Question

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

Answer 1

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.

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]);