gcc에서 구조체 pack size 조절하려면?

#include <stdio.h>
                                                                                
int main()
{
    struct A
    {
        char a;
    };
                                                                                
#pragma pack(1)
    struct B
    {
        char a;
        short b;
    };
#pragma pack()
                                                                                
    struct C
    {
        char a;
        short b;
    };
                                                                                
    printf("%d, %d, %d\n", sizeof(A), sizeof(B), sizeof(C));
                                                                                
    return 0;
}