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;
}#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;
}packedpacked 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
__attribute__((packed))的作用,布布扣,bubuko.com
原文地址：http://blog.csdn.net/lmh12506/article/details/25641853
