Print an int in binary representation using C

I'm looking for a function to allow me to print the binary representation of an int. What I have so far is;

char *int2bin(int a)
{
 char *str,*tmp;
 int cnt = 31;
 str = (char *) malloc(33); /*32 + 1 , because its a 32 bit bin number*/
 tmp = str;
 while ( cnt > -1 ){
      str[cnt]= '0';
      cnt --;
 }
 cnt = 31;
 while (a > 0){
       if (a%2==1){
           str[cnt] = '1';
        }
      cnt--;
        a = a/2 ;
 }
 return tmp;

}

But when I call

printf("a %s",int2bin(aMask)) // aMask = 0xFF000000

I get output like;

0000000000000000000000000000000000xtpYy (And a bunch of unknown characters.

Is it a flaw in the function or am I printing the address of the character array or something? Sorry, I just can't see where I'm going wrong.

NB The code is from here

EDIT: It's not homework FYI, I'm trying to debug someone else's image manipulation routines in an unfamiliar language. If however it's been tagged as homework because it's an elementary concept then fair play.

Here's another option that is more optimized where you pass in your allocated buffer. Make sure it's the correct size.

// buffer must have length >= sizeof(int) + 1
// Write to the buffer backwards so that the binary representation
// is in the correct order i.e.  the LSB is on the far right
// instead of the far left of the printed string
char *int2bin(int a, char *buffer, int buf_size) {
    buffer += (buf_size - 1);

    for (int i = 31; i >= 0; i--) {
        *buffer-- = (a & 1) + '0';

        a >>= 1;
    }

    return buffer;
}

#define BUF_SIZE 33

int main() {
    char buffer[BUF_SIZE];
    buffer[BUF_SIZE - 1] = '\0';

    int2bin(0xFF000000, buffer, BUF_SIZE - 1);

    printf("a = %s", buffer);
}

A few suggestions:

null-terminate your string
don't use magic numbers
check the return value of malloc()
don't cast the return value of malloc()
use binary operations instead of arithmetic ones as you're interested in the binary representation
there's no need for looping twice

Here's the code:

#include <stdlib.h>
#include <limits.h>

char * int2bin(int i)
{
    size_t bits = sizeof(int) * CHAR_BIT;

    char * str = malloc(bits + 1);
    if(!str) return NULL;
    str[bits] = 0;

    // type punning because signed shift is implementation-defined
    unsigned u = *(unsigned *)&i;
    for(; bits--; u >>= 1)
        str[bits] = u & 1 ? '1' : '0';

    return str;
}

Your string isn't null-terminated. Make sure you add a '\0' character at the end of the string; or, you could allocate it with calloc instead of malloc, which will zero the memory that is returned to you.

By the way, there are other problems with this code:

As used, it allocates memory when you call it, leaving the caller responsible for free()ing the allocated string. You'll leak memory if you just call it in a printf call.
It makes two passes over the number, which is unnecessary. You can do everything in one loop.

Here's an alternative implementation you could use.

#include <stdlib.h>
#include <limits.h>

char *int2bin(unsigned n, char *buf)
{
    #define BITS (sizeof(n) * CHAR_BIT)

    static char static_buf[BITS + 1];
    int i;

    if (buf == NULL)
        buf = static_buf;

    for (i = BITS - 1; i >= 0; --i) {
        buf[i] = (n & 1) ? '1' : '0';
        n >>= 1;
    }

    buf[BITS] = '\0';
    return buf;

    #undef BITS
}

Usage:

printf("%s\n", int2bin(0xFF00000000, NULL));

The second parameter is a pointer to a buffer you want to store the result string in. If you don't have a buffer you can pass NULL and int2bin will write to a static buffer and return that to you. The advantage of this over the original implementation is that the caller doesn't have to worry about free()ing the string that gets returned.

A downside is that there's only one static buffer so subsequent calls will overwrite the results from previous calls. You couldn't save the results from multiple calls for later use. Also, it is not threadsafe, meaning if you call the function this way from different threads they could clobber each other's strings. If that's a possibility you'll need to pass in your own buffer instead of passing NULL, like so:

char str[33];
int2bin(0xDEADBEEF, str);
puts(str);

Print an int in binary representation using C

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