Possibly dereferencing NULL in C

Question

I'm traversing a list of nodes which have two fields: next and size. There comes a point in the code where I need to link in a new node and I am having trouble. I've found where the code seg faults and it looks like this. Note that curr is the current node in the list and I need to link in temp between curr and curr->next.

    Node* temp = NULL;
    temp = ((curr + 1) + a_memory_offset_int);   //calculate the address where temp should go

    temp->next = curr->next;      //Seg faults here
    temp->size = some_other_int; //Seg faults here as well

    curr->next = temp;

Is there some way that I am trying to set the fields for a NULL node? Is there something wrong with the syntax (as I am confident the logic is correct)?

Answer 1

Without seeing more code, I suspect you might not understand what a_memory_offset_int is doing. It's doing the exactly same thing as the + 1, that is to say that it's doing pointer arithmetic. This:

temp = ((curr + 1) + a_memory_offset_int);

is equivalent to:

temp = (Node*)(((char *)curr + 1*sizeof(Node)) + a_memory_offset_int*sizeof(Node));

What you probably really want is:

temp = (Node*)(((char *)curr + 1*sizeof(Node)) + a_memory_offset_int);

Note the only difference is the multiplication of a_memory_offset_int by sizeof(Node). More simplified, this is what you want:

temp = (Node*)((char *)curr + a_memory_offset_int) + 1;

Answer 2

A linked list node's memory address isn't really important - you shouldn't be calculating it yourself, and instead calling malloc, then linking it in.

Something more like this:

Node* temp = NULL;
temp = malloc(sizeof(Node)); // create a new node, allocating fresh memor

temp->next = curr->next;     // should be fine now...
temp->size = some_other_int;

curr->next = temp;

Answer 3

Sample program for verification of pointer arithmetic.

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

typedef struct node {
    int size;
    struct node * next;
} Node;

int main() {
    void *memory = malloc(10*sizeof(Node));//allocate of Node * 10
    Node *curr = memory;
    Node *temp = NULL;

    temp = curr + 1;
    temp->size = 999;
    printf("%lu\n", sizeof(Node));
    //16
    printf("%p,%p\n", (void*)curr, (void*)temp);
    //00000000004D67B0,00000000004D67C0 <- difference is 16
    int a_memory_offset_int = 16;
    temp = curr + a_memory_offset_int;
    if(temp > &curr[9])//curr[9] is last element
        printf("It is outside the allocated memory\n");//this display
    temp = curr + a_memory_offset_int/sizeof(Node);
    printf("%d\n", temp->size);//999

   return 0;
}