segmentation fault when returning string from function

Question

I was wondering what this code should print. I wanted to check whether it prints we can, can we or we can, but instead i got segmentation fault.

Here is the code:

char* mysubstr() {
    char* x = "Yes we can, can we";
    char* y = "we can";
    char* tmp = x;
    
    while(*tmp) {
        if (*tmp == *y)
            return *tmp;
        tmp++;
    }
    return x;
}


void main() {
    printf("%s", mysubstr());
}

I think the wrong part is the return *tmp;, but why? What's wrong with that?

What has your compiler to say to this line `return *tmp;` ? What type is `tmp` and what is `*tmp`? — Gerhardh, Jan 31 '22 at 10:33
Your function name indicates you want to implement some substring searching. But your function will return as soon as the first character of `y` is found and returns a `'w'` then — Gerhardh, Jan 31 '22 at 10:34
i get a warning that says ```return makes pointer from integer without a cast```. Does it means the compiler thinks this ```*tmp``` is an integer? — ryden, Jan 31 '22 at 10:36
Does this answer your question? [Can a local variable's memory be accessed outside its scope?](https://stackoverflow.com/questions/6441218/can-a-local-variables-memory-be-accessed-outside-its-scope) — JHBonarius, Jan 31 '22 at 10:36
@JHBonarius that dupe is about returning address of local nonstatic buffer. No relation to this question. — Gerhardh, Jan 31 '22 at 10:36
@ryden the compiler does not only think it is an integer. It **is** an integer. Actually a `char` to be more precise. — Gerhardh, Jan 31 '22 at 10:36
@JHBonarius that is again a dupe about local buffers. Please read this question before suggesting similar wrong dupes. — Gerhardh, Jan 31 '22 at 10:37
The root problem is this [What compiler options are recommended for beginners learning C?](https://software.codidact.com/posts/282565) Ensure that you get compiler errors when writing invalid C, so you don't waste time troubleshooting bugs that the compiler has already found for you. — Lundin, Jan 31 '22 at 11:05

score 1 · Answer 1 · answered Jan 31 '22 at 10:47

Your compiler basically already told you what is wrong:

return makes pointer from integer without a cast

This is because you define a function returning a char * but you return a char.

With char *tmp = x; you define a pointer and in your return statement you dereference it. Hence you return (char*)'w'

If you use that return value for printf value 'w' is taken an address which causes your crash.

You should just do return tmp;

This is not the only issue in your code. Your function name indicates you want to make some substring function. Bur your code returns immediately it found a match of first letter in second string. You don't verify if the other character also are same.

@ryden be aware that the warning "return makes pointer from integer without a cast" is almost always an error. — Jabberwocky, Jan 31 '22 at 11:05

score 0 · Accepted Answer · answered Jan 31 '22 at 15:25

Following works:

char* mysubstr()
   {
       char* x = "Yes we can, can we";
       char* y = "we can";
       char* tmp = x;
  
       while(*tmp)
       {
           if(*tmp == *y)
           {
               return (char*) tmp;
           }
           tmp++;
       }
       return (char*) x;
   }
  
   int main()
   {
       printf("%s", mysubstr());
       return 0;
  }

And the answer is:

**We can, can we**

segmentation fault when returning string from function

2 Answers2