CODEWITHSUNDEEP
ctypesbit-shiftbitmask
rhlee
rhlee

Reputation: 4046

Bitshifting chars in c

I'm trying to get the bytes from an int into a series of chars in a portable way across all little endian systems.

I have the following code:

#include <stdio.h>

int
main()
{
  int i = 0xabcdef12;
  printf("i: %x\n", i);
  char a, b, c, d;
  a = (i >> 000) & 0xFF;
  b = (i >> 010) & 0xFF;
  c = (i >> 020) & 0xFF;
  d = (i >> 030) & 0xFF;
  printf("a b c d: %x %x %x %x\n", a , b, c, d);
  if(a == 0x12)
    printf("a is 0x12\n");
  if(b == 0xef)
    printf("b is 0xef\n");
  if(c == 0xcd)
    printf("c is 0xcd\n");
  if(d == 0xab)
    printf("d is 0xab\n");
  if(a == 0xffffff12)
    printf("a is 0x12\n");
  if(b == 0xffffffef)
    printf("b is 0xffffffef\n");
  if(c == 0xffffffcd)
    printf("c is 0xffffffcd\n");
  if(d == 0xffffffab)
    printf("d is 0xffffffab\n");
  return 0;
}

This piece of code compiles without any warning when using -Wall.

When run this gives:

i: abcdef12
a b c d: 12 ffffffef ffffffcd ffffffab
a is 0x12
b is 0xffffffef
c is 0xffffffcd
d is 0xffffffab

Here are some gcc prints:

Breakpoint 1, main () at test.c:14
14    if(a == 0x12)
(gdb) p/x a
$1 = 0x12
(gdb) p/x b
$2 = 0xef
(gdb) p/x c
$3 = 0xcd
(gdb) p/x d
$4 = 0xab

I'm pretty sure I'm doing something wrong. It would really help me understand what is going on if you could answer a few of the following questions:

And if anybody has a reliable (system independent, but endianess is not important) way of going from int to char[], that would be great.

Upvotes: 4

Views: 3024

Answers (2)

timrau
timrau

Reputation: 23058

During comparison, a, b, c and d are promoted as int. Since they are all char, the most significant bit is treated as sign bit. The promotion to int fills MSB to higher bits.

0x12's MSB is 0, and 0xef, 0xcd and 0xab's MSB is 1. That's why after promotion you get 0x00000012 but 0xffffffef, 0xffffffcd and 0xffffffab.

If you change

char a, b, c, d;

into

unsigned char a, b, c, d;

Then you can get what you expected.

Upvotes: 1

Mysticial
Mysticial

Reputation: 471559

Here's a quick fix.

Change:

char a, b, c, d;

to

unsigned char a, b, c, d;

The reason is that char is signed on your system. When you pass, a, b, c, d into printf(), they get promoted to int. They are sign-extended. That's why you get all those leading ffs.

GDB is reporting the correct values because it is reading the chars directly. (and thus no integer promotion)

Upvotes: 5

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