How to inverse an integer in C?

Question

I am trying to inverse a hexadecimal value. The result is wrong, however.

#include <stdio.h>
#include <stdint.h>
#include <netinet/in.h>

int main(void)
{
    uint32_t acc = 0xBBD1;

    printf("0x%X", htons(~acc));    // prints 0x2E44 
}

Let's do the inversion by hand:

0xBBD1 = 1011 1011 1101 0001
~1011 1011 1101 0001 =
 0100 0100 0010 1110
0100 0100 0010 1110 = 0x442E

This means, the code should actually print 0x442E instead of 0x2E44.

What's wrong with my code?

@cad This is a significant edit you've done. IMO, it loses some of OP's original issue. — chux - Reinstate Monica, Aug 23 '15 at 19:52

score 2 · Accepted Answer · answered Aug 23 '15 at 13:40

2

Technically, no. But why are you using htons? It changes the endianess of a 16 bit datum to big-endian (that is, network byte order). First is your variable not 16 bit but 32 bit, so uint16_t acc = 0xBBD1 would be more appropriate. Second, as your on a little-endian machine, the bytes are exchanged, hence the output.

answered Aug 23 '15 at 13:40

cadaniluk

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Thanks, I didn't know about endian until now, I fix my code to take endian into account. – Adriansyah Aug 24 '15 at 06:25

score 0 · Answer 2 · answered Aug 23 '15 at 13:40

0

htons() reverses the byte order on x86 machines. So your result is correct, just bytes swapped. Have a look at endianness.

answered Aug 23 '15 at 13:40

PhilMasteG

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How to inverse an integer in C?

2 Answers2