共享库中带有__attribute __（（constructor））的全局/静态变量初始化问题

本文介绍了共享库中带有__attribute __（（constructor））的全局/静态变量初始化问题的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 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.cppstruct 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 -ldlThe output:Calling dlopen...MyStruct, this: 0x121b200, s=1MyStruct, this: 0x121b220, s=1MyStruct, this: 0x121b240, s=1MyStruct, this: 0x7ffc19736910, s=2~MyStruct, this: 0x7ffc19736910, s=2&s1: 0x7fb1fe487190, &s2: 0x7fb1fe487198, &s3: 0x7fb1fe4871a0s1: 0x121b200, s2: 0x121b220, s3: 0x121b240s4: 0x7fb1fe4871a8, s4.s: 2MyStruct, this: 0x7fb1fe4871a8, s=1Calling foo()...&s1: 0x7fb1fe487190, &s2: 0x7fb1fe487198, &s3: 0x7fb1fe4871a0s1: 0x121b200, s2: (nil), s3: 0x121b240s4: 0x7fb1fe4871a8, s4.s: 1~MyStruct, this: 0x7fb1fe4871a8, s=1~MyStruct, this: 0x121b240, s=1The 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. 这篇关于共享库中带有__attribute __（（constructor））的全局/静态变量初始化问题的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！