在C语言中，确实指向一个结构，始终指向它的第一个成员？ [英] In C, does a pointer to a structure always point to its first member?

问题描述

假设我有一些C结构为，我想一组特定的功能来操作。

Suppose I have a number of C structs for which I would like a particular set of functions to operate upon.

我想知道如果下面是一个合法的方法：

I'm wondering if the following is a legitimate approach:

typedef struct Base {
     int exampleMember;
     // ...
} Base;

typedef struct Foo {
     Base base;
     // ...
} Foo;

typedef struct Bar {
     Base base;
     // ...
} Bar;

void MethodOperatesOnBase(void *);

void MethodOperatesOnBase(void * obj)
{
     Base * base = obj;
     base->exampleMember++;
}

在这个例子中你会发现，这两个结构富和酒吧开始以基础成员。

In the example you'll notice that both structs Foo and Bar begin with a Base member.

和，在 MethodOperatesOnBase ，我投了无效* 参数基础* 。

我想指针传递给酒吧，以及在富这种方法，依靠第一该结构的成员是一个基本结构。

I'd like to pass pointers to Bar and pointers to Foo to this method and rely on the first member of the struct to be a Base struct.

这是可以接受的，还是有我需要了解一些（可能是编译器特定的）问题是什么？ （如某种包装/填充方案中，将改变结构的第一个成员的位置？）

Is this acceptable, or are there some (possibly compiler-specific) issues I need to be aware of? (Such as some sort of packing/padding scheme that would change the location of the first member of a struct?)

推荐答案

是的，C标准明确保证，这将正常工作。

Yes, the C standard specifically guarantees that this will work.

（C1X§6.7.2.1.13：一个指向结构对象，适当转换，道出了它的初始成员......反之有可能是内，结构对象无名填充，而不是在其开始。 。）

(C1x §6.7.2.1.13: "A pointer to a structure object, suitably converted, points to its initial member ... and vice versa. There may be unnamed padding within as structure object, but not at its beginning.")

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