If understand my own code correctly I am allocating space for 3 pointers pointing to a data type ListNode and then looping three times and adding a number to each node. I know that the nodes are not connected to one another

now when I try to access any node from the allocated space I get the error commented in the bottom of the code

#include <stdio.h>
#include <stdlib.h>


typedef struct ListNode {
    int val;
    struct ListNode *next;
    } ListNode;

int main(void)
{    
    int nums[] = {2,3,4};

    ListNode * ptr = (ListNode*)malloc(3*sizeof(ListNode*));
    
    for (int i=0; i < sizeof(nums)/sizeof(nums[0]); i   )
    {
        ListNode new;
        new.val = nums[i];
        ptr[i] = new;
    }

    printf("%d \n",ptr[0].val);

    free(ptr);
    return 0; 
}
/*
Error: 
malloc(): corrupted top size
Aborted (core dumped)
*/

why do I get this error and how to properly access each node in the allocated space

CodePudding user response：

You're not allocating enough space:

ListNode * ptr = (ListNode*)malloc(3*sizeof(ListNode*));

Here you're attempting to dynamically allocate a 3 element of array of ListNode structs, but you're instead allocating space for 3 pointers to ListNode. These pointers are smaller that the structs, so you end up writing past the end of allocated memory, triggering undefined behavior and a crash.

You want to allocate space for 3 structs, not 3 pointers:

ListNode * ptr = (ListNode*)malloc(3*sizeof(ListNode));

Or better yet:

ListNode * ptr = (ListNode*)malloc(3 * sizeof *ptr);

As this doesn't depend on what the type of ptr is.

CodePudding user response：

    int nums[] = {2,3,4};
    size_t k = sizeof(nums)/sizeof(nums[0]);

    ListNode * ptr = malloc(k*sizeof(ListNode));
    

1. Do not cast the malloc's result for reasons you can find in many places, including on SO.

2. Allocate COUNT times sizeof(object), not sizeof(object*) bytes.