一般情况下C struct结构体内存分配

c语言中struct的结构体的内存分配解析测试环境:4.10.0-38-generic #42~16.04.1-Ubuntu SMP Tue Oct 10 16:32:20 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.11)

代码如下

#include<stdio.h>

#include<string.h>

struct example1

{

int a;

int b;

char c;

char d;

double e;

};

struct example2

{

int a;

char c;

int b;

double e;

char d;

};

int main()

{

int i;

struct example1 ex1 = {1, 2, 'a', 'b', 99.9};

struct example2 ex2 = {1, 'a', 2, 'b', 99.9};

printf("size of structure ex1 in bytes %d\n", sizeof(ex1));

printf("size of structure ex2 in bytes %d\n", sizeof(ex2));

printf("Address of ex1.a = %u\n", &ex1.a);

printf("Address of ex1.b = %u\n", &ex1.b);

printf("Address of ex1.c= %u\n", &ex1.c);

printf("Address of ex1.d = %u\n", &ex1.d);

printf("Address of ex1.e = %u\n",&ex1.e);

printf("Address of ex2.a = %u\n", &ex2.a);

printf("Address of ex2.c = %u\n", &ex2.c);

printf("Address of ex2.b= %u\n", &ex2.b);

printf("Address of ex2.e = %u\n", &ex2.e);

printf("Address of ex2.d = %u\n",&ex2.d);

return 0;

}

output:

size of structure ex1 in bytes 24

size of structure ex2 in bytes 32

Address of ex1.a = 1108104576

Address of ex1.b = 1108104580

Address of ex1.c= 1108104584

Address of ex1.d = 1108104585

Address of ex1.e = 1108104592

Address of ex2.a = 1108104608

Address of ex2.c = 1108104612

Address of ex2.b= 1108104616

Address of ex2.e = 1108104624

Address of ex2.d = 1108104632

struct example1 { int a; int b; char c; char d; double e; };

int a； 4字节 1108104576

int b： 4字节 1108104580

char c：1字节 1108104584

char d：1字节 1108104585

double e：8字节 1108104592 这里double类型是8字节类型，如果e直接放入到d后面保存，那么地址e必定不能以8字节对齐，所以char d后面将会填充6个

数据 byte达到8字节对齐的目的。

编译器按照成员列表的顺序一个接一个的地给每个成员分配内存。只有当存储成员需要满足正确的边界对齐要求时，成员间才需要填充内存空间。

分析结构体example2

int a; 4个字节

char c 1个字节，因为下个元素是int型，要季节对齐，所以填充3个byte空数据。

int b 4个字节，下个元素double类型，8字节对齐，这里填充4个byte空数据。

double e 8个字节

char d 1个字节，以结构体中最大类型字节对齐，这里结构体是最大字节类型double类型，所以需要8字节对齐。所以这里要填充7个byte空数据。

所以结构体example2占用内存为 4+4+8+8+8 = 32 byte。

来源：oschina

链接：https://my.oschina.net/u/2614497/blog/3161685

标签

CStruct