decimal to binary

binary.c

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
 
int digit(unsigned long n)
{
	if ( n == 0 ) return 1;
        int k = (int)(log2(n));
        return k+1;
}
 
int main(int argc,char *argv[])
{
        if ( argc < 2 ) {
                fprintf(stderr,"Usage : %s number\n",argv[0]);
                return -1;
        }
        unsigned long decimal = atol(argv[1]);
        printf("dec = %ld\n",decimal);
        int d = digit(decimal);
        int length = d + (d/4);
        printf("digit = %d\n",d);
        char *binary = malloc(length+1);
        binary[length] = 0;
        for ( int i = 0, s = 0; i < d; i++ ) {
		binary[length-i-1-s] = '0' + (decimal>>(i))%2;
		if ( (i+1)%4 == 0 ) {
			binary[length-i-1-(++s)] = ' ';
		}
        }
        printf("binary = %s\n",binary);
        return 0;
}