Linked list compiler error

Question

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

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

Node* head;

void AddEntry(){
    int x;
    Node* temp;
    temp = head;
    while(temp != NULL){
        temp = temp->next;
    }
    Node* temp1 = (Node*)malloc(sizeof(Node));
    temp->next = temp1;
    printf("What is the value for this entry?\n");
    scanf("%d",&x);
    temp1->data = x;
    temp1->next = NULL;
}
void PrintList(){
    Node* temp;
}



int main(void){

}

When I compile this code I get the compiler error:

pointertest.c: In function ‘AddEntry’:

pointertest.c:16:8: warning: assignment from incompatible pointer type [enabled by default]
   temp = temp->next;
        ^
pointertest.c:19:13: warning: assignment from incompatible pointer type [enabled by default]
  temp->next = temp1;

I am not understanding why that is. I’ve seen this done in my textbook and elsewhere. I thought it was assigning the pointer temp to address saved in temp next.

Thanks for your help

score 3 · Accepted Answer · 2015-04-16 19:12:12Z

3

Your structure definition is bogus. Try this instead:

typedef struct tagNode {
  int data;
  struct tagNode *next;
} Node;

In your code, there is no such thing as a struct Node, there is just an unnamed structure with an "alias" (typedef) called Node.

By defining it this way, you can declare of variable of this type by doing either:

struct tagNode foo;

Or:

Node foo;

But when I typedef a struct like this, I avoid using the tag to avoid confusion.

It has been brought up in comments that it might be confusing why I chose to use struct tagNode instead of struct Node. Both work equally well, but my personal preference is to use different names to avoid confusion later on. I find it easier to visually disambiguate struct tagNode *foo from Node *foo then if I had just used struct Node.

edited Apr 16, 2015 at 19:12

answered Apr 16, 2015 at 18:56

user21926

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7 Comments

MS535 Over a year ago

So that extra term tagNode is used when you need to reference the struct inside the declaration?

user21926 Over a year ago

Yes. There are other ways to do the same thing, but that is one way of doing it.

MS535 Over a year ago

I was always wondering why that was only sometimes included. Makes perfect sense, thanks so much!

Alex Celeste Over a year ago

There's no compelling reason to use a different name for the typedef. This answer is potentially misleading as it implies a different name is needed for the incomplete type reference inside the struct definition.

Iharob Al Asimi Over a year ago

I don't think that can cause confusion, what does cause confusion is typedefing struct Node *.

|

Iharob Al Asimi · Accepted Answer · 2015-04-16 19:10:35Z

3

There is no struct Node in your code.

This is a typedef for an anonymous structure

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

which is valid in c, but up to here there is no struct Node declared at all, you need

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

If you are going to typedef it anyway, you can do it like this too

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

edited Apr 16, 2015 at 19:10

answered Apr 16, 2015 at 18:57

Iharob Al Asimi

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