深入解析C语言中结构体的内存对齐机制及其优化方法

为了让CPU能够更舒服地访问到变量&＃xff0c;struct中的各成员变量的存储地址有一套对齐的机制。这个机制概括起来有两点&＃xff1a;第一&＃xff0c;每个成员变量的首地址&＃xff0c;必须是它的类型的对齐值的整数倍&＃xff0c;如果不满足&＃xff0c;它与前一个成员变量之间要填充(padding)一些无意义的字节来满足&＃xff1b;第二&＃xff0c;整个struct的大小&＃xff0c;必须是该struct中所有成员的类型中对齐值最大者的整数倍&＃xff0c;如果不满足&＃xff0c;在最后一个成员后面填充。

各种类型的变量的align值如下&＃xff0c;参考的是wikipedia的页面&＃xff1a;Data structure alignment

The following typical alignments are valid for compilers from Microsoft, Borland, and GNUwhen compiling for 32-bit x86:

• A char (one byte) will be 1-byte aligned.
• A short (two bytes) will be 2-byte aligned.
• An int (four bytes) will be 4-byte aligned.
• A float (four bytes) will be 4-byte aligned.
• A double (eight bytes) will be 8-byte aligned on Windows and 4-byte aligned on Linux.
• A long double (twelve bytes) will be 4-byte aligned on Linux.
• Any pointer (four bytes) will be 4-byte aligned on Linux. (eg: char*, int*)

The only notable difference in alignment for a 64-bit linux system when compared to a 32 bit is:

• A double (eight bytes) will be 8-byte aligned.
• A long double (Sixteen bytes) will be 16-byte aligned.
• Any pointer (eight bytes) will be 8-byte aligned.

这里写了个程序来验证这些事&＃xff1a;

#include struct s {char a;short b;char c;double d;char e;
};int main() {struct s s1;printf("%d, %d, %d, %d, %d\n",(char*)(&s1.a) - (char*)(&s1),(char*)(&s1.b) - (char*)(&s1),(char*)(&s1.c) - (char*)(&s1),(char*)(&s1.d) - (char*)(&s1),(char*)(&s1.e) - (char*)(&s1));printf("%d\n", sizeof(struct s));return 0;
}

在64位linux下面运行这段代码的结果是&＃xff1a;