test.c

#include <stdio.h>
#include <stdlib.h>

typedef unsigned char byte;
typedef char* string;

string dec2bin(byte x)
{
    int i;
    string bin = (string)malloc(sizeof(char)*9);
    bin[8] = '\0';

    for (i = 7; i >= 0; i--) {
        bin[7-i] = (x&(1<<i))?'1':'0';
    }

    return bin;
}

int main (int argc, char** argv) {
    byte num;
    string bin;
    char buf[4];

    printf("Enter a number: ");

    fgets(buf, 4, stdin);
    num = atoi(buf);
    bin = dec2bin(num);

    printf("The number you entered was: %i\n", num);
    printf("Binary: %s\n", bin);
    
    free(bin);
    return 0;
}