C code not output the right number for long long int

Question

I'm trying to get the size limits of long long int with the following code:

int main()
{
    long long int from, to;
    printf("signed long long int: ");
    bytes=sizeof(long long int);
    bits = 8* bytes;
    from = -(1 << (bits-1));
    to =  (1 << (bits-1)) - 1;
    printf(" %d bytes from %lld to %lld\n", bytes, from, to);
}

the output is:

signed long long int:  8 bytes from -2147483648 to 2147483647

I expect it to be -9223372036854775808 to 9223372036854775807. What's wrong with the code above?

Answer 1

What's wrong with the code above?

1 << (bits-1) is shifting the integer constant of type int outside its range and leads to integer overflow and undefined behavior.

You may want to consider shifting a long long rather than an int.

1LL << (bits-1)

Yet this too is integer overflow and undefined behavior as it shifts the 1 outside the long long range.

The result of E1 << E2 is E1 left-shifted E2 bit positions; ... If E1 has a signed type and nonnegative value, and E1 × 2^E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined. C11dr §6.5.7 4

---

To portable find the range of long long, use LLONG_MIN, LLONG_MAX from <limit.h>.

#include <limit.h>

printf("%d bytes from %lld to %lld\n", bytes, LLONG_MIN, LLONG_MAX);

---

If long long lacks padding, (something very very common, by not required by C), the below will print the max value of long long.

#include <limit.h>
int main(void) {
  long long max;
  int bytes = sizeof(long long);
  int bits = CHAR_BIT * bytes;
  long long max_halfish = 1LL << (bits-2);
  long long max = (max_halfish - 1) + max_halfish;
  printf("max %lld\n", max);
}

min is simply (-max - 1) if we assume the very common 2's complement coding.

Answer 2

You need to coerce the 1 before shifting, otherwise is it treated as an int constant:

#include <stdio.h>

int main()
{
    long long int from, to;
    printf("signed long long int: ");

    int bytes = sizeof(long long int);
    int bits = 8 * bytes;

    from = -(((long long)1) << (bits-1));
    to =  (((long long)1) << (bits-1)) - 1;

    printf(" %d bytes from %lld to %lld\n", bytes, from, to);
}

Or use the more readable 1LL as @phuclv pointed out.

Output:

signed long long int:  8 bytes from -9223372036854775808 to 9223372036854775807

Answer 3

1 is an int literal, therefore 1 << (bits-1) is done in int type, which invokes undefined behavior since you're shifting more than the type's width. Use 1LL instead for a long long value

Answer 4

1 is an integer constant, change to 1LL to get the result in long long int.

int main() {

  long long int from, to;
  int bytes,bits;

  printf("signed long long int: ");
  bytes=sizeof(long long int);
  bits = 8* bytes;
  from = -(1LL << (bits-1));
  to =  (1LL << (bits-1)) - 1;
  printf(" %d bytes from %lld to %lld\n", bytes, from, to);
}