linux i386系统中struct 的数据对齐(From: todaygood.cublog.cn)-优快云博客

本文探讨了Linux i386系统中struct类型的内存对齐方式，并通过实例展示了gcc编译器如何进行对齐处理。通过对不同数据类型结构体的分析，明确了该平台的最大对齐单位为4字节。

linux i386系统中struct 的数据对齐

在linux 系统中的应用程序，struct 并非是采用自然对齐，而是gcc强制对齐

何为自然对齐：在数据项大小的整数倍（例如：8字节数据项存入8的整数倍的地址）的地址处存储
                           数据项。

#include <stdio.h>
#include <sys/utsname.h>
#include <linux/types.h>

/*
* Define several data structures, all of them start with a lone char
* in order to present an unaligned offset for the next field
*/
struct c   {char c; char      t;} c;
struct s   {char c; short     t;} s;
struct i   {char c; int       t;} i;
struct l   {char c; long      t;} l;
struct ll {char c; long long t;} ll;
struct p   {char c; void *    t;} p;
struct u1b {char c; __u8      t;} u1b;
struct u2b {char c; __u16     t;} u2b;
struct u4b {char c; __u32     t;} u4b;
struct u8b {char c; __u64     t;} u8b;

int main(int argc, char **argv)
{
    struct utsname name;

    uname(&name); /* never fails :) */
    printf("arch Align: char short int long   ptr long-long "
       " u8 u16 u32 u64/n");
    printf(       "%-12s %3i   %3i   %3i   %3i   %3i   %3i      "
       "%3i %3i %3i %3i/n",
       name.machine,
       /* note that gcc can subtract void * values, but it's not ansi */
       (int)((void *)(&c.t)   - (void *)&c),
       (int)((void *)(&s.t)   - (void *)&s),
       (int)((void *)(&i.t)   - (void *)&i),
       (int)((void *)(&l.t)   - (void *)&l),
       (int)((void *)(&p.t)   - (void *)&p),
       (int)((void *)(&ll.t) - (void *)&ll),
       (int)((void *)(&u1b.t) - (void *)&u1b),
       (int)((void *)(&u2b.t) - (void *)&u2b),
       (int)((void *)(&u4b.t) - (void *)&u4b),
       (int)((void *)(&u8b.t) - (void *)&u8b));
    return 0;
}

[root@hujunlinux driver_pratice]# ./dataalign
arch Align: char short int long   ptr long-long u8 u16 u32 u64
i686              1     2    4     4     4     4       1   2    4      4

可见最大的对齐单位是4

int main()
{
        struct A{ char c;
                   long long b;
                }one;

        printf("sizeof(one)=%d,sizeof(c)=%d/n",sizeof(one),
        (int)((void*)(&(one.b))-(void*)(&one))
        );


        return 0;

}
./charSize
sizeof(one)=12,sizeof(c)=4

int main()
{
        struct A{ char c;
                 short b;
                }one;

        printf("sizeof(one)=%d,sizeof(c)=%d/n",sizeof(one),
        (int)((void*)(&(one.b))-(void*)(&one))
        );


        return 0;

}

./charSize
sizeof(one)=4,sizeof(c)=2

int main()
{
        struct A{ char c;
                   int b;
                }one;

        printf("sizeof(one)=%d,sizeof(c)=%d/n",sizeof(one),
        (int)((void*)(&(one.b))-(void*)(&one))
        );


        return 0;

}
./charSize
sizeof(one)=8,sizeof(c)=4