一个比较偏的类型转换有关问题

一个比较偏的类型转换问题

得到一个field在结构体(struct)中的偏移量
#define FPOS(type,field) ((dword)&((type *)0)->field)



这里我有个疑问，(type *)0 把0进行强制转换，这样的依据是什么，转换后的内存地址是多少，这个地址是不是有效的？







[解决办法]
偏移量是相对值，所以可以那样做。你甚至可以这样做：

C/C++ code

#define FPOS(type,field) ((int)&((type *)10000)->field - 10000)// 1. 假定一个type变量的起始地址在10000，((type *)10000)就是将该地址开始，后面的sizeof(type)的内//    存中的内容，看成是type。// 2. (int)&((type *)10000)->field，这个是取field的绝对地址，并将其转换为int(大致和你上面的dword相当)// 3. (int)&((type *)10000)->field - 10000，这个是用field的绝对地址减去起始地址，自然就是偏移量了// 4. 很明显，将10000改成0，就可以省去减号及其后面的部分，看起来更简洁一些而已。typedef struct _A{    int a;    int b;    char c;    char d;    char e;    char f;} A;int main(int argc, char** argv){    cout << FPOS(A, d) << endl;    return 0;}
[解决办法]
探讨


它要的只是地址，并没有真正去访问这个存储单元，这么做是没问题的，从0取值才能得到偏移量，如果从某结构体变量取值，那就成绝对线性地址了，这个应该好理解的！

[解决办法]
ISO C11 N1570
6.3.2.3
3 An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.66) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.
4 Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal.
5 An integer may be converted to any pointer type. Except as previously specified, the result is implementation-defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation.67)
6 Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type.
7.19
3 The macros are
NULL
which expands to an implementation-defined null pointer constant; and
offsetof(type, member-designator) which expands to an integer constant expression that has type size_t, the value of which is the offset in bytes, to the structure member (designated by member-designator), from the beginning of its structure (designated by type). The type and member designator shall be such that given
static type t;
then the expression &(t.member-designator) evaluates to an address constant. (If the specified member is a bit-field, the behavior is undefined.)
某种意义上来说，正因为这个impl-def造成实现的不确定性，所以标准库有必要包含offsetof。


[解决办法]
#define FPOS(type,field) ((int)&((type *)10000)->field - 10000)

编译器在展开这个宏时，并不会真正去访问这个地址(10000)的,而是假设10000是某个type类型的变量的地址，然后得这个变量的field字段的的偏移，在减去这个变量的地址，就得到field字段相对于结构体第一个程序的偏移。

实际上，这个宏，编译器在编译期就可其确定其值，而不是在运行期计算的。下面是我做的一个实验，分别给出我实验的源代码和汇编代码

源代码
#define FPOS(type,field) ((int)&((type *)10000)->field - 10000)

typedef struct
{
int f1;
char f2;
int f3;
char f4;
}MY_ST;

int main()
{
int n=FPOS(MY_ST,f2);
printf("n=%d\n",n);
return 0;

}

汇编代码
_mainPROC; COMDAT
; 17   : int n=FPOS(MY_ST,f2);
; 18   : printf("n=%d\n",n);

push4
pushOFFSET ??_C@_05HKPJIDL@n?$DN?$CFd?6?$AA@
callDWORD PTR __imp__printf
addesp, 8

; 19   : return 0;
xoreax, eax
}