Unknown Malloc stack overflow c

Question

I have written a piece of code that takes several integers (as many as 100 000 int) as input from a file and stores them in a "recursive" struct.

As long as I run this code on my PC everything is fine.

Here is the code:

typedef struct node{
    int data;
    struct node* next;
} node;

...

node* create(void){
    node* list = (node*)malloc(sizeof(node));
    return list;
}

node* insert(node* list, int temp){
    if(list == NULL){
        list = create();
        list->data = temp;
        list->next = NULL;
        return list;
    }
    list->next = insert(list->next, temp);
    return list;
}

int main(void){
    ...
    node* list = NULL;
    while(there is still data to input){
        list = insert(list, data);
    }
}

However, when I try to run this code on my Android phone, I get a

malloc stack overflow error

(I know that the stack space reserved on a phone is less then the one on a PC).

The problem is that, to my knowledge, this program should use a lot of stack memory.

This is what I think is happening inside my program (please correct me if I am wrong):

1). node* list = NULL ==> Space for a pointer (8 byte) is allocated on the stack;

2). list = insert(list, temp) ==> Goes to the end of data stream.

3). list = create() ==> The create() function is called;

4). node* list = (node*)malloc(sizeof(node)) ==> Space for a pointer is allocated on the stack (8 byte) and space for the struct is allocated on the heap (16 byte);

5). return list ==> create() function is closed, therefore the variable node* list on the stack is "freed" while the space allocated on the heap remains.

So my program should be using a lot of heap memory, but just 8 byte of stack memory (the ones needed for the first pointer in main ==> node* list = NULL), how is it possible that I get error:

malloc stack overflow

?

Thank you

Lorenzo

P.s. Sorry guys but I was trying to make my code shorter, but what I had written was no sense. I fixed it now (or I hope so).

list->next = NULL; list = list->next; does not look very good to me — Sourav Ghosh
– Sourav Ghosh, Commented Jan 15, 2018 at 14:33
As long as I run this code on my PC everything is fine. - I really doubt it given the above. — Eugene Sh.
– Eugene Sh., Commented Jan 15, 2018 at 14:34
Your code probbaly invokes undefined behaviour (see upvoted comments above). So it may run apparently fine on one platform and crash on an other one. — Jabberwocky
– Jabberwocky, Commented Jan 15, 2018 at 14:41
My guess is that you are over stressing the heap allocator and whatever algorithm is used internally is using the whole stack. — JeremyP
– JeremyP, Commented Jan 15, 2018 at 14:41
Wth is a "malloc stack overflow error"? Whatever tool that spat out that text is crap and should not be used. — Lundin
– Lundin, Commented Jan 15, 2018 at 15:08

afic · Accepted Answer · 2018-01-15 14:47:47Z

1

You are overusing the variable list.

You need to retain a pointer your current node instead of overwriting it with the line:

list = create();

consider the following or similar:

int main(void){
    ...
    node* list = NULL;
    node* current = NULL;
    node* next = NULL;
    while(...){
        ...
        next = create();
        if(list == NULL)   //list empty case
        {
            list = next;
            current = next;
        }
        current->next = next;
        next->next = NULL;
        current = next;
    }
}

I encourage you to wrap some of this logic in a function separate from main().

The actual cause of the segmentation fault is not in the code you showed, but in your current code when every you try to use list it is NULL, which is probably your undefined behavior.

edited Jan 15, 2018 at 14:47

answered Jan 15, 2018 at 14:42

afic

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2 Comments

Physics Koan Over a year ago

Sorry, you are right. I was trying to oversimplify my code to make it easier to read, and I wrote something that has no sense. I have update my code with my original (which actually has the function you are talking about).

JeremyP Over a year ago

I agree with everything you say here. However "malloc stack overflow" is not a segmentation violation or at least not the error I would expect from dereferencing a null pointer as a result of running out of heap space. In any event, the fact that your code allows the memory to eventually be reclaimed probably does not mitigate the problem.

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