Example:

#pragma pack(2)
struct One
{
    char c;
    short b;
    char a;
};

print: 6

#pragma pack(2)

struct One

{

　　char a;   

　　char c;    

　　short b;    

};

print: 4

#pragma pack(4)
struct One
{
    int a;
    short b;
    char c;
    char d;
};

print: 8

#pragma pack(4)
struct One
{
    int a;
    char c;
    short b;
    char d;
};

print: 12

#pragma pack(4)
struct One
{
    char c;
    int a;
    char d;
    short b;
};

print: 12

#pragma pack(8)
struct One
{
    double a;
    short c;
    int b;
    short d;
};

print: 24

struct alignment