Printing character array error in C

Question

While trying this code in C, I'm seeing garbage value in main method. What mistake I'm doing?

I hope that I should use "%s" in main method also but seems like wrong. Why?

#include <stdio.h>
#include <stdlib.h>
char* minimumMultiple(int num);

int main(void) {
// your code goes here
printf("%s", minimumMultiple(13)); // prints  -  *p·ä…d¿“¿Eƒ (GARBAGE) - WHY?
return 0;
}

char* minimumMultiple(int num) {
int chars1s = 1, rem = 1;
while (rem != 0) {
    rem = (rem*10+1) % num;
    chars1s++;
}
char chars[chars1s];
int i;
printf("%d\n", chars1s); // prints 6 
for (i=0;i<chars1s;i++) {
    chars[i] = '1';
}
chars[i] = '\0';
    printf("%s\n",chars); // prints "111111"
return chars;
}

`chars` is local storage which should not be returned for further usage outside of its scope ( as the stack can and will overwrite it ) — amdixon, Aug 16 '15 at 10:41
`chars[i] = '\0';` writes one past the array invoking Undefined Behavior. Fix it by changing `char chars[chars1s];` to `char chars[chars1s + 1];` — Spikatrix, Aug 16 '15 at 10:56

score 2 · Accepted Answer · edited Aug 16 '15 at 10:57

2

You are returning an address for locale variable chars in minimumMultiple.

test_c.c:25:5: warning: function returns address of local variable [-Wreturn-local-addr]
     return chars;

You should define chars as a pointer.

 char *chars = malloc(chars1s + 1);

edited Aug 16 '15 at 10:57

melpomene

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answered Aug 16 '15 at 10:41

Assem

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I see. What are some ways to solve this? – unknown_boundaries Aug 16 '15 at 10:44
define chars as a pointer, see the updated answer – Assem Aug 16 '15 at 10:47
Now how it's solving the problem. It's just storing the variable in heap I guess? – unknown_boundaries Aug 16 '15 at 10:49
Ah I think earlier it was going in function's stack which was vanished after the call. – unknown_boundaries Aug 16 '15 at 10:51
@Prakhar Yes. You are right. Don't forget to free the `malloc`ed memory. Another way to solve this problem would be to use a global or a static variable. Both have `static` storage duration and exists as long as the program does. – Spikatrix Aug 16 '15 at 10:51
@Prakhar Yeah, this allows you to access it from main, before you couldn't. Just want to add that this is leaking memory. – wallek876 Aug 16 '15 at 10:52
@wallek876 Oh yep nice point you raised. I should nullify it after the things. Should I use free or initialize with null? – unknown_boundaries Aug 16 '15 at 10:53
@Prakhar Both are different. By assigning the pointer to `NULL`, you lose the allocated memory, leading to a memory leak. **`free` it**. That's the right way to do it. – Spikatrix Aug 16 '15 at 10:54

Printing character array error in C

1 Answers1