__attribute__ 问题

测试程序
#include <stdio.h>
struct A{
        char a;
        int b;
        unsigned short c;
        long d;
        unsigned long long e;
        char f;
};

struct B{
        char a;
        int b;
        unsigned short c;
        long d;
        unsigned long long e;
        char f;
}__attribute__((aligned));

struct C{
        char a;
        int b;
        unsigned short c;
        long d;
        unsigned long long e;
        char f;
}__attribute__((aligned(1)));


struct D{
        char a;
        int b;
        unsigned short c;
        long d;
        unsigned long long e;
        char f;
}__attribute__((aligned(4)));

struct E{
        char a;
        int b;
        unsigned short c;
        long d;
        unsigned long long e;
        char f;
}__attribute__((aligned(8)));

struct F{
        char a;
        int b;
        unsigned short c;
        long d;
        unsigned long long e;
        char f;
}__attribute__((packed));

int main(int argc, char **argv)
{
        printf("A = %d, B = %d, C = %d, D = %d, E = %d, F = %d\n",
                sizeof(struct A), sizeof(struct B), sizeof(struct C), sizeof(struct D), sizeof(struct E), sizeof(struct F));
        return 0;
}
在fedora 7下的测试结果：
A = 28, B = 32, C = 28, D = 28, E = 32, F = 20
A：不使用__attribute__ 默认4字节对齐
B：__attribute__((aligned))
      the compiler automatically sets the alignment for the declared variable or field to the largest alignment which is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables or fields that you have aligned this way.
   最大对齐方式，此例中为16字节对齐，同E
C:__attribute__((aligned(1))) 不支持，除了packed 不能减小对齐字节数，以默认对齐方式对齐
D:__attribute__((aligned(4))) 四字节对齐
E：__attribute__((aligned(8))) 八字节对齐
F：__attribute__((packed))
     the aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well.
     The packed attribute specifies that a variable or structure field should have the smallest possible alignment―one byte for a variable, and one bit for a field, unless you specify a larger value with the aligned attribute.

Here is a structure in which the field x is packed, so that it immediately follows a:

          struct foo
          {
            char a;
            int x[2] __attribute__ ((packed));
          };
   变量以字节对齐，结构体域以位对齐   

cygwin下的测试结果：
A = 32, B = 32, C = 32, D = 32, E = 32, F = 20
从测试结果上看默认8字节对齐？或是只支持packed,未知
可参考的文档：
http://developer.apple.com/documentation/DeveloperTools/gcc-3.3/gcc/Variable-Attributes.html
http://www.skynet.org.cn/archiver/?tid-87.html