main 无法从另一个文件中找到方法

Question

How can I use a method from another file in main.c?

I created a method in my list.c file and try to print it out in my main.c file. When I try that it gives me the error "current undeclared,current2 undeclared,head undeclared".

• added list.h

Main.c

#include <stdio.h>
#include <stdlib.h>
#include "list.h"
    
typedef struct list_struct *List;
    
    
int main(){
    
            
createList();
printf("%d %d %d" , head->value, current->value, current2->value);
return 0;
        
        
}
    

list.c

#include <stdio.h>
#include <stdlib.h>
#include "list.h"
            
typedef struct list_struct *List;
            
struct node {
int value;
struct node *next;
};
            
List createList(){
            
struct node *head = malloc(sizeof(struct node));
                head->value = 45;
                head->next = NULL;
            
struct node *current = malloc(sizeof(struct node));
                current->value = 98;
                current->next = NULL;
                current->next = current;
            
struct node *current2 = malloc(sizeof(struct node));
                current2->value = 3;
                current2->next = NULL;
                return 0;
            
}

list.h

#ifndef LIST_H_
#define LIST_H_

#ifndef BOOLEAN
    #define BOOLEAN
    typedef enum {false, true} Bool;
#endif
/**
 * Forward pointer declaration to an internal implementation specific hidden list structure.
 */
typedef struct list_struct *List;
/**
 * List life-cycle functions create and delete list or remove a single node with index
 */
List createList();
void deleteList(List head);
List removeAt(List head, int index);

#endif /* LIST_H_ */

Answer 1

In main you do

printf("%d %d %d" , head->value, current->value, current2->value);

which means that head , current and current2 must be available as global variables from another compilation unit.

But you don't do that... you define them as local variables inside createList .

You need to move them out of the function like:

struct node *head;

List createList(){
            
    head = malloc(sizeof(struct node));
    head->value = 45;
    head->next = NULL;

and then in list.h you need:

extern struct node *head;

And the same for current and current2

That said... Don't use globals...

Define the variables in main and pass them as arguments to the function.