C language: Structure -- a detailed explanation of memory byte alignment

Preface ：

We're learning c Language memory management is always amazing , Its smart memory management strategy , Complain over and over about its incomprehensible methods , Online information should not be too detailed , Or I didn't explain it clearly , Today, let's learn based on 32 Bit operating system Of course C Language ： Memory byte alignment

tip: stay 32 In bit compile mode , Default to 4 Byte alignment ; stay 64 In bit compile mode , Default to 8 Byte alignment .

Catalog

1.  What is alignment ？

2.  Why should computers be aligned ？

 3. Let's go straight to the code

Case a

  Case 2

Alignment rule  

A term is used to explain ：

There are two rules for alignment ：

---

1.  What is alignment ？

　　 In modern computers, memory space is in bytes （byte） The division of , Theoretically, it seems that access to any type of variable can start at any address , But the actual situation is that when you access a specific variable, you often access it at a specific memory address , This requires All types of data are arranged spatially according to certain rules , and Instead of discharging sequentially one by one , This is alignment .

2.  Why should computers be aligned ？

　　 Each hardware platform has a big difference in the processing of storage space . Some platforms can only access certain types of data from certain addresses , Other platforms may not have this situation . But if the data storage is not aligned according to the requirements suitable for its platform , Loss of access efficiency .

For example, one int Data of type , If the address is 8, So it's easy , The number is 8 Memory addressing can be done once . If the number is 10, It's more troublesome ,CPU It is necessary to number 8 Memory addressing , Read 4 Bytes , Get the first half of the data , Then number it as 12 Memory addressing , Read 4 Bytes , Get the second half of the data , Then join the two parts together , To get the value of the data .

Try to put a data in one step , Avoid storing across steps , This is called memory alignment . stay 32 In bit compile mode , Default to 4 Byte alignment ; stay 64 In bit compile mode , Default to 8 Byte alignment .

 3. Let's go straight to the code

Case a

#include <stdio.h>

void main(){

    struct A{
        char a;
        short b;
        int c;
    };

    printf( "size of struct A = %d \n", sizeof(struct A) );

}

The output is ：8 byte .

Let's look at the picture

First step ：

First, the self alignment value is the largest ( That is to say int type ) yes 4  32 The alignment value of bit operating system is also 4, So the effective alignment value is 4

The second step ：

The starting address of the storage member must be an integer multiple of the effective alignment value of the member

a yes char Type data , Occupy 1 Byte memory ;short Type data , Occupy 2 Byte memory ;int Type data , Occupy 4 Byte memory . therefore , Structure A The self alignment value of is 4. therefore ,a and b Make up 4 Bytes , So as to meet with c Of 4 Byte alignment . and a Only 1 Bytes ,a And b A byte is empty between . We know , The structure type data is stored in order, and the structure is arranged backward one by one

  Case 2

#include <stdio.h>

void main(){

    struct A{
        short b;
        int c;
        char a;
    };

    printf( "size of struct A = %d \n", sizeof(struct A) );

}

The output is ：12 byte .

Let's look at the picture

First step ：

The self alignment value is the largest ( That is to say int type ) yes 4  32 The alignment value of bit operating system is also 4, So the effective alignment value is 4

The second step ：

The starting address of the storage member must be an integer multiple of the effective alignment value of the member

b yes short Type data , Occupy 2 Byte memory ;chart Type data , Occupy 1 Byte memory ;int Type data , Occupy 4 Byte memory . therefore , Structure A The self alignment value of is 4. therefore ,b One data account for 4 byte ( It was originally two bytes ),c One data account for 4 byte ,a One data account for 4 byte （ Originally a byte ）

Analogy like this （ Aligned appearance ）

Alignment rule  

A term is used to explain ：

Here we introduce four concepts

The alignment value of the data type itself ,

Specify the alignment value ,

The self alignment value of a structure or class ,

Data member 、 Valid alignment values for structs and classes

　　1） The alignment value of the data type itself ： Is the self alignment value of the basic data type , such as char The self alignment value of type is 1 byte ,int The self alignment value of type is 4 byte .

　　2） Specify the alignment value ： Precompiled orders #pragma pack （value） Specified alignment value value.

　　3） The self alignment value of a structure or class ： The value with the largest self alignment value among its members , Like the above struct A The alignment value for is 4.

　　4） Data member 、 Valid alignment values for structs and classes ： The smaller of the self alignment value and the specified alignment value .

There are two rules for alignment ：

1、 First   The self alignment value of the structure and Alignment value of the operating system comparison , take Small the Is the valid alignment value

Like this one down here   The self alignment value of the structure is 4  The alignment value of the operating system is also 4, So the effective alignment value is 4   

#include <stdio.h>

void main(){

    struct A{
        short b;
        int c;
        char a;
    };

    printf( "size of struct A = %d \n", sizeof(struct A) );

}

2、 The starting address of the storage member must be an integer multiple of the effective alignment value of the member .

Generally speaking, it's Each data type and Compare the valid alignment values                  Take the larger one Namely The last valid alignment value

#include <stdio.h>

void main(){

    struct A{
        short b;
        int c;
        char a;
    };

    printf( "size of struct A = %d \n", sizeof(struct A) );

}

Or the code above ,b by short type （1 byte ） And valid alignment values （4 byte ） The larger value is 4 byte