那些遇到过的问题

在C++中实现全局变量和静态成员变量的动态初始化

San*_*low 7 c++ linker static initialization

关于链接和可执行模块加载过程,我对C++中的变量初始化的实现有一些疑问.我主要关心的是全局变量和静态成员变量的动态初始化,其中初始化过程涉及代码的执行.我正在寻找解决Windows和Linux问题的答案.

我已经明白在静态初始化的情况下:

- 在编译期间将初始值放入其自己的部分

- 这些部分由OS模块加载器映射到内存中

- 通过应用DIR32类型重定位,为变量分配初始值的内存地址的位置

这是我的问题.

  1. 编译器将哪些信息放入与链接器要使用的全局变量的动态初始化相关的生成的目标文件中？请尽可能详细地了解相关部分和生成的符号.与非静态全局变量相比,静态成员变量有何不同？

  2. 链接器在链接过程中将哪些信息放入最终链接模块中,以便OS模块加载器能够正确初始化所有变量(包括动态初始化的全局/静态成员变量,这些变量使函数调用作为初始化的一部分)？

  3. 在动态变量初始化期间需要执行的函数如何映射到需要使用该代码初始化的特定变量？

  4. 加载可执行文件或动态链接模块时,如何执行变量的动态初始化？

  5. 与常规静态成员变量和函数的实现相比,C++ 11常量表达式(由constexpr说明符标记)的实现是否涉及任何特殊注意事项？

我有一个具体的例子,我希望答案可以在上述问题的框架内提及,因为我觉得有一个具体的例子来获取一个目标文件,识别相关的部分/符号以及这个特定的代码将如何链接和加载,以便可以执行静态变量的成功初始化将使答案更容易理解.此示例适用于使用MSVC作为编译器的Windows; 请提及gcc/linux的具体差异,无论它们存在于何处.

这是一个简单的C++代码示例,涉及一个常规变量和一个静态成员变量,根据我的理解需要在main之前由OS加载器动态初始化,因为它调用函数作为其初始化的一部分:

class Test
{
public:

    static int testFunction()
    {
        return 10;
    }

    static int memberVar;
};

int Test::memberVar = Test::testFunction();

int foo()
{
    return 5;
}

int var = foo();


int main(int argc, char* argv[])
{
    var;

    Test::memberVar;

    return 0;
}
Run Code Online (Sandbox Code Playgroud)

这里是MSVC使用在调试模式下编译的上述代码生成的目标文件的节和符号的转储(转储是使用llvm-readobj创建的,这是一个llvm/clang附带的实用程序):

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非常感谢您考虑我的问题; 一个彻底的答案将不胜感激.

Ric*_*ton 2

这与其说是链接器问题，不如说是编译器/运行时问题。当然，完整的答案因系统而异，但对于 Linux 上的 gcc/clang 来说是这样的。我提到的任何特定符号或部分均适用于 ARM，其他处理器可能有所不同。

我将您的小示例程序复制到ELLCC 演示（这是一个基于 clang 的发行版）中，并针对 ARM 进行了编译。我必须关闭优化才能看到任何有趣的东西，因为未使用您的初始化变量。

通过查看汇编语言，您将看到编译器将生成代码来执行需要在源文件中完成的任何初始化。正如您所指出的，初始化为链接时常量值的事物可以通过在节中放置适当的符号及其初始值来初始化（通常称为 .data 表示可写内容， .const 表示只读内容。）在编译或链接时计算由编译器生成的函数初始化，该函数在进入 main() 之前执行。如果您编译示例并查看程序集，靠近末尾处有几行如下所示：

.section    .init_array,"aw",%init_array
.align  2
.long   _GLOBAL__sub_I__6873_0.cc(target1)
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这里的神奇之处在于 .init_array 部分对于编译器和运行时系统具有特殊含义。编译器已将内部生成的函数的地址放置在 .init_array 部分中。该函数执行该源文件所需的任何初始化。如果源代码中有静态 C++ 构造函数，它们也会从这里调用。x86 处理器有一个类似的部分，称为 .ctors，它具有类似的语义。

现在是运行时部分。当程序启动时，运行时系统首先获得控制权。它执行诸如初始化库、加载动态库等操作，并获取 .init_array 中的每个函数指针并执行它。这会让你的变量初始化。

请注意，除了将 .init_array 中的所有函数指针收集到一个位置以便运行时系统可以为它们提供资金之外，链接器实际上不需要执行任何操作。

您现在可能已经猜到了，当程序尝试退出处理全局析构函数时，我们还会使用另一个名为 .fini_array （或 x86 世界中的 .dtors）的部分。

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