C语言结构体对齐，超详细，超易懂

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结构体对齐规则

• 结构体的第一个成员总是存储在结构体变量开辟的空间的起始地址。
• 其它成员变量要存储在一个名叫对齐数的整数倍的地址。
  

  • 不同的编译器有着不同的对齐数，例如Visual Studio Code的编译器的对齐数是8，Linux的编译器的对齐数是4。
  • 结构体的对齐数是变量大小和编译器对齐数的比较的较小值，例如在Linux下char变量的对齐数是1，int变量的对齐数是4，char *变量的对齐数是4。

• 结构体总大小为最大对齐数的整数倍，结构体内每一个变量都会产生一个对齐数，取其最大的对齐数。
• 如果是嵌套结构体，那么嵌套的结构体存储在自己成员的最大对齐数的整数倍地址处，嵌套结构体大小为所有成员（包括嵌套的结构体的成员）的最大对齐数的整数倍。
  

结构体Tmp与结构体Cmd，结构体Msg的变量在内存的存放位置

 结构体Tmp，大小为12

结构体Cmd，大小为8

结构体Msg，大小为8 

示例

#include <stdio.h> #include <stdlib.h> #include <string.h> struct Tmp { char c1; int a; char c2; }; struct Cmd { char c1; char c2; int a; }; struct Msg { int a; char c1; char c2; }; int main(int argc,char *argv[]) { struct Tmp t; struct Cmd c; struct Msg m; memset(&t,0,sizeof(struct Tmp)); memset(&c,0,sizeof(struct Cmd)); memset(&m,0,sizeof(struct Msg)); printf("int size is %ld\n",sizeof(int)); //4 printf("char size is %ld\n",sizeof(char)); //1 printf("int* size is %ld\n",sizeof(int *)); //8 printf("char* size is %ld\n",sizeof(char *)); //8 printf("Tmp struct size is %ld\n",sizeof(struct Tmp)); //12 printf("Tmp struct variable size is %ld\n",sizeof(t)); //12 printf("Cmd struct size is %ld\n",sizeof(struct Cmd)); //8 printf("Cmd struct variable size is %ld\n",sizeof(c)); //8 printf("Msg struct size is %ld\n",sizeof(struct Msg)); //8 printf("Msg struct variable size is %ld\n",sizeof(m)); //8 return 0; }

嵌套结构体对齐

 嵌套结构体Msg1，大小为2​​4

 嵌套结构体Msg2，大小为16

#include <stdio.h> #include <stdlib.h> #include <string.h> struct Tmp // size = 16 { char tc1; int ta; char tc2; int tc3; }; struct Cmd // size = 8 { char cc1; char cc2; int ca; }; struct Msg1 { int a; char c1; char c2; struct Tmp t; }; struct Msg2 { int a; char c1; char c2; struct Cmd c; }; int main(int argc, char *argv[]) { struct Tmp t; struct Cmd c; struct Msg1 m1; struct Msg2 m2; memset(&t, 0, sizeof(struct Tmp)); memset(&c, 0, sizeof(struct Cmd)); memset(&m1, 0, sizeof(struct Msg1)); memset(&m2, 0, sizeof(struct Msg2)); printf("Tmp struct size is %ld\n", sizeof(struct Tmp)); // 16 printf("Cmd struct size is %ld\n", sizeof(struct Cmd)); // 8 printf("Msg1 struct size is %ld\n", sizeof(struct Msg1)); // 24 printf("Msg1 struct variable size is %ld\n", sizeof(m1)); // 24 printf("Msg2 struct size is %ld\n", sizeof(struct Msg2)); // 16 printf("Msg2 struct variable size is %ld\n", sizeof(m2)); // 16 return 0; }