c - 使用自身地址初始化对象

Question

这是在C99和C11中定义的吗？

struct A
{
    struct A* first;
    int value;
};

{    // inside a function
    struct A a = { &a }; 
    a.first->value = 123;
}

并使用说明符静态:

{    // inside a function
    static struct A a = { &a }; 
    a.first->value = 123;
}

Answer 1

这是明确定义的行为。

根据 C 标准 §6.2.4(强调我的):

An object exists, has a constant address, and retains its last-stored value throughout its lifetime.

非静态结构的生命周期从进入声明它的 block 开始(称为自动存储持续时间):

For such an object that does not have a variable length array type, its lifetime extends from entry into the block with which it is associated until execution of that block ends in any way.

如果在与声明关联的 block 中指定初始化:

[the initialization] is performed each time the declaration is reached in the execution of the block; otherwise, the value becomes indeterminate each time the declaration is reached.

因此在进入 block 时，该结构已保证存储空间和一个常量地址，您可以在该结构的初始化中自由使用它，因为初始化保证在该结构的生命周期之后发生结构已经开始。

对于静态结构，

Its lifetime is the entire execution of the program and its stored value is initialized only once, prior to program startup.

因此，一旦程序开始执行，该结构就保证了存储空间和常量地址，并且它的存储值在该生命周期内但在标准执行协议(protocol)(调用 main() 等)之前被初始化).

来源:C99 draft .这些引用文献的 C11 文献是相同的。

Here是一个演示。这在 -std=c99 -pedantic 和 -std=c11 -pedantic 下编译时没有警告。