C project file error: structure is being duplicated in 2 files

Question

I have a project file in C where I declare a structure in file.H, and 2 files includes file.h,
the content of .H file:

#ifndef SORT_H
#define SORT_H

struct lnode {
 int data;
 struct lnode *next;
} *head, *visit;

void add(struct lnode **q, int num);


/*typedef struct lnode NODE; */



#endif

++++ Content of ADD.C +++++

#include "SORT.H"

void add(struct lnode **q, int num)
{
     struct lnode *temp;

     temp = *q;

     /* if the list is empty, create first node */
     if(*q == "") {
      *q = malloc(sizeof(struct lnode));
       temp = *q;
     } else {
      /* go to last node */
      while(temp->next != "")
       temp = temp->next;

       /* add node at the end */
       temp->next = malloc(sizeof(struct lnode));
       temp = temp->next;
     }

     /* assign data to the last node */
     temp->data = num;
     temp->next = "";
}

++++ Content of SORT.C ++++

#include "SORT.h"
#include<conio.h>
#include<stdio.h>

void main(){

    struct lnode *newnode = NULL;
    char choice;
    int lim, i = 0, num;

    clrscr();

     /*get max value*/
     printf("Input no. of values: ");
     scanf("%d",&lim);

     for(i = 0; i <= lim; i++){
        printf("[%i] = ",i);
        scanf("%d",&num);
        add(&newnode, num);
     }


    head = newnode;
    for(;;){
        printf("\n\n\nMENU: \n");
        printf("[A]Selection Sort\n[B]Insertion Sort\n[C]Exchange Sort\nChoice: ");
        scanf("%s",&choice);

        switch(toupper(choice)){
            case 'A':
                      clrscr();
                      printf("Selection Sort: \n");
                      break;
            case 'B':
                      clrscr();
                     printf("Insertion Sort: \n");

                      break;
            case 'C':
                      clrscr();
                        printf("Exchange Sort: \n");
                      break;
            case 'D':
                      exit();
                      break;
            default:
                      clrscr();
                      printf("Incorrect choice!\n\n\n");
                      break;
        }
    }
}

And i get the linker_error: _head and _visit is being duplicated from file1.c to file2.c.

and i need this structure in my 2 files, anyway for me to do this, also if you need more information just say so.

@DennisMeng its already on the .h file and the error still occurs — hushpuppies, Jan 12 '14 at 08:12
No need to add `add.c` and `sort.c`; other than the `#include`s, their contents aren't going to have anything to do with the error you're getting. — Dennis Meng, Jan 12 '14 at 08:24
@DennisMeng i just figured that it might be of some use. do you have any idea as to why is this happening? thank you :) — hushpuppies, Jan 12 '14 at 08:26
See http://stackoverflow.com/questions/1433204/how-do-i-share-a-variable-between-source-files-in-c-with-extern-but-how — Chris Culter, Jan 12 '14 at 08:29
Just for next time, don't include code in the question unless it's absolutely necessary to show what's wrong/reproduce the error. In your case, all that's really relevant is that you're doing `#include "SORT.h"` at the top of both files; the contents of `add()` and `main()` don't matter. As for the actual error, it looks like others are on top of it. — Dennis Meng, Jan 12 '14 at 08:29
@DennisMeng will keep that in mind, thank you. also chris i will take a look at that link — hushpuppies, Jan 12 '14 at 08:30
`#include ` for `malloc` and `exit`, also `exit()` -> `exit(1)` and `temp->next = ""` --> `temp->next = NULL` — BLUEPIXY, Jan 12 '14 at 08:34
In your .h files you should generally not have definitions but only declarations. Look in your .h file and you have this: `struct { ... } *head, *visit;` Make it a declaration by saying `struct { ... };` Put the definitions in one of your .c files — Brandin, Jan 12 '14 at 08:41

score 2 · Accepted Answer · answered Jan 12 '14 at 08:32

2

Variables head and visit are defined as global variables in the header file which is included in two files. Hence they are double defined. You will need to declare those vars in the header and define them in one of .c files.

#ifndef SORT_H
#define SORT_H

struct lnode {
   int data;
   struct lnode *next;
};

extern struct lnode  *head, *visit;
...
#endif

and in SORT.C

struct lnode  *head, *visit;

answered Jan 12 '14 at 08:32

Marian

7,402
2
22
34

thanks this worked for me, though why re declare the struct inside the store.c file? thanks – hushpuppies Jan 12 '14 at 08:47
@hushpuppies In the C file `struct lnode *foo` defines foo as being a pointer to struct lnode. In the .h file you declare what `struct lnode` is – Brandin Jan 12 '14 at 11:00

C project file error: structure is being duplicated in 2 files

1 Answers1