定义宏打印某位域共有多少位

#include <stdio.h>   
  
#define bits(p, d)  { \   
    int _tmp=p->d, _bits=0; \   
    for (p->d=1; p->d; p->d<<=1) \   
        _bits++; \   
    p->d=_tmp; \   
    printf("%s->%s has %d bits", #p, #d, _bits); \   
}   
  
typedef struct _s{   
    int a:4;   
} S;   
int main()   
{   
    S tmp, *s = &tmp;   
    bits(s,a);   
}   
  
/* 输出： s->a has 4 bits */