How to assign a value to a string outside the declaration in C ? Bus error/segfault

So for the sake of training I'm trying to recode the equivalent of an strstr function, after a few try I got it right, here the code:

(I know there are simpler way to do it which I did, but this time I wanted to try by using a 3rd string to stock the occurence)

 6char *my_strstr(char *s1, char *s2)
 7{
 8  int i, j;
 9  char *tmp;
 10  i = (j = 0);
 11
 12  if (s1 != '\0' && s2 != '\0')
 13    {
 14      while (s1[i] != '\0' && s2[j] != '\0')
 15        {
 16          if (s1[i] == s2[j])
 17            {
 18              tmp[j] = s1[i];
 19              j++;
 20            }
 21          i++;
 22        }
 23        printf("tmp = %s\n", tmp);
 24    }
 25  return (tmp);
 26}
 27
 28
 29int main()
 30{
 31  char a[] = "test Point123";
 32  char b[] = "Point123";
 33  char *ret;
 34
 35  ret = my_strstr(a, b);
 36  printf("ret = %s\n",ret);
 37
 38  return (0);
 39}

I get the output I wanted:

tmp = Point123
ret = Point123

But then just to be sure I tried with a longer string, and thats where the problems started. Here the string I tried,

char a[] = "test Point123456789";
char b[] = "Point123456789";

and the output I got with it:

tmp = Point123456?"1
ret = Point123456?"1
Abort trap: 6

With longer string I get sometimes segfault, sometimes bus Error 10. On some other post I figured that the Bus error 10 sometimes replace a segfault on mac OS (on which I'm coding for the 1st time, I'm used to code on linux), I didnt find anything about the trap tho.

Anyway I figured its more a code problem that my compiler and I'd like to know why my code function on smaller string but not bigger ones, and I read that it could be how I affected value to the strings I'm using but I dont understand where I'm making an error.

So if anyone could give me a clue on what I'm doing wrong I'd greatly appreciate it :)

via Chebli Mohamed