CODEWITHSUNDEEP
cpointerslinked-listscope
user3407513
user3407513

Reputation: 45

Scope and Linked Lists in C

I'm trying to get more comfortable building and using linked lists in C, and for the most part I think I have the basics down. However, I am running into problems in regards to Scope, specifically the linkage of a Pointer to a Pointer of structs.

In my linked_list.c implementation I am using a main function to test out and play around with the linked list functions I've built. But the eventual goal is to run Main from another file, and only reference the few functions I'd need externally from a header file.

The implementation I'm using right now DOES work, but it seems to rely on me declaring the struct list** inside my (temporary)) main function.

It's my understanding that if you declare a variable/pointer outside of a block, you can control the linkage to be internal, accessible to other functions in the file, using the static command. What I would like to be able to do is call a function like void initialize_list() from OUTSIDE the program and have it initialize this internally linked list**.

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

struct node {
   int value;
   struct node *next;
};

struct list {
    struct node *head;
    struct node *tail;
    int size; //  Maintains current size of list
};

static struct list** work_list;

void dirty_init(void){
    struct list* new_list = malloc(sizeof(struct list));
    new_list->head = NULL;
    new_list->tail = NULL;
    new_list->size = 0;
    *work_list = new_list;
}

Running my dirty_init function returns a segfault.

The version of code that DOES work is quite similar, but it seems to rely on the **work_list being declared inside of my main function before passing it to initialize_list. 

void initialize_list(struct list **out_list){
    struct list* my_list = malloc(sizeof(struct list));
    my_list->head = NULL;
    my_list->tail = NULL;
    my_list->size = 0;
    *out_list = my_list;
}
int main(void){
    int i;
    struct list   **work_list;
    initialize_list(work_list);
}

This is all well and good, but it has the drawback of making my **work_list pointer internally linked to the Main block. Any ideas where I am going wrong trying to get the dirty_init function to work?

Upvotes: 0

Views: 527

Answers (1)

Jens
Jens

Reputation: 72657

This here:

int main(void){
    int i;
    struct list   **work_list;
    initialize_list(work_list);
}

can't work. C is pass by value and you pass an uninitialized value, which is then used as a result pointer. Did you mean

int main(void){
    int i;
    struct list   *work_list;
    initialize_list(&work_list);
}

instead?

Upvotes: 2

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