Links to the original text: http://harlon.org/2018/04/05/cpluscplusmemorypack/

C/C++ Memory Alignment Byte Alignment Strategy

Strct memory completion is one of the compulsory questions in the written test, but sometimes the way stuct memory alignment is very confusing. Here we comb the strategy of memory alignment.

strategy

• Rule 1: The front address must be an integral multiple of the back address, not aligned.
• Rule 2: The entire address must be an integer multiple of the largest byte.
• Rule 3: Specify # pragma pack(n), the entire address is an integer multiple of n.

Example

Take 32-bit environment as an example (64-bit environment pointer is 8 bytes, the rule is the same):

struct s {
    char *p; // 4 byte
    char c; // 4 byte rule 1
    int a; // 4 byte
};
sizeof(s) = 12;
struct s {
    char *p; // 4 byte
    char c; // 1 byte
    char ch[2]; // 3 byte rule 1
    int x; // 4 byte
};
sizeof(s) = 12;
struct s {
    char p; // 4 byte rule 1
    int x; // 4 byte
    char c; // Rule 2 of 4 bytes
};
sizeof(s) = 12;
struct s {
    char p; // 4 byte rule 1
    int x; // 4 byte
    char c; // 4 byte
    struct a {
        char *b; // 4 byte
        long y; // 4 byte
    } inner; // 8 byte
};
sizeof(s) = 20;
struct s {
    char p;
    char x;
    struct a {
        char b;
        int y[10];
    } inner;
    char c;
};
sizeof(s) = 56;
struct s {
    char p;
    char x;
    int f:1;
    int b:2;
    char c:1;
};
sizeof(s) = 12;
struct s {
    char c; // 4 byte rule 1
    union u {
        char *p; 
        short x;
    } inner; // Four byte
};
sizeof(s) = 8;
union s {
    char c;
    struct u {
        char p;
        int x;
    } inner;
};
sizeof(s) = 8;
#pragma pack(2)
struct s {
    char *p; // 4 byte
    char x; // 2 byte rule 3
    int a; // 4 byte
};
sizeof(s) = 10;
#pragma pack(1)
struct s {
    char *p;
    char x;
    int a;
};
sizeof(s) = 9;
#pragma pack(8)
struct s {
    char *p;
    char x;
    int a;
};
sizeof(s) = 16;