C Data Structure

[markfisher]

Hello everyone

I have been working on generic C data structure (Doubly Linked List) and I have a question:

------ List.c -----

struct node {
void *data;
struct node *next;
struct node *prev;
};

struct list {
unsigned int length;
struct node *head;
struct node *tail;
};
typedef struct list List;

// I also have functions ;-) and one of them is "init()" which initialize the list.

Now here is my problem, users can define a variable "List *list". Right now list is a BAD POINTER because it is not equal to (Point to) any memory address. Now I want to force users to call "init()" function first before calling any other functions in list.c

How can I do that?
"I thought about having global variable in list.c and make it as a flag "static int flag = 0" and if they call init() i change it to '1' but that does not work well ;-)"

Can someone point me to the right direction.

Thanks
Mark

[gerard4143]

I'm not sure why you need to force the users to call init()- a well commented header file or docs should suffice or is this part of the assignment requirements..

[nezachem]

The only thing which comes to mind is to use handles (aka descriptors). The handle-based code does not expose a struct list in any form. An init function returns an opaque handle instead. Of course it needs to maintain an internal registry of allocated handles and a handle-to-list map.
As an example of such approach look at the file descriptors and their open/read/write/close system calls.

[markfisher]

Thanks for the reply

gerard4143: It's not a school project or assignment. All I'm trying to do is to create my own data structure library which it must be generic.

I will use that library for my other projects (Again not a school one :-))

nezachem:
Thanks for your reply. You have a really good point (Idea). But I'm not sure how to implement it (I've never done C programming that way). Can you direct me to any documentation / tutorial on how to do it (OR explain more)?

Again thanks a lot for all your help.
Mark

[adcodingmaster]

Thanks for the reply

gerard4143: It's not a school project or assignment. All I'm trying to do is to create my own data structure library which it must be generic.

I will use that library for my other projects (Again not a school one :-))

nezachem:
Thanks for your reply. You have a really good point (Idea). But I'm not sure how to implement it (I've never done C programming that way). Can you direct me to any documentation / tutorial on how to do it (OR explain more)?

Again thanks a lot for all your help.
Mark

According to my understanding write your init() function. now write another function my_malloc() in that my_malloc function u call your init() function. whenever user wantto allocate memory to list he will call my_malloc() function. i think that will solve your problem.

[nezachem]

Quick and dirty:

static struct List * list_array[MAX_LISTS];

int create_list()
{
    int i;
    for(i = 0; (i < MAX_LISTS) && (list_array[i] != 0); i++)
        ;
    if(i == MAX_LISTS)
        return -1;
    list_array[i] = init_list(); // This is your regular init_list()
    return i;
}

int delete_list(int id)
{
    if(id < 0 || id >= MAX_LIST)
        return -1;
    struct List * list = list_array[i];
    if(list == 0)
        return -1;
    really_delete_list(list); // Your regular memory deallocation etc
    list_array[id] = 0;
    return 0;
}

// etc

[markfisher]

Thanks for your reply nezachem
As I can see in your code, you instead of giving users list *, you will give them an id, and list.c keeps track of those ids. id one of them is zero they need to allocate it.
Am I right?

Now here is a problem:
I can call delete_list without allocating. Users are not forced to use create_list fuction and I want to force them in order to use other functions, they must call create_list for every List type variable they created.

Thanks
Mark

[WaltP]

When they pass in the list address to delete, if it hasn't been initialized, don't delete it. Return an error code.

[markfisher]

I got it. Thank you all.

Mark