c bitfields strange behaviour with long int in struct

Question

i am observing strange behaviour when i run the following code. i create a bitfield by using a struct, where i want to use 52 bits, so i use long int. The size of long int is 64 bits on my system, i check it inside the code. Somehow when i try to set one bit, it alwas sets two bits. one of them is the one i wanted to set and the second one is the index of the first one plus 32. Cann anybody tell me, why is that?

#include <stdio.h>

typedef struct foo {
  long int x:52;
  long int:12;
};

int main(){
  struct foo test;
  int index=0;
  printf("%ld\n",sizeof(test));
  while(index<64){
    if(test.x & (1<<index))
      printf("%i\n",index);
    index++;
  }
  test.x=1;
  index=0;
  while(index<64){
    if(test.x & (1<<index))
      printf("%i\n",index);
    index++;
  }
  return 0; 
}

Sry forgot to post the output, so my question was basicly not understandable... The Output it gives me is the following:

8

0

32

The code you've posted has undefined behavior. (You access `test.x` before initializing it.) So anything can happen. — James Kanze, Jul 01 '13 at 19:04

Keith Thompson · Accepted Answer · 2013-07-02T00:32:55.163

index is of type int, which is probably 32 bits on your system. Shifting a value by an amount greater than or equal to the number of bits in its type has undefined behavior.

Change index to unsigned long (bit-shifting signed types is ill-advised). Or you can change 1<<index to 1L << index, or even 1LL << index.

As others have pointed out, test is uninitialized. You can initialize it to all zeros like this:

 struct foo test = { 0 };

The correct printf format for size_t is %zu, not %ld.

And it wouldn't be a bad idea to modify your code so it doesn't depend on the non-portable assumption that long is 64 bits. It can be as narrow as 32 bits. Consider using the uint_N_t types defined in <stdint.h>.

I should also mention that bit fields of types other than int, unsigned int, signed int, and _Bool (or bool) are implementation-defined.

Thanks alot, this respond was realy informative and solved my problem entirely, i will also use your advisis! — zabeltech, Jul 01 '13 at 19:31

score 1 · Answer 2 · answered Jul 01 '13 at 19:03

1

You have undefined behavior in your code, as you check the bits in text.x without initializing the structure. Because you don't initialize the variable, it will contain random data.

answered Jul 01 '13 at 19:03

Some programmer dude

400,186
35
402
621

True, but that's probably not the cause of the symptoms he's seeing. – Keith Thompson Jul 01 '13 at 19:12
@KeithThompson Well, it's undefined behavior, so there's really no way to tell... ;) – Some programmer dude Jul 01 '13 at 19:13
1

True in principle, but if we assume that `test` happens to be all zero bits initially, and that `int` is 32 bits, then the *other* undefined behavior of `1 << index` explains the behavior he's seeing. – Keith Thompson Jul 01 '13 at 19:15

c bitfields strange behaviour with long int in struct

2 Answers2