structure pointer declaration giving error

Question

struct node {
int data;
struct node  *next;
};
struct node * list,root;
list=NULL;
root=NULL;

Above is what I am trying to create in my program and my program when I compile gives error

kbc.c:8: warning: data definition has no type or storage class
kbc.c:8: error: conflicting types for ‘list’
kbc.c:7: note: previous declaration of ‘list’ was here
kbc.c:8: warning: initialization makes integer from pointer without a cast
kbc.c:9: warning: data definition has no type or storage class
kbc.c:9: error: conflicting types for ‘root’
kbc.c:7: note: previous declaration of ‘root’ was here
kbc.c:9: warning: initialization makes integer from pointer without a cast

Here is the full program if some one wants to see I am just making a program to write a link list

#include<stdio.h>
#include<stdlib.h>
struct node {
int data;
struct node  *next;
};
struct node * list,root;
list=NULL;
root=NULL;
struct node * create_node(int );
int main ()
{
int i,j,choice;
printf("Enter a number this will be root of tree\n");
scanf("%d",&i);
printf("Give us choice 1 to enter more numbers 0 to quit\n");
scanf("%d",&choice);
switch (choice)
        {
        case 1:break;

        case 2:break;
        }
}
// end of function main
struct node * create_node(int data)
{
struct node *temp;
t    emp = (struct node *)malloc(sizeof(struct node));
return temp;
}

root is not a pointer. You may want struct node *list, *root; — pmg
– pmg, Commented Jun 19, 2011 at 15:59
My guess is that list=NULL; root=NULL; should probably be done inside of main. Globals are a code smell... pass them as parameters. — Taryn East
– Taryn East, Commented Jun 19, 2011 at 16:06
By the way, since list and root are "global" variables, they will automatically be initialized to 0 (NULL) if you write struct node *list, *root;. — Alok Singhal
– Alok Singhal, Commented Jun 19, 2011 at 18:34

Jonathan Leffler · Accepted Answer · 2011-06-19 16:35:55Z

3

Part of the problem is that you have declared one pointer and one structure. Your declarations are equivalent to:

struct node *list;
struct node  root;

This is why the '*' indicating a pointer belongs with the variable name (declarator in standardese) and not with the type.

The other part of the problem is that you can only initialize a variable at global scope with a (constant) initializer; you cannot write assignments at global scope.

There are multiple ways to fix this:

typedef struct node *NodePtr;
NodePtr list = NULL, root = NULL;

struct node *list = NULL, *root = NULL;

struct node *list = NULL;
struct node *root = NULL;

Note that one of the many reasons for not using a #define in place of typedef is that:

#define NodePtr struct node *        // BAD!
NodePtr list = NULL, root = NULL;    // BAD!

creates exactly the erroneous situation in the question.

Addressing some of the error messages:

kbc.c:8: warning: data definition has no type or storage class

This is referring to the line:

list = NULL;

Because you are writing at global scope, this is interpreted as a variable definition, with no explicit type. This is horrid style nowadays, but was allowed in original (pre-standard) C, using the 'implicit-int' rule to infer the type - which also explains some of the subsequent messages. The 'implicit-int' interpretation was obsolescent in C89 and not officially supported by C99, but compilers still recognize it (with a warning at least in C99 mode).

So, this is interpreted as:

int list = NULL;

These messages are now explained:

kbc.c:8: error: conflicting types for ‘list’
kbc.c:7: note: previous declaration of ‘list’ was here

You have written code that the compiler must interpret as a second declaration of list.

kbc.c:8: warning: initialization makes integer from pointer without a cast

And this is explained; since the second list is an int and NULL is a null pointer constant in this implementation (likely #define NULL ((void *)0)), there is a conversion from a pointer to an integer without a cast.

The second set of warnings apply to analogously to root.

edited Jun 19, 2011 at 16:35

answered Jun 19, 2011 at 16:26

Jonathan Leffler

760k145 gold badges961 silver badges1.3k bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Registered User Over a year ago

actually when I used struct node *list,*root; then also I had the same error.How ever I used following now typedef struct node * nodePtr; nodePtr list,root; list = NULL; root = NULL; and I still got the same error.

pmg Over a year ago

Some people do not like to hide the pointerness of a type behind a typedef :)

Jonathan Leffler Over a year ago

@pmg: Good point - I wondered whether to bring that out and decided not to. I don't like hiding pointer-ness inside typedefs when (as here) I'm going to dereference the type to get at the data. When the type is opaque, then I have no problem with a typedef hiding a pointer; I'm blissfully unaware of it.

Jonathan Leffler Over a year ago

@Registered User: you will continue to have problems while you try to write your 'initializer' separate from the declaration. Inside the scope of a function, you can do that; at global or file scope, you cannot. At global or file scope, you must initialize as you declare the variable.

John Zwinck · Accepted Answer · 2011-06-19 16:16:52Z

3

This is not OK at global scope:

struct node * list,root;
list=NULL;
root=NULL;

Try this:

struct node *list=NULL, *root=NULL;

The reason is that you can't put statements to execute (like list=NULL) outside of a function.

edited Jun 19, 2011 at 16:16

answered Jun 19, 2011 at 16:10

John Zwinck

252k44 gold badges346 silver badges459 bronze badges

2 Comments

Registered User Over a year ago

Yes you were right and I saw the error disappearing but why is that thing I mean why can I not put list=NULL outside of a function?

John Zwinck Over a year ago

It's just not how C works. In some languages (including most scripting languages), you can just put statements at global scope like that. But in C (and many C-like languages), the global scope supports declaration (and even initialization) of variables, but you can't just write your whole program in the global scope. That's what main() is for.

Collectives™ on Stack Overflow

structure pointer declaration giving error

2 Answers 2

4 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related