CODEWITHSUNDEEP
cpointersgcc
LeedMx
LeedMx

Reputation: 434

Value at pointer only getting updated if used

just noticed some weird behavior, can anyone explain and tell me what I'm doing wrong?

I want to implement a Hashset using an array of linked list pointers using C, however, when I add my linked list node, the data value only gets updated if I print it out first.

Here's my code:

#include <stdio.h>
#define print(ref) printf(#ref" = %d\n",ref);
#define HASH_MODULE 13

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

void addToSet(int,listNode**);
int hashCode(int);


int main(){
    listNode*linkedListHashset[HASH_MODULE];

    addToSet(10,linkedListHashset);
    print(linkedListHashset[10]->data);

    return 0;
}

void addToSet(int value, listNode**set){
    int bucket = hashCode(value);
    print(bucket);
    listNode newNode = {value};
    newNode.next = set[bucket];
    set[bucket] = &newNode;
    //print(set[bucket]->data);
}

int hashCode(int value){
    return value%HASH_MODULE;
}

You can see there's a commented out line, if I compile it like this, the value won't get stored, giving this output (which is not what I would expect):

bucket = 10
linkedListHashset[10]->data = 0

However, when I include the commented out line here's the output, it does reflect the desired update, here the data at the pointer behaves as I expected:

bucket = 10
set[bucket]->data = 10
linkedListHashset[10]->data = 10

I'm thinking this might be a compiler artifact or something, I compile by doing:

$gcc -Wall -Werror -O -o  source.c

Therefore no warnings, everything seems in order. What do you think? What am I missing?

Upvotes: 0

Views: 31

Answers (1)

Joseph Sible-Reinstate Monica
Joseph Sible-Reinstate Monica

Reputation: 48647

In addToSet, you're declaring newNode on the stack, then storing its address in set[bucket] and returning. It then becomes a dangling pointer, which leads to undefined behavior when you dereference it in main.

Upvotes: 1

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