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

/*
Converts an integer value to a null-terminated string using 
the specified radix and stores the result in the given buffer.
If radix is 10 and value is negative the string is preceded by 
the minus sign (-). With any other radix, value is always considered 
unsigned. buffer should be large enough to contain any possible value: 
(sizeof(int)*8+1) for radix=2, i.e. 17 bytes in 16-bits platforms 
and 33 in 32-bits platforms.
Parameters:
 value    Value to be represented as a string. 
 buffer   Buffer where to store the resulting string. 
 radix    Numeral radix in which value has to be represented, between 2 and 36. 
Return Value:
 A pointer to the string. 
No error checking is performed.
*/
char* itoa (int value, char* buffer, int radix)
{
        if ( radix == 10 ) {
                sprintf (buffer, "%d", value);
        } else {
                char tmp_buffer[ sizeof(int)*8 + 1 ];
                char *ptr = &tmp_buffer[ sizeof(int)*8 ];
                unsigned n = value;
                *ptr-- = 0;
                do {
                        *ptr-- = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ" [n % radix];
                        n /= radix;
                } while ( n );
                strcpy (buffer, ++ptr);
        }
}

int main ()
{
        int  value, radix;
        char buffer[sizeof(int)*8 + 1];
        while (1) {
                printf ("Value (decimal): "); scanf ("%d", &value);
                printf ("Radix (2 .. 36): "); scanf ("%d", &radix);
                if ( radix < 2 || radix > 36 ) {
                        printf("Good bye!\n");
                        return 0;
                }
                itoa (value, buffer, radix);
                printf("Result: %s\n", buffer);
        }
}