学步园

 
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 
 
#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;
}

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。