对于__u8的显示

#include <asm/types.h>
#include <stdio.h>
#include <sstream>
#include <string>
using namespace std;

typedef __s8        Int8;
typedef __u8        Uint8;
typedef __s16       Int16;
typedef __u16       Uint16;
typedef __s32       Int32;
typedef __u32       Uint32;
typedef __s64       Int64;
typedef __u64       Uint64;

typedef struct S1
{
    Uint8 a;
    Int8  a2;
    Int32 b;
    Uint16 c;
}S1;

#pragma pack(1)
typedef struct S2
{
    Uint8 a;
    Int8 a2;
    Int32 b;
    Uint16 c;
}S2;
#pragma pack()

int main(int argc, char** argv)
{
    S1 data1;
    memset(&data1,0,sizeof(data1));
    S2 data2;
    memset(&data2,0,sizeof(data2));
    printf("sizeof(S1):%d\n",sizeof(S1));
    printf("sizeof(S2):%d\n",sizeof(S2));

    ostringstream oss1,oss2,oss3,oss4;
    oss1 << (int)data1.a << data1.a2 << data1.b << data1.c;
    oss2 << data2.a << data2.a2 << data2.b << data2.c;
    oss3 << (int)data1.a << (int)data1.a2 << data1.b << data1.c;
    oss4 << data2.a << (int)data2.a2 << data2.b << data2.c;
    printf("%s\n",oss1.str().c_str());
    printf("%s\n",oss2.str().c_str());
    printf("%s\n",oss3.str().c_str());
    printf("%s\n",oss4.str().c_str());

    return 0;
}

输出结果：

sizeof(S1):12
sizeof(S2):8
0

0000

gdb调试：

(gdb) p data1
$1 = {a = 0 '\000', a2 = 0 '\000', b = 0, c = 0}
(gdb) p data2
$2 = {a = 0 '\000', a2 = 0 '\000', b = 0, c = 0}

结构体的成员变量a，虽然我们可以把它当作整型使用，但是ostringstream应该是当作字符处理了，ascii码表的值为0的元素实际上就是空。
gdb调试里，显示为a = 0 '\000'，可能是系统对于a做了两种显示而已。