c - What is the proper way of implementing a good "itoa()" function?

Question

I was wondering if my implementation of an "itoa" function is correct. Maybe you can help me getting it a bit more "correct", I'm pretty sure I'm missing something. (Maybe there is already a library doing the conversion the way I want it to do, but... couldn't find any)

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

char * itoa(int i) {
  char * res = malloc(8*sizeof(int));
  sprintf(res, "%d", i);
  return res;
}

int main(int argc, char *argv[]) {
 ...

Answer 1

// Yet, another good itoa implementation
// returns: the length of the number string
int itoa(int value, char *sp, int radix)
{
    char tmp[16];// be careful with the length of the buffer
    char *tp = tmp;
    int i;
    unsigned v;

    int sign = (radix == 10 && value < 0);    
    if (sign)
        v = -value;
    else
        v = (unsigned)value;

    while (v || tp == tmp)
    {
        i = v % radix;
        v /= radix;
        if (i < 10)
          *tp++ = i+'0';
        else
          *tp++ = i + 'a' - 10;
    }

    int len = tp - tmp;

    if (sign) 
    {
        *sp++ = '-';
        len++;
    }

    while (tp > tmp)
        *sp++ = *--tp;

    return len;
}

// Usage Example:
char int_str[15]; // be careful with the length of the buffer
int n = 56789;
int len = itoa(n,int_str,10);