How to convert struct to char array in C
- by falcojr
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;
}
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:
ffffff80 00 00 00
-128 0 0 0
255:
ffffffff 00 00 00
-1 0 0 0
256:
00 01 00 00
0 1 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
Am I forgetting to do something in the conversion process or am I forgetting something about integer representation?
*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.
*Edit: formatting
