歌词雨还在下滴滴答答:C 语言中 __packed
来源:百度文库 编辑:中财网 时间:2024/04/27 22:08:40
关键字: __packed ,C语言,内存对齐
【正文】
简单的说,__packed 用于表示C语言中结构的压缩,即:没有填充和对齐。
定义如下:
__packed struct P { ... };
struct P pp; /* pp is a packed struct */
__packed也可以定义为:
struct my_unpacked_struct { char c; int i; } __attribute__ ((__packed__));
如果一个结构体定义为 __packed,那么其子结构体也必须为__packed(如果有的话)例如:
struct S {...};
__packed struct P {...};
struct T {
struct S ss; /* OK */
struct P pp; /* OK */
};
__packed struct Q {
struct S ss; /* faulted - sub-structs must be packed */
struct P pp; /* OK */
};
__packed struct P { char c; int x; };
__packed struct {
struct P X;
char z;
struct P Y;
}Q;
此时,Q的内存映像如下:
<-------------------- Q -------------------->
<-------- X --------> <-------- Y -------->
+---+---+---+---+---+---+---+---+---+---+---+
byte | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10|
| c | ------x------ | z | c | ------x------ |
+---+---+---+---+---+---+---+---+---+---+---+
参考文献:
[1] http://www.iyonix.com/tools/packed.shtml
[2] http://axelio.wordpress.com/2007/07/24/be-careful-with-packed-structures/
[3] http://sig9.com/articles/gcc-packed-structures
[4] http://en.wikipedia.org/wiki/Da