", s1, s2.get(), s3.get());
  printf("s4: %p, s4.s: %d\n", &s4, s4.s);
}

main.cpp

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

using Foo = void(*)(void);

int main()
{
  printf("Calling dlopen...\n");
  void* h = dlopen("./libshared.so", RTLD_NOW | RTLD_GLOBAL);
  Foo f = reinterpret_cast<Foo>(dlsym(h, "foo"));
  printf("\nCalling foo()...\n");
  f();
  return 0;
}

Compiled with

$ g++ -fPIC -shared -std=c++14 shared.cpp -o libshared.so
$ g++ -std=c++14 -o main main.cpp -ldl

The output:

Calling dlopen...
MyStruct, this: 0x121b200, s=1
MyStruct, this: 0x121b220, s=1
MyStruct, this: 0x121b240, s=1
MyStruct, this: 0x7ffc19736910, s=2
~MyStruct, this: 0x7ffc19736910, s=2
&s1: 0x7fb1fe487190, &s2: 0x7fb1fe487198, &s3: 0x7fb1fe4871a0
s1: 0x121b200, s2: 0x121b220, s3: 0x121b240
s4: 0x7fb1fe4871a8, s4.s: 2
MyStruct, this: 0x7fb1fe4871a8, s=1

Calling foo()...
&s1: 0x7fb1fe487190, &s2: 0x7fb1fe487198, &s3: 0x7fb1fe4871a0
s1: 0x121b200, s2: (nil), s3: 0x121b240
s4: 0x7fb1fe4871a8, s4.s: 1
~MyStruct, this: 0x7fb1fe4871a8, s=1
~MyStruct, this: 0x121b240, s=1

The value of s1 and s3 are expected.

But s2 and s4 behave weird.

• s2.get() should be 0x121b220, but in foo() it becomes nullptr;
• s4's value is printed as s4.s: 2 in onLoad(), but after that its constructor is called with default value s=1, then in foo() its value is s=1.

Putting the variables in anonymous namespace has the same result.

What's wrong with s2 and s4?

My OS: Ubuntu 16.04.2, GCC: 5.4.0

解决方案

As per the discussion on this GCC bug report and this follow-up doc patch it seems that what you're seeing is unspecified behavior in GCC (not a bug).

However, the order in which constructors for C++ objects with static storage duration and functions decorated with attribute constructor are invoked is unspecified. In mixed declarations, attribute init_priority can be used to impose a specific ordering.

It seems in this case that a segfault was narrowly avoided, as assigning to an uninitialized std::unique_ptr could cause delete to be invoked for an uninitialized pointer member. GCC's unspecified behavior translates into undefined behavior (in this particular case) according to the C++ specification, because it's undefined behavior to read from an uninitialized variable (except for an uninitialized unsigned char).

Anyway, to correct this problem you do indeed need to use __attribute((init_priority)) to order initialization of your statically-declared objects before the constructor function.

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