Question

I want to print an integer number in binary mode using following function:

#include <stdio.h>

void print_binary(int n)
{
    int i = 0;

    for (i = sizeof(n)*8 - 1; i >= 0; i--)
    {
        printf("%d", ((n & ((1 << (i + 1)) - 1)) >> i) ? 1 : 0);
    }
}   

main.c

int main(int argc, char *argv[])
{
    printf("%d in binary:\n", atoi(argv[1]));
    print_binary(atoi(argv[1]));
    printf("\n");

    printf("%d in hex: 0x%x\n", atoi(argv[1]), atoi(argv[1]));

    return 0;
}   

if I passed -1 into it, the output is incorrect, what is wrong?

-1 in binary:
01111111111111111111111111111111
-1 in hex: 0xffffffff

What causes the most significant bit to become 0?

Answer 1

This expression 1 << (i + 1) is undefined behavior when i + 1 == 32 (i.e., >= CHAR_BIT * sizeof (int) in your platform.

(C99, 6.5.7p3) “If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined.”