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。
posted on 2008-05-06 17:33
shosh 阅读(236)
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