Memory alignment of structure

Memory alignment of structures is a Especially popular Knowledge points of ！

Cited example ：

#include<iostream>
using namespace std;
struct S
{
    
	char c;		// 1
	int a;		// 4
	char d;		// 1
};
int main()
{
    
	struct S s = {
     'a',2,'y'};
	cout << sizeof(struct S) << endl;// 12
	cout << sizeof(s) << endl; 		 // 12
	return 0;
}

Structure memory alignment rules

1. Structure of the The first variable , Always put it at the beginning of the structure The offset for the 0 The place of .
2. Structural members Start with the second member , The remaining members are always placed at an offset of an integer multiple of the alignment number .
Align numbers = The compiler's default alignment number is the smaller value of the variable's own size (VS The default number of alignments is 8,Linux There is no default alignment number )
3. The total size of the structure , It must be an integer multiple of the maximum number of alignments in each member of the structure
4. If nesting The situation of the structure , The nested structure is aligned to an integral multiple of its maximum alignment , The overall size of the structure is the maximum number of alignments ( The number of alignments with nested structures ) Integer multiple .

Meet the first point ：

Meet the second point ：


Meet the third step ：


In order to meet the third point , We need memory alignment , The total size of the structure must be 4 Multiple , Now only 12 It can meet the requirements .

But we have some Waste of space ah ！

So why have memory alignment ？

Most of the references say that ：

1. Platform reasons ( Reasons for transplantation )：
   Not all hardware platforms can access any data on any address ; Some hardware platforms can only access certain types of data at certain addresses , Otherwise, a hardware exception will be thrown .
2. Performance reasons ：
   data structure ( Especially stacks ) It should be aligned as far as possible on the natural boundary .
   The reason lies in , To access unaligned memory , The processor needs to make two memory accesses ; The aligned memory access only needs one access .

for example ： Some scenes can only be read at a time 4byte Space ： Compare alignment and misalignment , There may be errors when reading data .

On the whole ：
The memory alignment of the structure is Trade space for time How to do it .

How to optimize ？

  When designing structures , We have to satisfy the alignment , And save space , How to do it ？ That is to let Members with small space shall be gathered together as much as possible .

// for example ：
struct S1
{
    
	char c1;
	int i;
	char c2;
};
struct S2
{
    
	char c1;
	char c2;
	int i;
};

S1 and S2 The type of members as like as two peas , however S1 and S2 There are some differences in the amount of space taken up .

Change the default alignment number

use #pragma Change the default alignment number

#include <stdio.h>
#pragma pack(8)// Set the default alignment number to 8
struct S1
{
    
	char c1;
	int i;
	char c2;
};
#pragma pack()// Unset the default number of alignments , Restore to default 
#pragma pack(1)// Set the default alignment number to 1
struct S2
{
    
	char c1;
	int i;
	char c2;
};
#pragma pack()// Unset the default number of alignments , Restore to default 
int main()
{
    
	// What is the result of the output ？
	printf("%d\n", sizeof(struct S1));
	printf("%d\n", sizeof(struct S2));
	return 0;
}

Conclusion ：
When the structure is not aligned properly , We can change the default alignment number by ourselves .