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。