__attribute__((packed))的作用

__attribute__((packed))的作用  

在结构体变量的声明中，经常可以看到__attribute__((packed))修饰符。这是做什么用的呢？请看一下程序：

#define u8 unsigned char
#define u16 unsigned short
#define u32 unsigned int

int main()
{
    struct {
        u16 reg;
        u32 test2;
        u8 test1;
        u8 val[256];
    } msg = {                                                                                                                                  
        .reg = 0x8001,
        .test1 = 0xff,
        .test2 = 0x71727374,
        .val = {0x11, 0x12, 0x13, 0x14},
    };
    u8* ptr = (u8*) &msg;
    int i;
    for (i=0; i<0x10; i++)
        printf("%02x ", ptr[i]);
    return 0;
}

程序的输出为：

01 80 00 00 74 73 72 71 ff 11 12 13 14 00 00 00

如果在msg前加上__attribute__((packed)) ，程序变为：

#define u8 unsigned char
#define u16 unsigned short
#define u32 unsigned int

int main()
{
    struct {
        u16 reg;
        u32 test2;
        u8 test1;
        u8 val[256];
    } __attribute__((packed)) msg = {                                                                                                                                  
        .reg = 0x8001,
        .test1 = 0xff,
        .test2 = 0x71727374,
        .val = {0x11, 0x12, 0x13, 0x14},
    };
    u8* ptr = (u8*) &msg;
    int i;
    for (i=0; i<0x10; i++)
        printf("%02x ", ptr[i]);
    return 0;
}

程序的输出为：

01 80 74 73 72 71 ff 11 12 13 14 00 00 00 00 00

因此可以看到，packed属性修改了编译器对结构体成员的布局，尽可能压缩存储空间。默认情况下，gcc会为了效率考量，会让char或者short独占一个双字（4字节）。

gcc手册的说明：

packed

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));           };

有时间也看看这个：

http://stackoverflow.com/questions/8568432/is-gccs-attribute-packed-pragma-pack-unsafe