Question

I’m trying to implement tree algorithms in C. I have declared a extern struct in a header file that is completely independent (b_tree_ds.h). Now I plan to import the file in all source files that want to use this struct. So I must declare it using extern in header.

Now the problem is thhat I want to define its typedef as well. the compiler gives error of multiple storage classes. How should I do that.

typedef extern struct node {
    struct node* left;
    struct node* right;
    int key;    // contains value
}NODE;

The actual problem is as follow, which I still can’t fix???
I recently learnt how can we use multiple source files with header files to make code portable and hierarchical. In order to do so, I tired creating my tree program using this principal. Here are my files

b_tree_ds.h – This will contain a declaration of datastructure of node of a tree, which can be called to various functions implementing different functionality of the tree (which may be in different source files)

typedef struct node {
    struct node* left;
    struct node* right;
    int key;    // contains value
}NODE;

When i try adding a extern as in typedef extern struct node it gives a error of multiple storage class but if I miss it, I get error for multiple definitions.

Here are my other source files

traverse.h – contains declaration of traverse function

void traverse_print (NODE* p);

Here also I get error for unknown identifier NODE

traverse.c – contains definition to that function

#include <stdio.h>
#include "b_tree_ds.h"
#include "traverse.h"

void traverse_print(NODE* p)
{
    if(p->left != NULL)
    {
        traverse_print(p->left);
    }

    if (p->right != NULL)
    {
        traverse_print(p->right);
    }

    printf ("\n%d",p->key);
}

Finally main.c

#include <stdio.h>
#include "traverse.h"

void main()
{
    // input
    NODE p;

    printf("\nInput the tree");
    input_tree (&p);

    printf("\n\nThe tree is traversing ...\n")
    traverse_print(&p);
}

void input_tree (NODE *p)
{
    int in;
    int c;
    NODE *temp;

    printf("\n Enter the key value for p: ");
    scanf("%d", &in);
    p->key  =in;
    printf ("\n\nIn relation to node with value %d",in);
    printf ("Does it have left child (Y/N): ")
    if ((c = getchar()) == Y);
    {
        //assign new memory to it.
        temp = (NODE *)malloc(sizeof(NODE));
        input_tree(temp);
    }
    printf ("\n\nIn relation to node with value %d",p->key);

    printf ("\nDoes it have right child (Y/N): ")
    if ((c = getchar()) == Y);
    {
        //assign new memory to it.
        temp = (NODE *)malloc(sizeof(NODE));
        input_tree(temp);
    }
}

This is my first attempt to such practice, please suggest is the structuring of my program good or should I try something else.

Answer 1

You can’t make a struct extern. Just define it in an include-guard protected header and include that header everywhere you need it.

EDIT for SquareRootOfTwentyThree

I use those therms in the following way:

A structure type definition describes the members that are part of the
structure. It contains the struct keyword followed by an optional
identifier (the structure tag) and a brace-enclosed list of members.

A structure declaration has the same form as a structure definition
except the declaration does not have a brace-enclosed list of members.

So "definition" is exactly what I meant.