offsetof

最新推荐文章于 2025-01-29 15:17:53 发布

michaelrun

最新推荐文章于 2025-01-29 15:17:53 发布

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分类专栏： C Tricks

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本文深入解析了C语言中结构体的内存布局，包括如何获取结构体内成员的地址和偏移量，并通过示例代码进行演示。

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

struct foobar{
	unsigned int foo;
	char bar;
	char boo;
};

int main(int argc, char** argv){

	struct foobar tmp;

	printf("address of &tmp is= %p\n\n", &tmp);
	printf("address of tmp->foo= %p \t offset of tmp->foo= %lu\n", &tmp.foo, (unsigned long) &((struct foobar *)0)->foo);
	printf("address of tmp->bar= %p \t offset of tmp->bar= %lu\n", &tmp.bar, (unsigned long) &((struct foobar *)0)->bar);
	printf("address of tmp->boo= %p \t offset of tmp->boo= %lu\n\n", &tmp.boo, (unsigned long) &((struct foobar *)0)->boo);

	printf("computed address of &tmp using:\n");
	printf("\taddress and offset of tmp->foo= %p\n",
	      (struct foobar *) (((char *) &tmp.foo) - ((unsigned long) &((struct foobar *)0)->foo)));
	printf("\taddress and offset of tmp->bar= %p\n",
	      (struct foobar *) (((char *) &tmp.bar) - ((unsigned long) &((struct foobar *)0)->bar)));
	printf("\taddress and offset of tmp->boo= %p\n",
	      (struct foobar *) (((char *) &tmp.boo) - ((unsigned long) &((struct foobar *)0)->boo)));

	return 0;
}

same as #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
/** 
 * container_of - cast a member of a structure out to the containing structure 
 * @ptr:        the pointer to the member. 
 * @type:       the type of the container struct this is embedded in. 
 * @member:     the name of the member within the struct. 
 * 
 */  
#define container_of(ptr, type, member) ({                      \   
        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \   
        (type *)( (char *)__mptr - offsetof(type,member) );})