如何转换结构，在C字符数组 [英] How to convert struct to char array in C

问题描述

我想一个结构转换为字符数组通过网络发送。但是，我从字符数组一些奇怪的输出，当我这样做。

I'm trying to convert a struct to a char array to send over the network. However, I get some weird output from the char array when I do.

#include <stdio.h>

struct x
{
   int x;
} __attribute__((packed));


int main()
{
   struct x a;
   a.x=127;
   char *b = (char *)&a;
   int i;
   for (i=0; i<4; i++)
      printf("%02x ", b[i]);
   printf("\n");
   for (i=0; i<4; i++)
      printf("%d ", b[i]);
   printf("\n");
   return 0;
}

下面是输出a.x的各种值（在x86上使用gcc）：结果
127：结果
7F 00 00 00结果
127 0 0 0结果

Here is the output for various values of a.x (on an X86 using gcc):
127:
7f 00 00 00
127 0 0 0

128：结果
ffffff80 00 00 00结果
-128 0 0 0结果

128:
ffffff80 00 00 00
-128 0 0 0

255：结果
FFFFFFFF 00 00 00结果
-1 0 0 0结果

255:
ffffffff 00 00 00
-1 0 0 0

256：结果
00 01 00 00结果
0 1 0 0结果

256:
00 01 00 00
0 1 0 0

我理解为127和256的值，但为什么数字将128当改变？为什么不只是为：
80 00 00 00
128 0 0 0

I understand the values for 127 and 256, but why do the numbers change when going to 128? Why wouldn't it just be: 80 00 00 00 128 0 0 0

我忘记做一些事情在转换过程中还是我忘了一些关于整数重新presentation？

Am I forgetting to do something in the conversion process or am I forgetting something about integer representation?

*注意：这只是一个小的测试程序。在实际的程序我有更多的结构，更好的变量名，我转化为little-endian的。结果
*编辑：格式

*Note: This is just a small test program. In a real program I have more in the struct, better variable names, and I convert to little-endian.
* formatting

推荐答案

的 X 本身的格式说明说，该参数是一个 INT ，并自号为负，的printf 需要8个字符显示 INT -sized值。在 0 修饰符告诉填充用零输出，而 2 修饰符说，的最小输出应该是两个字符。据我所知，的printf 不提供一种方式来指定的最大的宽度，除了字符串。​​

The x format specifier by itself says that the argument is an int, and since the number is negative, printf requires eight characters to show all four non-zero bytes of the int-sized value. The 0 modifier tells to pad the output with zeros, and the 2 modifier says that the minimum output should be two characters long. As far as I can tell, printf doesn't provide a way to specify a maximum width, except for strings.

现在那么，你只传递一个字符，所以裸 X 告诉使用全功能 INT 即得到通过，而不是 - 因为默认参数推广 ... 参数。尝试 HH 修饰符告诉函数参数视为只是一个字符而不是：

Now then, you're only passing a char, so bare x tells the function to use the full int that got passed instead — due to default argument promotion for "..." parameters. Try the hh modifier to tell the function to treat the argument as just a char instead:

printf("%02hhx", b[i]);

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