Macro giving different values for different compilers

Question

On gcc-4.8.5

#include<iostream>
#include<stdio.h>
using namespace std;

#define CONVERT(a, str) (sprintf(str , "%llu", a))

int main()
{
    char str[10];
    CONVERT(10, str);

    cout<<"  Str = "<<str<<endl;
    return 0;
}

Output - Str = 10

But the same code on DevC++ prints some junk value (always same). Not able to find the issue.

EDIT :- If i do - CONVERT((unsigned long long)10, str); - it works fine. But still question remains? Why this difference between these two compilers? Just there implementaion or something hidden in this?