Segmentation fault error in tail recursive function?

Question

I am writing a function in C that converts a number to a different base such as octal, binary, hexadecimal, etc. We are required to write it recursively and the function must return a char *. However, I am getting a segmentation fault error on this snip pend of code. I am also not sure if the conversion is working properly because I cannot get past this error. I am brand new to C so I apologize if there is anything I am obviously missing.

#include <stdio.h>

char *convertToBaseOut(short int num, int base);

int main()
{
    printf("%s", convertToBaseOut(256, 8));
    return 0;
}

char *convertToBaseOut(short int num, int base) {
    if (num == 0) {
        return '0';
    }
    return convertToBaseOut(num/base, base) + (num % base);
}

Segmentation fault

...Program finished with exit code 139

Answer 1

In C you can't use + to concatenate strings.

Here you have simple recursive function:

size_t tostring(char *str, unsigned num, unsigned base)
{
    static const char digits[] = "0123456789ABCDEF";

    size_t pos;
    if(!num)
    {
        return 0;
    }
    else
    {
        pos = tostring(str, num / base, base);
        str[pos++] = digits[num % base];
        str[pos] = 0;
    }
    return pos;
}

int main()
{

    char str[20];
    printf("%u\n" , tostring(str,0xabcd, 2));
    printf("%s\n", str);

    return 0;
}

It reruens length of the string as well.