C Typedef - Incomplete Type

Question

So, out of the blue, the compiler decides to spit this in face: "field customer has incomplete type".

Here's the relevant snippets of code:

customer.c

#include <stdlib.h>
#include <string.h>

#include "customer.h"

struct CustomerStruct;
typedef struct CustomerStruct
{
    char id[8];
    char name[30];
    char surname[30];
    char address[100];
} Customer ;

/* Functions that deal with this struct here */

customer.h

A header file for customer.h

#include <stdlib.h>
#include <string.h>

#ifndef CUSTOMER_H
#define CUSTOMER_H

    typedef struct CustomerStruct Customer;

    /* Function prototypes here */

#endif

This is where my problem is:

customer_list.c

#include <stdlib.h>
#include <string.h>

#include "customer.h"
#include "customer_list.h"

#include "..\utils\utils.h"


struct CustomerNodeStruct;
typedef struct CustomerNodeStruct
{
    Customer customer; /* Error Here*/
    struct CustomerNodeStruct *next;
}CustomerNode;



struct CustomerListStruct;
typedef struct CustomerListStruct
{
    CustomerNode *first;
    CustomerNode *last;
}CustomerList;

/* Functions that deal with the CustomerList struct here */

This source file has a header file, customer_list.h ,but I don't think its relevant.

My Problem

In customer_list.c, at the line with the comment /* Error Here */, the compiler complains about field customer has incomplete type.

I've been googling this problem all day, and now im at the point of pulling out my eyeballs and blending them with strawberries.

What is the source of this error ?

Thanks in advance :)

[P.S. if I forgot to mention something, let me know. Its been a stressful day for me, as you might tell ]

Answer 1

Move the struct declaration to the header:

customer.h
typedef struct CustomerStruct
{
...
}

Answer 2

In C, the compiler needs to be able to figure out the size of any object that is referenced directly. The only way that the sizeof(CustomerNode) can be computed is for the definition of Customer to be available to the compiler when it is building customer_list.c.

The solution is to move the definition of the struct from customer.c to customer.h.

Answer 3

What you have is a forward declaration of Customer structure that you are trying to instantiate. This is not really allowed because compiler has no idea about the structure layout unless it sees it definition. So what you have to do is move your definition from the source file into a header.

Answer 4

It seems that something like

typedef struct foo bar;

won't work without the definition in the header. But something like

typedef struct foo *baz;

will work, as long as you don't need to use baz->xxx in the header.