Unable to Access Casted Void Pointer - c

Question

I am creating a linked list that contains nodes with the "dataItem" of a void pointer. The purpose of this is in order to be able for the Node to contain any kind of data. However, I am unable to access the void pointer's data, even when that data is casted to the correct type.
My code looks like the following:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct Student
{
char stuName[51];
char stuMajor[5];
double GPA;
char stuID[10];
}student;
typedef struct Node
{
union{
void *dataPtr;
int countr;
}dataItem;
int link;
struct Node* next;
}node;
void readData(struct Node *);
void main(){
node head;
node temp;
readData(&temp);
student *ptr = (student *)(temp.dataItem.dataPtr);
printf("%s %d", ptr->stuName, ptr->GPA);//breaks here because unable to access memory
}
void readData(struct Node *link)
{
link = (node *)malloc(sizeof(node));
student *ptr = (student *)malloc(sizeof(struct Student));
printf("enter the student name : ");
fflush(stdin);
scanf("%[^\n]", ptr->stuName);
printf("enter the student's major : ");
fflush(stdin);
scanf("%[^\n]", ptr->stuMajor);
printf("enter the student GPA : ");
scanf("%lf", &(ptr->GPA));
printf("enter the student ID : ");
fflush(stdin);
scanf("%[^\n]", ptr->stuID);
link->dataItem.dataPtr = ptr;
}
I know I definitely have a pointer wrong somewhere I'm not unsure how. I also have the node within my readData function point to a new malloc of Node because I want a new node each time readData is called for when I implement the linked list further.

Answer 1

Your code is very broken,
You didn't include any header file, you need at least stdlib.h for malloc(), and stdio.h for printf() and scanf().
Your main() definition is wrong, because main() must return int.
You fflush(stdin) which is undefined behavior.
You ignore the return value from scanf().
You assume that malloc() always returns a valid pointer.
You called readData() which is not declared yet.
But the most important mistake, is that you passed node temp's address to readData() and you malloc()ed it, but you didn't return a pointer to it, thereby losing all the changes made inside readData() which will not work anyway because it's undeclared at the moment you called it.
I fixed your code, because I know that you didn't like my answer, but check out the fixes which are related to the answer, and now it works as I expected
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct Student
{
char stuName[51];
char stuMajor[5];
double GPA;
char stuID[10];
} student;
typedef struct Node
{
union{
void *dataPtr;
int countr;
} dataItem;
int link;
struct Node* next;
} node;
void readData(struct Node **link);
int main()
{
node *head;
student *ptr;
readData(&head);
ptr = head->dataItem.dataPtr;
if (ptr != NULL)
printf("%s\t%g", ptr->stuName, ptr->GPA);
return 0;
}
void readData(struct Node **link)
{
student *ptr;
if (link == NULL)
return;
*link = malloc(sizeof(node));
if (*link == NULL)
return;
memset(*link, 0, sizeof(node));
ptr = malloc(sizeof(struct Student));
if (ptr == NULL)
return;
printf("enter the student name : ");
if (scanf("%50[^\n]%*c", ptr->stuName) != 1)
ptr->stuName[0] = '\0';
printf("enter the student's major : ");
if (scanf("%4[^\n]%*c", ptr->stuMajor) != 1)
ptr->stuMajor[0] = '\0';
printf("enter the student GPA : ");
if (scanf("%lf%*c", &(ptr->GPA)) != 1)
ptr->GPA = 0;
printf("enter the student ID : ");
if (scanf("%9[^\n]%*c", ptr->stuID) != 1)
ptr->stuID[0] = 0;
(*link)->dataItem.dataPtr = ptr;
}
I also added some safety fixes to scanf() adding the length modifier to prevent buffer overflow, and also remove the trailing '\n' with "%*c" specifier, it will not work if multiple spaces follow the value, but you can test it meanwhile just pressing Enter/Return, if you want more sofisticated input, you should use something else instead of scanf().

Answer 2

This:
void readData(struct Node *link)
{
link = (node *)malloc(sizeof(node));
You throw away the link parameter value that was passed in, instead assigning it a new allocation via malloc. This is your most significant problem, I think. A simple fix would be to delete the line performing the malloc.

Answer 3

You want to pass a pointer to a pointer (or address of a pointer) to a node to the readData() function. readData allocates a new node and fills it; what happens in your code is that readData() gets a copy of the address of temp, overwrites the copy of this address with a new address, the one obtained from malloc (which is invisible to the calling code in main()) and then fills that malloc'ed object. Upon return from readData the malloc'ed object is inaccessible.
What you wanted to do is this:
void main(){
node head;
node *tempAddr; // pointer
readData(&tempAddr); // pass address of that pointer
and then use *tempAddr in main instead of temp.
void readData(struct Node **linkAddr)
{
node *link = *linkAddr = (node *)malloc(sizeof(node));
[...]
unwind would complain that you should not cast the result of malloc, but I think it's fine. I also do not care about the return value of main, or its arguments.