自己的offsetof实现会产生警告 [英] Own offsetof implementation produces warning

问题描述

我写了一小段代码来了解offsetof宏在后台如何工作.代码如下:

I wrote a small piece of code to understand how the offsetof macro works in the background. Here is the code:

#include <stdio.h>   
#include <stdlib.h>  

int main(void)
{
    /* Getting the offset of a variable inside a struct */
    typedef struct {
        int a;
        char b[23];
        float c;
    } MyStructType;

    unsigned offset = (unsigned)(&((MyStructType * )NULL)->c);

    printf("offset = %u\n", offset);

    return 0;
}

但是，如果我运行它，则会收到警告消息:

However, if I run it I get a warning message:

警告:从指针强制转换为不同大小的整数[-Wpointer-to-int-cast]

WARNING: cast from pointer to integer of different size [-Wpointer-to-int-cast]

但是，如果我查看c中原始的offsetof宏，代码将如下所示:

However, if I look at the original offsetof macro in c, the code looks like this:

#include <stdio.h>   
#include <stdlib.h> 
#include <stddef.h> 

int main(void)
{
    /* Getting the offset of a variable inside a struct */
    typedef struct {
        int a;
        char b[23];
        float c;
    } MyStructType;

    unsigned offset = offsetof(MyStructType, c);

    printf("offset = %u\n", offset);

    return 0;
}

那么当我强制转换为unsigned时，为什么会收到警告?它似乎是offsetof宏的类型.这让我感到困惑.

So why do I get the warning as I cast to unsigned ? It appears to be the type for the offsetof macro. This is puzzling me.

推荐答案

正如mch所评论的， unsigned 不是正确的类型.在几乎所有实际系统中，它都是32位的. offsetof 宏应该产生类型为 size_t 的结果，这是您应该"投射到的结果.我认为您对将结果存储到 unsigned 类型的对象中的代码感到困惑；只要确定值很小就可以，但这并不表示 offsetof(MyStructType，c); 的类型是 unsigned .C允许您将较大的整数类型静默分配给较小的整数类型.

As mch commented, unsigned is not the right type; it's 32-bit on pretty much all real-world systems. The offsetof macro is supposed to produce a result of type size_t, which is what you "should" be casting to here. I think you're confused by the code you found storing the result into an object of type unsigned; that's okay as long as they're sure the value is small, but it doesn't mean the type of the expression offsetof(MyStructType, c); was unsigned. C allows you to silently assign a larger integer type into a smaller one.

但是，无论您做什么，这不是 offsetof 的有效实现，并且具有未定义的行为(通过应用-> 指向无效的指针).没有UB的有效 offsetof 的方法是 #include< stddef.h> .

However, no matter what you do, this is not a valid implementation of offsetof and has undefined behavior (via applying -> to an invalid pointer). The way you get a working offsetof without UB is #include <stddef.h>.

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