print binary representation of a number

Question

I want to print the binary representation of an int. My solution seems to work for both int and unsigned int in Visual Studio, but somebody told me it's wrong. Does anyone see an error? If so, why does my program seem to work for me?

void printbin(int n)
{
    unsigned int i = 1<<31;

    for (int j=0; j<32; j++)
    {
        if ((n & i) != 0)
            printf("1");
        else
            printf("0");
        i = i>>1;
    }

    printf("\n");
}

Answer 1

1<<31 shifts a bit passed the value bits and potentially into a sign (or padding) bit. This is undefined behavior in C.

n & i is attempting to "and" bits of an unsigned int and the sign of a signed int.

OP's use of 32 assumes an int is 32-bits wide.

Following is an example the prints the sign and the variable number of bits - works [INT_MIN...INT_MAX].

#include <limits.h>
void printbin_c(int n) {
  char buf[CHAR_BIT * sizeof n + 1];
  char *p = &buf[sizeof buf - 1];
  *p = '\0';

  int i = n;
  if (i > 0) {
    i = -i;
  }

  do {
    p--;
    *p = '0' - i%2;
    i /= 2;
  } while (i);

  if (n < 0) {
    p--;
    *p = '-';
  }

  puts(p);
}

[Edit] Cope with 1's complement @John Bollinger

Using the negative absolute value with if (i > 0) i = -i; as the positive absolute value does not work well with INT_MIN 2's complement.