此属性可以强制修改gcc的对齐方式
一般软件的2进制协议中,会定义一系列的规范,32位机一般会定义4字节对齐的协议,这样对于32位机来说速度是最快的。
最近发现gcc一个问题,在一个结构体包含unsignd long long (64位)类型时会导致结构体8字节对齐,且__attribute__ ((aligned(4)))
竟然无效
本例子用的x86 编译器版本
#gcc --version gcc (GCC) 4.1.3 20080704 (prerelease) (Debian 4.1.2-25) Copyright (C) 2006 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
# arm-linux-gcc --version arm-linux-gcc ((gcc4.4-290+uclibc_0.9.32.1+eabi+linuxpthread)) 4.4.1 Copyright (C) 2009 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
直接上例子程序
#include <stdio.h> typedef unsigned long long ullong; typedef struct { char a; ullong b; int c; /*} __attribute__ ((packed)) test ;*/ /*} __attribute__ ((aligned (4))) test ;*/ } test; int main ( int argc, char *argv[] ) { printf("file:%s, line:%d, sizeof(test):%d\n", __FILE__, __LINE__, sizeof(test)); return 0; }
x86 32运行结果
file:test.c, line:15, sizeof(test):16
arm contex A9 运行结果
file:test.c, line:15, sizeof(test):24
结果说明在arm编译器成了8字节对齐 x86编译器还是4字节对齐
为了让该结构体4字节对齐,我们采用
__attribute__ ((aligned (4)))
强制对齐
输出结果还是
file:test.c, line:15, sizeof(test):24
无奈使用单字节对齐
__attribute__ ((aligned (4))) test
输出结果为
file:test.c, line:15, sizeof(test):13
这样虽然解决了我们4字节对齐的问题,但是这样 我们得严格注意结构体对齐的问题。