How do the double pointers (struct tree_st **root) actually work?

Question

We have to create a binary tree with car models and modify it in different ways. The main problem for me is the use of a double pointer (struct tree_st **root).

Why can't I just use a standard, single pointer?

Here's the code if you need more details of what I'm talking about:

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define MAX_SIZE 20+1

typedef struct auto_st   //auto as in "car"
{
    char marka[MAX_SIZE];  //marka as in "car model"
    int kubikaza;  // "cubage"
    int godiste;  // "year of production"
    struct auto_st *left, *right;

}AUTO;

FILE *safe_fopen(char *filename, char *mode, int exit_code);
void init(AUTO **root);
AUTO *create_new(char marka[], int kubikaza, int godiste);
void add_location(AUTO *new, AUTO **root);
void read_file(FILE *in, AUTO **root);
void write_one(FILE *out, AUTO *root);
void write_all(FILE *out, AUTO *root);
void find(AUTO *root, char marka[]);
AUTO *newest(AUTO *root, int max_kub);

int main(int arg_num, char *args[])
{
    AUTO *root;
    if(arg_num!=4)
    {
        printf("WRONG NUMBER OF ARGUMENTS!\n");
        exit(1);
    }

    int max_kub = atoi(args[1]);
    char *in_filename = args[2];
    char *out_filename = args[3];

    FILE *in = safe_fopen(in_filename,"r",10);
    FILE *out = safe_fopen(out_filename,"w",11);

    init(&root);
    read_file(in,&root);
    write_all(out,root);

    AUTO *best = newest(root, max_kub);
    if(best==NULL)
    {
        printf("CAR DOESN'T EXIST!\n");
    }
    else
    {
        write_one(out,best);
    }

    find(root,"Ferrari");

    fclose(in);
    fclose(out);
    return EXIT_SUCCESS;
}

FILE *safe_fopen(char *filename, char *mode, int exit_code)
{
    FILE *pf = fopen(filename,mode);
    if(pf==NULL)
    {
        printf("CAN'T OPEN FILE %s\n", filename);
        exit(exit_code);
    }
    return pf;
}

void init(AUTO **root)
{
    *root = NULL;
}

AUTO *create_new(char marka[], int kubikaza, int godiste)
{
    AUTO *new = (AUTO*)malloc(sizeof(AUTO));
    if(new == NULL)
    {
        printf("NOT ENOUGH RAM!!\n");
        exit(5);
    }

    strcpy(new->marka,marka);
    new->kubikaza = kubikaza;
    new->godiste = godiste;

    new->left = NULL;
    new->right = NULL;

    return new;
}

void add_location(AUTO *new, AUTO **root)
{
    if(*root==NULL)
    {
        *root = new;
    }
    else if(strcmp((*root)->marka,new->marka)==1)
    {
        add_location(new,&((*root)->left));
    }
    else if(strcmp((*root)->marka,new->marka)==-1)
    {
        add_location(new,&((*root)->right));
    }
}

void read_file(FILE *in, AUTO **root)
{
    char tmarka[MAX_SIZE];
    int tkubikaza;
    int tgodiste;

    while(fscanf(in,"%s %d %d",tmarka, &tkubikaza, &tgodiste)!=EOF)
    {
        AUTO *new = create_new(tmarka, tkubikaza, tgodiste);
        add_location(new,root);
    }
}

void write_one(FILE *out, AUTO *root)
{
    fprintf(out,"%s %d %d\n",root->marka, root->kubikaza, root->godiste);
}

void write_all(FILE *out, AUTO *root)
{
    if(root!=NULL)
    {
        write_all(out,root->left);
        write_one(out,root);
        write_all(out,root->right);
    }
}

void find(AUTO *root, char tmarka[])
{
    if(root!=NULL)
    {
        if(strcmp(root->marka,tmarka)==0)
        {
            printf("%s %d %d\n",root->marka, root->kubikaza, root->godiste);
        }
        else if(strcmp(root->marka,tmarka)==1)
        {
            find(root->left, tmarka);
        }
        else if(strcmp(root->marka,tmarka)==-1)
        {
            find(root->right, tmarka);
        }
    }
}

AUTO *newest(AUTO *root, int max_kub)
{
    if(root==NULL)
    {
        return NULL;
    }

    AUTO *best = NULL;
    if(root->kubikaza <= max_kub)
    {
        best = root;
    }

    AUTO *left = newest(root->left, max_kub);
    if(left!=NULL)
    {
        if(best==NULL || left->godiste > best->godiste)
        {
            best = left;
        }
    }

    AUTO *right = newest(root->right, max_kub);
    if(right!=NULL)
    {
        if(best==NULL || right->godiste > best->godiste)
        {
            best = right;
        }
    }

    return best;
}

Answer 1

A pointer is a pointer, and nothing more. It points to something else.

Take for example

int int_value = 1;
int *ptr_to_int = &int_value;

The variable ptr_to_int points to the location where int_value exists.

Now to have a "double pointer":

int **ptr_to_ptr_to_int = &ptr_to_int;

The variable ptr_to_ptr_to_int points to where ptr_to_int exists.

More "graphically" the above could be seen as something like

+-------------------+      +------------+      +-----------+
| ptr_to_ptr_to_int | ---> | ptr_to_int | ---> | int_value |
+-------------------+      +------------+      +-----------+

---

There are basically three uses of "double pointers", both related to arrays.

• The first is to have a dynamic array of pointers to objects (mostly structures).
• The second is to have a dynamic array of dynamic arrays of objects (a.k.a. a dynamic "2d array").
• To emulate pass by reference for a pointer.

In the case you show, with the functions init and read_file taking a "double pointer", it is the last case, to emulate pass by reference.

Answer 2

Unless explicitly defined as a reference, all parameters in C/C++ are passed by value, including pointers. When you change the contents of the pointer, you don't change the pointer itself, which is really just a memory address. If you have to be able to change the pointer, then you have to either pass it by reference, or as a double pointer.

In your code:

void init(AUTO **root);

init the value of the pointer to NULL, thus changing it. Sending AUTO *root would not allow this.

void add_location(AUTO *new, AUTO **root)

add_location can also set the base pointer if it is the first location added.

void read_file(FILE *in, AUTO **root);

read_file changes the pointer by creating a new instance.