GCC:__attribute__和#pragma的对齐设置 [英] GCC: Alignment settings with __attribute__ and #pragma
问题描述
如何将#pragma pack(2)
定义为结构属性?
How can I define #pragma pack(2)
as a structure attribute?
我已阅读此处,该__attribute__((packed,aligned(4)))
大致等于#pragma pack(4)
.
I've read here that __attribute__((packed,aligned(4)))
is roughly equivalent to #pragma pack(4)
.
但是,如果我尝试使用它(至少使用2而不是4),则会得到不同的结果.示例:
However, if I try to use this (at least with 2 instead of 4), I get different results. Example:
#include <stdio.h>
#pragma pack(push, 2)
struct test1 {
char a;
int b;
short c;
short d;
};
struct test1 t1;
#pragma pack(pop)
struct test2 {
char a;
int b;
short c;
short d;
} __attribute__((packed,aligned(2)));
struct test2 t2;
#define test(s,m) printf(#s"::"#m" @ 0x%04x\n", (unsigned int ((char*)&(s.m) - (char*)&(s)))
#define structtest(s) printf("sizeof("#s")=%lu\n", (unsigned long)(sizeof(s)))
int main(int argc, char **argv) {
structtest(t1);
test(t1,a);
test(t1,b);
test(t1,c);
test(t1,d);
structtest(t2);
test(t2,a);
test(t2,b);
test(t2,c);
test(t2,d);
}
输出为(在x86或x86x64上编译,Linux,gcc 4.8.4):
Output is (compilation on x86 or x86x64, Linux, gcc 4.8.4):
sizeof(t1)=10
t1::a @ 0x0000
t1::b @ 0x0002
t1::c @ 0x0006
t1::d @ 0x0008
sizeof(t2)=10
t2::a @ 0x0000
t2::b @ 0x0001
t2::c @ 0x0005
t2::d @ 0x0007
b,c和d成员的地址在两种情况下都不相同.
The adresses of members b, c and d are not the same in both cases.
我还要添加另一个__attribute__
吗?我什至在gcc文档中找不到关于这些属性的详尽文档.
Is there another __attribute__
I have to add? I can't even find a fine granular documentation on these attributes in the gcc documentation.
推荐答案
如@Nominal Animal在评论中所述,解决方案是将__attribute__((__aligned__(s)))
添加到每个struct成员,因为它可用于分别为每个struct设置对齐方式成员.
As noted by @Nominal Animal in the comments the solution is to add __attribute__((__aligned__(s)))
to each struct member as it can be used to set the alignment individually for each member.
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