C++中全局变量和静态成员变量动态初始化的实现

【问题标题】：Implementation of dynamic initialization for global variables and static member variables in C++C++中全局变量和静态成员变量动态初始化的实现
【发布时间】：2015-06-30 11:40:19
【问题描述】：

关于链接和可执行模块加载过程，我对 C++ 中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。我正在寻找解决我对 Windows 和 Linux 的问题的答案。

我已经明白在静态初始化的情况下：

-编译时将初始值放入自己的section中

-这些部分被 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位置

这是我的问题。

  1. 编译器会将哪些信息放入生成的目标文件中，这些信息与全局变量的动态初始化相关，以供链接器使用？请尽可能详细地了解相关部分和生成的符号。静态成员变量与非静态全局变量有什么区别？

  2. 链接器在链接过程中将哪些信息放入最终链接的模块中，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，这些变量将函数作为一部分调用初始化）？

  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用该代码初始化的特定变量？

  4. 加载可执行或动态链接模块时，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考虑？

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参考，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被链接和加载，以便可以执行静态变量的成功初始化，这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 存在的具体差异。

这是一个涉及常规变量和静态成员变量的 C++ 代码的简单示例，据我了解，它需要在 ma​​in 之前由操作系统加载程序动态初始化，因为它调用了一个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;
}

这里是 MSVC 使用上面在调试模式下编译的代码生成的目标文件的部分和符号的转储（转储是使用 llvm-readobj 创建的，它是 llvm/clang 附带的实用程序）：

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非常感谢您考虑我的问题；将不胜感激。

【问题讨论】：

  • 我不确定，但我认为您误解了静态函数。静态函数是仅对同一文件中的其他函数可见的函数（翻译单元）。除了关键字“static”之外，它们与静态数据没有任何关系。
  • var 需要动态初始化，因为它的初始化器不是常量表达式（它调用非 constexpr 函数）。正如上面的评论所说，该函数是否为static 完全无关紧要。变量是否为static 也完全无关紧要。您的问题是关于具有 static storage duration 的变量，但所有全局变量（线程局部变量除外）都具有静态存储持续时间，而不仅仅是标记为 static 的变量。
  • 您在前四个问题中提出的问题是特定于实现的。您想了解 Windows 和 Linux 的这些详细信息吗？您基本上是在问两个不同的问题：Windows 的所有这些细节是什么，Linux 的所有这些细节是什么。
  • Jonathan Wakely，你是对的，我已经调整了问题以反映你的 cmets。
  • CrazyCasta，我正在寻找 Linux 和 Windows 的详细信息。修改了问题以使其更清楚。

标签： c++ static linker initialization


【解决方案1】：

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，就是这样。我将提到的任何特定符号或部分都是针对 ARM 的，其他处理器可能会有所不同。

我将您的小示例程序复制到ELLCC demo，这是一个基于 clang 的发行版，并为 ARM 编译了它。我不得不关闭优化才能看到任何有趣的东西，因为没有使用您的初始化变量。

通过查看汇编语言，您会看到编译器将生成代码来执行任何需要在源文件中完成的初始化。正如您所指出的，初始化为链接时常量值的事物可以通过将适当的符号及其初始值放在一个部分中来初始化（通常称为 .data 用于可写的东西， .const 用于只读的东西。）不能的值在编译或链接时计算由一个编译器生成的函数初始化，该函数在进入 main() 之前执行。如果您编译您的示例并查看程序集，则在接近结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)

这里的神奇之处在于 .init_array 节是对编译器和运行时系统具有特殊意义的节。编译器已将内部生成函数的地址放在 .init_array 部分中。该函数执行此源文件所需的任何初始化。如果你在源代码中有静态 C++ 构造函数，它们也会从这里被调用。 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。这会初始化您的变量。

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做任何事情。

您现在可能已经猜到了，还有一个称为 .fini_array（或 x86 世界中的 .dtors）的部分，我们在程序尝试退出时使用它来处理全局析构函数。

【讨论】：

  • 非常感谢理查德，感谢您的信息，我能够进行动态初始化。根据您提供的信息，我还在msdn.microsoft.com/en-us/library/bb918180.aspx 和gonwan.com/?p=9 找到了有关 MSVC 动态初始化的更多信息。这些文章解释了 CRT 如何在 main 之前调用函数指针数组（如您所述）。在 MSVC 下，.CRT$XCU 部分包含负责初始化的函数。你的帖子让我走上了正轨，我真的很感激！
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