offsetof [英] offsetof
问题描述
你好，
我的实现没有定义offsetof，所以我设计了一个小的
程序，试图找到相对位置
结构中的成员。它只是不起作用。
我可以得到一些关于我做错的指示（除了编码
如此深夜）。 />
#include< stdlib.h>
#include< stdio.h>
int main（void ）
{
struct new
{
int a;
char b ;
char c;
} x;
size_t num;
num =（size_t）（（struct new *）& xc - & x）;
printf（"％u\\\
"，num）;
返回EXIT_SUCCESS;
}
总是返回0.
谢谢。
-
Alejo
Hello,
My implementation does not define offsetof, so I have designed a little
program that ''attempts'' to find the relative position of a member in its
structure. It just does not work.
Could I get some pointers on what I am doing wrong (apart from being coding
so late at night).
#include <stdlib.h>
#include <stdio.h>
int main( void )
{
struct new
{
int a;
char b;
char c;
} x;
size_t num;
num = (size_t)((struct new *)&x.c - &x);
printf( "%u\n", num );
return EXIT_SUCCESS;
}
It always returns 0.
Thanks.
--
Alejo
推荐答案
Alejo写道：
你好，
我的实现没有定义offsetof，所以我设计了一个小小的
程序，试图找到找到相对位置
结构中的成员。它只是不起作用。
我可以得到一些指示我做错了什么（除了编码如此深夜）。
#include < stdlib.h>
#include< stdio.h>
int main（无效）
{
struct new
{
int a;
char b;
char c;
} x;
size_t num;
num =（size_t）（ （struct new *）& xc - & x）;
printf（"％u \ n"，num）;
返回EXIT_SUCCESS;
}
它总是返回0.
Hello,
My implementation does not define offsetof, so I have designed a little
program that ''attempts'' to find the relative position of a member in its
structure. It just does not work.
Could I get some pointers on what I am doing wrong (apart from being coding
so late at night).
#include <stdlib.h>
#include <stdio.h>
int main( void )
{
struct new
{
int a;
char b;
char c;
} x;
size_t num;
num = (size_t)((struct new *)&x.c - &x);
printf( "%u\n", num );
return EXIT_SUCCESS;
}
It always returns 0.
把它放在一边。你为什么要这样做呢？
成员的偏移量不是非常便携的数量。因此，如果这已经与
加载/保存到文件[或内存缓冲区]，你应该重新考虑你的
设计。
对于一般问题，也许.c是第一个由编译器打包的元素。尝试执行
x.c = 4;
并获取编译器生成的汇编代码。我的编译器[gcc
3.3.1预发布]产生
main：
pushl％ebp
movl％esp，％ebp
subl
Leave that aside. Why are you doing this anyways? The offset of
members is not a terribly portable quantity. So if this has todo with
loading/saving to a file [or memory buffer] you ought to rethink your
design.
As to the general question, maybe .c is the first element as packed by
the compiler. Try doing
x.c = 4;
and get the assembler code the compiler produces. My compiler [gcc
3.3.1 pre-release] produces
main:
pushl %ebp
movl %esp, %ebp
subl
8，％esp
andl
8, %esp
andl
-16，％esp
movb
-16, %esp
movb
这篇关于offsetof的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
