查看结构体成员的大小和偏移地址的方法

这个是不小心被我撞见的，看到#define宏定义比较特别，仔细看了一下，发现原来有如此作用（请不要怪我少见多怪哦）。自己编写一小程序试之，贴出代码与运行结果与大家共享。 代码：

#include <stdio.h>

#define PACKVALUE 4
#pragma pack(push)
#pragma pack(PACKVALUE)        
typedef struct
{
    char sa;
    double sb;
    int sc;
} innerS;

typedef struct
{
    int a;
    char b;
    short c;
    innerS d[2];
} testS;

#pragma pack(pop)

typedef unsigned long dword;

#define FSIZE(type, field) sizeof(((type*)0)->field)
#define FPOS(type, field) ((dword) & ((type*)0)->field)

int main(void)
{
    printf("#pragma pack(%d):/nsizeof(char)=%d; sizeof(short)=%d; sizeof(int)=%d; sizeof(double)=%d/n/n",

                        PACKVALUE, sizeof(char), sizeof(short), sizeof(int), sizeof(double));
    

    printf("FSIZE = %d, FPOS = %d/n", FSIZE(testS, a), FPOS(testS, a));
    printf("FSIZE = %d, FPOS = %d/n", FSIZE(testS, b), FPOS(testS, b));
    printf("FSIZE = %d, FPOS = %d/n", FSIZE(testS, c), FPOS(testS, c));
    printf("FSIZE = %d, FPOS = %d/n", FSIZE(testS, d), FPOS(testS, d));
    printf("FSIZE = %d, FPOS = %d/n", FSIZE(testS, d[0]), FPOS(testS, d[0]));
    printf("FSIZE = %d, FPOS = %d/n", FSIZE(testS, d[0].sa), FPOS(testS, d[0].sa));
    printf("FSIZE = %d, FPOS = %d/n", FSIZE(testS, d[0].sb), FPOS(testS, d[0].sb));
    printf("FSIZE = %d, FPOS = %d/n", FSIZE(testS, d[0].sc), FPOS(testS, d[0].sc));
    printf("FSIZE = %d, FPOS = %d/n", FSIZE(testS, d[1]), FPOS(testS, d[1]));
    printf("FSIZE = %d, FPOS = %d/n", FSIZE(testS, d[1].sa), FPOS(testS, d[1].sa));
    printf("FSIZE = %d, FPOS = %d/n", FSIZE(testS, d[1].sb), FPOS(testS, d[1].sb));
    printf("FSIZE = %d, FPOS = %d/n", FSIZE(testS, d[1].sc), FPOS(testS, d[1].sc));
       

   return 0;
}

 

运行结果：

看来这是一个很不错的用来研究结构体对齐问题的辅助工具，比我上一次自己编写的那个查看内存的函数好用很多。

不过对于#define FPOS(type,field) ((dword) & ((type*) 0)->field)是如何实现的还是不懂，还望高人指点。