Why is my program crashing when I try to initialize my pointer?

Question

So my program runs the numElements method but than crashes, am I declaring my variables wrong or is my pointer variable wrong?

Header.h

typedef struct Node {
int number;
struct Node *pNext;
}Node;

Other.h

void LinkedList(Node *pNode){

int i,j,c,numElem;
time_t t;
srand((unsigned) time(&t));
numElem = numElements();
pNode[numElem];

for(i = 0; i < numElem; i++){

    pNode[i].number = (rand() % (MAX - MIN)) + MIN;
    //c = (rand() % (MAX - MIN)) + MIN;
    printf("P is %d\n",pNode[i].number);
    printf("C = %d",c);

   }
}

Main.c

Node *pNode;
LinkedList(pNode);

@litelite pNode is a pointer of Node that I pass through the functon — Cmi, Oct 25 '17 at 17:13
@Cmi and how does the caller initialize the memory for `pNode`? — litelite, Oct 25 '17 at 17:14
@Cmi you did not initialize any memory. You just created a dangling pointer. So (unless there is any other code in that function) `pNode[numElem];` will just access some random memory which is undefined behavior. — litelite, Oct 25 '17 at 17:21
@Cmi it reads a random adress in memory, that's what it does. — litelite, Oct 25 '17 at 17:22
@Cmi,litelite More precisely, it _would_ read a random address in memory, but the value isn't used, so the compiler won't bother generating code to do anything at all because of that statement. — zwol, Oct 25 '17 at 17:26
@zwol True, but it would in debug mode where the code is not optimized. — litelite, Oct 25 '17 at 17:35

Cmi · Accepted Answer · 2017-10-25T17:26:45.140

2

I needed this pNode = malloc(numElem * sizeof(Node));

edited Oct 25 '17 at 17:26

answered Oct 25 '17 at 17:22

Cmi

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Good for you figuring this out for yourself, but do you realize that you have created an array, not a linked list? (Maybe you _mean_ to make an array of linked list heads?) – zwol Oct 25 '17 at 17:24
@zwol I am making a linked list but am starting slow though – Cmi Oct 25 '17 at 17:26
@zwol by the way is it better to use realloc? – Cmi Oct 25 '17 at 17:50
realloc is for when you already have a chunk of memory allocated (by malloc) and you need to change how big it is. – zwol Oct 25 '17 at 18:25

asam · Answer 2 · 2021-05-14T17:02:50.050

-1

You need to allocate memory for pNode, for your case (C language) it could be,

pNode = malloc( numElem * sizeof(Node) );

The cast is not required for strict C, but for C++ it is.

edited May 14 '21 at 17:02

answered Oct 25 '17 at 17:23

asam

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[Don't cast the result of `malloc`](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). – zwol Oct 25 '17 at 17:23
zwol, that depends on the compiler. so your classification is wrong! if, for example, it is necessary to compile with C++ also, it is requested to use the cast. I add C++ to my answer! – asam Oct 25 '17 at 17:26
No, it doesn't depend on the compiler. – zwol Oct 25 '17 at 17:27
The question is about C only. Casting the result of malloc is an actively bad practice in C and should never be encouraged. Only when someone specifically requires code that can be compiled as both C and C++ should you include that cast. – zwol Oct 25 '17 at 17:35
No, it is not. The question is about C only. – zwol Oct 26 '17 at 20:57

Why is my program crashing when I try to initialize my pointer?

2 Answers2