There are two bugs in printd():

1. The last character is an l instead of a semicolon.
2. If n is negative, it prints a negative sign before each digit.

Your itoa() also has a bug in handling negative input. It always stringifies the absolute value of n.

Your strategy to use the return value to determine which array element to write to seems like a good idea. I think it would be more user friendly, though, to return the strlen() of the output.

int itoa(int n, char s[]) {
    int i = 0;

    if (n < 0) {
        s[0] = '-';
        return 1 + itoa(-n, s + 1);
    }

    if (n / 10) {
        i = itoa(n / 10, s);
    }
    s[i] = n % 10 + '0';
    s[++i] = '\0';

    return i; /* strlen(s), i.e. the next free slot in array */
}

There are two bugs in printd():

1. The last character is an l instead of a semicolon.
2. If n is negative, it prints a negative sign before each digit.

Your itoa() also has a bug in handling negative input. It always stringifies the absolute value of n.

Your strategy to use the return value to determine which array element to write to seems like a good idea. I think it would be more user friendly, though, to say that you are returning the strlen() of the output.

int itoa(int n, char s[]) {
    int i = 0;

    if (n < 0) {
        s[0] = '-';
        return 1 + itoa(-n, s + 1);
    }

    if (n / 10) {
        i = itoa(n / 10, s);
    }
    s[i] = n % 10 + '0';
    s[++i] = '\0';

    return i; /* strlen(s), i.e. the next free slot in array */
}