Checking if a Linked list is Empty in C - c

Question

I have a simple linked list (Which I'm given for a assignment), and I'm trying to check if it's empty (It has no header node), but my implementation doesn't seem to work.
I have the .c file:
#include <stdio.h>
#include <stdlib.h>
#include "test.h"
int main(void) {
List * node;
node = NULL;
node = malloc(sizeof(List));
addToList(node);
return 0;
}
void addToList(List * node) {
if(node == NULL) {
node->value = 3;
node->next = NULL;
}
else {
//add to end of the list:
List * temp = malloc(sizeof(List));
temp->value = 4;
while(node->next != NULL)
node = node->next;
node->next = temp;
temp->next = NULL;
}
}
And the Linked list in the struct (In the .h file)
typedef struct List {
int value;
List * next;
}List;
However, the if statement never executes, even though the list is empty. The struct I'm given doesn't have a header node(I can't change it), so how would I go about fixing this?
Any help would be much appreciated.

Answer 1

The problem is in addToList
node == NULL
should be
node->next == NULL
then you'll also need to insure that function isn't given a null pointer.
Currently you are providing a valid pointer the function and so it is never == NULL
See:
node = malloc(sizeof(List));
addToList(node);
here node is assigned a pointer to a List and provided to addToList.
I suspect what you are really looking for doesn't include that if/else at all and should look something like this (NOTE: You must initialize your list node by hand before calling this function. malloc just gives you a pointer to garbage).
//add to end of the list:
void addToList(List *node) {
if (node == NULL)
return;
List * temp = malloc(sizeof(List));
temp->value = 4;
while(node->next != NULL)
node = node->next;
node->next = temp;
temp->next = NULL;
}

Answer 2

As it was already said the problem is that at first you allocated a node
node = malloc(sizeof(List));
and it got the address of the allocated memory and only after that you called the function
addToList(node);
with this node as an argument.
As result this condition within the function
if(node == NULL) {
equal to false. node is not equal to NULL. It is equal to the address of the allocated memory.
It is much better when the function allocates itself each new node. However in this case you have to declare the function differently.
It and its call can look the following way
#include <stdio.h>
#include <stdlib.h>
#include "test.h"
void addToList( List ** node, int value );
// ^^^^^^^ ^^^^^^^^^
int main(void)
{
List * node NULL;
addToList( &node, 4 );
return 0;
}
void addToList( List ** node, int value )
{
while ( *node ) node = &( *node )->next;
List * temp = malloc( sizeof( List ) );
if ( temp != NULL )
{
temp->value = value;
temp->next = NULL;
*node = temp;
}
}

Answer 3

The condition node == NULL is always false because, before accessing the addToList function, you write node = malloc(sizeof(List)).
Try remove this declaration and change the if statement in
if(node == NULL) {
node = malloc(sizeof(List));
node->value = 3;
node->next = NULL;
}