offsetof

定义

在stddef.h头文件中，该宏的完整说明如下：

#ifdef __cplusplus

#ifdef _WIN64

#define offsetof(s,m) (size_t)( (ptrdiff_t)&reinterpret_cast((((s *)0)->m)) )

#else

#define offsetof(s,m) (size_t)&reinterpret_cast((((s *)0)->m))

#endif

#else

#ifdef _WIN64

#define offsetof(s,m) (size_t)( (ptrdiff_t)&(((s *)0)->m) )

#else

#define offsetof(s,m) (size_t)&(((s *)0)->m)

#endif

#endif /* __cplusplus */

功能

在msdn上，该宏被写作：

size_t offsetof( structName, memberName );

第一个参数是结构体的名字，第二个参数是结构体成员的名字。该宏返回结构体structName中成员memberName的偏移量。偏移量是size_t类型的。

程序示例

#include

#include

typedef struct

{

int iVal;

int iVal2;

}Test;

typedef struct

{

char ch;

int iNum;

}Test2;

int main(void)

{

Test t = {1, 2};

Test2 t2 = {'t', 100};

printf("\naddress of t : %p\naddress of t.iVal : %p\naddress of t.iVal2: %p\n\n", &t, &(t.iVal), &(t.iVal2));

printf("offset of iVal in t: %d\n", offsetof(Test, iVal));

printf("offset of iVal2 in t: %d\n", offsetof(Test, iVal2));

printf("\naddress of t2 : %p\naddress of t2.ch : %p\naddress of t2.iNum: %p\n\n", &t, &(t2.ch), &(t2.iNum));

printf("offset of ch in t2: %d\n", offsetof(Test2, ch));

printf("offset of iNum in t2: %d\n", offsetof(Test2, iNum));

return 0;

}

在VS2005中输出：

address of t : 0012FF10

address of t.iVal : 0012FF10

address of t.iVal2: 0012FF14

offset of iVal in t: 00000000

offset of iVal2 in t: 00000004

address of t2 : 0012FF00

address of t2.ch : 0012FF00

address of t2.iNum: 0012FF04

offset of ch in t2: 00000000

offset of iNum in t2: 00000004

需要注意的是，Test2中iNum成员的偏移量是4而不是1，这涉及到C语言中内存对齐机制。