using an array member in a data structure

Question

I have the following data structure:

typedef struct node
{
    int key;
    int *v;
}node;

... and the global variables:

node *multiway[10];
int contor=0;

I am trying to insert inside this structure all the nodes of a multiway tree, and all the kids of each node. In order to do this, I made this function:

int * getKids(int value,int n) //returneaza vectorul de copii ai unui nod
{
    //value-nodul parinte
    //n- numarul de noduri ale vectorului
    int *result=(int*)malloc(n*sizeof(int));
    int counter=0;
    int i;
    for(i=1;i<=n;i++)
    {
        if(a[i]==value)
        {
            counter++;
            result[counter]=i;
            printf("%d ",result[counter]);
        }
    }
    int copii[counter]; //in vector pun toti copiii valorii date, value
    for(i=1;i<=counter;i++)
    {
        copii[i]=result[i];
    }
    contor++;
    multiway[contor]=(node*)malloc(sizeof(node)); //added this line after a comment
    multiway[contor]->key=value; //SEGMENTATION FAULT
    multiway[contor]->v=copii;
    return result;
}

My code compiles with no warnings, but when I run, it crashes. When I debug, I get a segmentation fault at the line which I commented with "Segmentation fault". Any idea of what I did wrong? Thank you.

Arrays in C run from `0` to `length-1`, not from `1` to `length`. — Jongware, Apr 24 '15 at 21:55
`multiway` is an array of 10 pointers to `node`. Where did you allocate each element in the array? And what is the value of `counter`. Is it 10 or greater? — Marius Bancila, Apr 24 '15 at 21:56
You can't dynamically allocate `copii` with: `int copii[counter];`. You should use `malloc`. — imp25, Apr 24 '15 at 21:59
I added the following line before assigning an element of the multiway: multiway[contor]=(node*)malloc(sizeof(node)); I get a warning but at least my code doesn`t crash. Am I doing it ok? — Sportler, Apr 24 '15 at 22:02
@SouravGhosh `multiway[contor]->v=copii;` set local variable. So _You should use malloc._ — BLUEPIXY, Apr 24 '15 at 22:04

score 0 · Accepted Answer · edited May 23 '17 at 12:06

Many issue with your code. To point out some of them

Array indexing

for(i=1;i<=n;i++)
{
    if(a[i]==value)

arrays in c has 0 based index. So, basically it should be for(i=0;i< n;i++). You also did not show us what a is.

In case of
```
int copii[counter];
```

what if if(a[i]==value) never becomes TRUE?

In case of

multiway[contor]->key=value; //SEGMENTATION FAULT
multiway[contor]->v=copii;

what if contor is more than 9? You'll be overrunning the memory.

a function local variable's lifetime is till the function finishes execution. Once the function is finished, that variable won't be present anymore. Trying to access that variable produces undefined behavior.
```
multiway[contor]->v=copii;
```

in the above case, copii is local to getKids(), and after getKids() finishes execution, trying to access multiway[contor]->v will lead to UB.

Note: Do not cast the return value of malloc() and family.

using an array member in a data structure

1 Answers1