Compiler error with struct definition in header file

Question

I keep getting a compiler issue when trying to use a struct I defined in a header file.

I have two files: main.c:

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

     int main(){
         struct NODE node;
         node.data = 5;
         printf("%d\n", node.data);
         return 0;
     }

as well as node.h:

#ifndef NODE
#define NODE
    struct NODE{
        int data;
        struct NODE *next;
    };

#endif

I was writing a small program to practice some modular programming in C, but I've been getting the following compiler error:

node.h:5:21: error: expected ‘{’ before ‘*’ token
         struct NODE *next;
                     ^

I got the main.c to compile and do what I would like it to do when I define the struct directly in the file main.c, but for some reason it won't work if I place the definition in a header file and then try to include it in main.c. It's very frustrating, and I'm sure it's a minor thing, but could someone please tell me why this isn't working? From what I've been reading, I should be able to do this, no?

Thanks a lot!

@StoryTeller why should changing the name in the include guard help this code compile? — P. Gillich, Apr 28 '19 at 19:56
Because `#define NODE` means "replace any occurrence of the token`NODE` by nothing". For a header file `header.h`, normal conventions for the header guard include `HEADER_H` or `HEADER_H_INCLUDED` — and you'll see system headers using leading underscores, but they're required to do that and you're required NOT to do that. — Jonathan Leffler, Apr 28 '19 at 19:57
Because your struct has the same name as the your include guard `#define`, so the preprocessor is expanding it into an empty string. — Pierce Griffiths, Apr 28 '19 at 19:57
As my fellows explained, this is why the header definition needs to be more unique than others. Always define your headers with unique suffix such as ```_H_``` or whatever you like. — Adir Ratzon, Apr 28 '19 at 20:00
Note that you should not, in general, create function, variable, tag or macro names that start with an underscore. Part of [C11 §7.1.3 Reserved identifiers](https://port70.net/~nsz/c/c11/n1570.html#7.1.3) says: — _All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use._ — _All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces._ See also [What does double underscore (`__const`) mean in C?](https://stackoverflow.com/a/1449301) — Jonathan Leffler, Apr 28 '19 at 20:01

Pierce Griffiths · Answer 1 · 2019-05-03T15:31:58.533

6

The preprocessor is expanding NODE to nothing because you've defined it with a macro. Change your header file to look like this:

#ifndef NODE_H
#define NODE_H
    struct NODE{
        int data;
        struct NODE *next;
    };

#endif

edited May 03 '19 at 15:31

answered Apr 28 '19 at 19:56

Pierce Griffiths

733
3
15

score 3 · Answer 2 · answered Apr 28 '19 at 19:57

3

You defined a macro NODE as nothing. From that point on, every NODE in your source code is replaced with nothing. So your header file is actually:

struct{
    int data;
    struct *next;
};

That should answer your question why changing the include guard from NODE to NODE_H fixes it.

answered Apr 28 '19 at 19:57

gnasher729

51,477
5
75
98

OOOOOOOOOH I see now. It was a name collision! works now, thanks! – P. Gillich Apr 28 '19 at 19:58

Compiler error with struct definition in header file

2 Answers2