C programming recursion question generating segfault - c

Question

trying to convert any number to any base in a recursion with C:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char* anybase(int n, int b)
{
char * s;
int len;
if(n==0) {strcpy(s,""); return s;}
s = anybase(n/b, b);
len=strlen(s);
s[len] = "0123456789ABCEDFGHIGKLMNOPQRSTUVWXYZ"[n%b];
s[len+1]='\0';
printf ("%s, %d, %d\n", s, n, b);
/* return s; */
}
int main(){
char *s;
s = anybase(900000, 18);
/*printf ("%s, %d, %d\n", anybase(90000, 18), 90000, 18);*/
}
is it the recursion having a problem? not sure why one can't return a value in the function call. what would be needed to call this recursion function and return a value instead of void.
Why is it generating seg fault each time running it? Thanks!

Answer 1

You were pretty much there, except you never allocated any memory for the string you're trying to produce. Here's a version which (apparently) runs correctly:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char* anybase(int n, int b)
{
char *s;
int len;
if(n == 0)
{
s = malloc(200);
s[0] = '\0';
}
else
{
s = anybase(n/b, b);
len=strlen(s);
s[len] = "0123456789ABCEDFGHIGKLMNOPQRSTUVWXYZ"[n%b];
s[len+1]='\0';
}
return s;
}
int main()
{
char *result;
result = anybase(900000, 18);
printf("%s\n", result);
free(result);
}
Note that since the buffer returned by anybase is allocated dynamically (using the malloc library function) it must be free'd after use.

Answer 2

As others mentioned the issue is s is just a pointer that points to no where.
But instead of allocating memory for s inside of your anybase function a better approach is to allocate the memory in the main function. This avoids possible memory leaks when calling the function multiple times.
#include <stdio.h>
#include <string.h>
void anybase(int n, int b, char* s)
{
int len;
if(n==0) {strcpy(s,""); return;}
anybase(n/b, b, s);
len=strlen(s);
s[len] = "0123456789ABCEDFGHIGKLMNOPQRSTUVWXYZ"[n%b];
s[len+1]='\0';
printf ("%s, %d, %d\n", s, n, b);
printf("%d\n", len +1);
return;
}
int main(){
char s[33]; /* enough for all ints with base 2 */
anybase(90000, 18, s);
return 0;
}

Answer 3

The string buffer can be created statically in the main function but must then be passed to each recursive function call such that it can be written in to.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFFER_SIZE 200
const char *selection = "0123456789ABCEDFGHIGKLMNOPQRSTUVWXYZ";
void anybase(int n, int b, char *s)
{
if (n) {
anybase(n / b, b, s);
strncat(&s[strlen(s)], &selection[n % b], sizeof(char));
printf("%s, %d, %d\n", s, n, b);
}
}
int main()
{
char buf[BUFFER_SIZE];
memset(buf, 0, sizeof(char) * BUFFER_SIZE);
anybase(900000, 18, buf);
printf("Ouput: %s, N: %d, B: %d\n", buf, 90000, 18);
}