编译器 gcc，vc++使用的编译器，clang 经过测试，默认字节对齐方式均为8字节

代码

#include <iostream>
using namespace std;


#pragma pack (8)
struct v1{
    int       a;
    long long b;
    char      c;
};
#pragma pack (4)
struct v2{
    int       a;
    long long b;
    char      c;
};
#pragma pack (2)
struct v3{
    int       a;
    long long b;
    char      c;
};
#pragma pack (1)
struct v4{
    int       a;
    long long b;
    char      c;
};
#pragma pack ()

int main(int argc, const char * argv[]) {
    cout<<sizeof(v1)<<endl;
    cout<<sizeof(v2)<<endl;
    cout<<sizeof(v3)<<endl;
    cout<<sizeof(v4)<<endl;
    return 0;
}

输出结果为：

24
16
14
13
Program ended with exit code: 0