C 结构的位域大小

Question

#include <stdio.h>
int main() {
    struct on_off {
        unsigned light      : 1;
        unsigned toaster    : 1;
        int count;/* 4 bytes */
        unsigned ac         : 4;
        unsigned            : 4;
        unsigned clock      : 1;
        unsigned            : 0;
        unsigned flag       : 1;
    } kitchen;
    struct box_props {
         unsigned int opaque       : 1;
         unsigned int fill_color   : 3;
         unsigned int              : 4; 
         unsigned int show_border  : 1;
         unsigned int border_color : 3;
         unsigned int border_style : 2;
         unsigned int              : 2; 
    } s;

    printf("\n\nSize of struct on_off = %d\n", sizeof(struct on_off));
    printf("\nSize of box_props = %d\n", sizeof(struct box_props));

    return 0;
}

编译此程序时，struct on_off 的大小报告为 16，而 box_props 的大小报告为 4。谁能解释为什么会这样？

Answer 1

对于第一个结构

struct on_off{
    unsigned light      : 1;
    unsigned toaster    : 1;  // 1 byte 
                              // 3 bytes - packing to align to int border
    int count;                // 4 Bytes        
    unsigned ac         : 4;
    unsigned ss         : 4;  // 1 Byte
    unsigned clock      : 1;  // 1 byte
    unsigned            : 0;  // 2  byte -- A size of zero forces alignment to next boundary.
    unsigned flag       : 1;  // 1 byte
                              // 3 bytes -- Packing at end of struct to align to word boundary.
}
                              // 16 bytes -- Total

对于第二个结构

struct box_props{
     unsigned int opaque       : 1;
     unsigned int fill_color   : 3;
     unsigned int              : 4;   // 1 byte
     unsigned int show_border  : 1;
     unsigned int border_color : 3;   // 1 byte
     unsigned int border_style : 2;   
     unsigned int              : 2;   // 1 byte
                                      // 1 byte - packing at the end of structure to align to word boundary.
}
                                      // 4 bytes Total